5.6. The Boxes

5.6. The Boxes
For all lovers of the electronics, the box where their device is to be put is the famous “production weak link”. The finished boxes are either impossible to purchase, or they can be bought but their dimensions or shape is inappropriate, or they are too expensive, or... In cases like this one should be quick-witted enough to find some square-shaped box that is being used at household, or some packaging box or similar. That is how it’s done in the “whole white world”. Two years ago, in the famous electrotechnical magazine ETI TOP PROJECTS the article named “TIC TAC RADIO” was printed, where a receiver with ZN414 that is placed in the transparent plastic box of TIC TAC mints (In the abstract, it was written that making this device serves well as an excuse for buying candies, which is probably meant for the readers that are on a diet for aesthetic reasons).
However, the “finishing touch” is of great importance for everything. The majority of your friends will be more impressed by a lovely box where the receiver is placed, than the reproduction quality, type of modulation and other technical characteristics. And a nice, appropriate box cannot be bought, it is up to you to make it. It can be something as on pic.3.11 or similar. The idea can be also found in some catalogue of radio receivers’ manufacturers, or you can think of something of your own. As far as the author of these lines is concerned, he likes best the wooden boxes from the 20’s and 30’s of the previous century, from the times of the charleston, E. H. Armstrong and Al Capone. They looked something like those on the pic.5.17 and can serve you as an inspiration for your personal design.
The mid button is for the variable capacitor for station tuning, the right one is for the potentiometer for volume regulation. The button on the left can be a rotary switch for turning on/off (S). It can also be a tone regulation button, and for the reaction-type receivers it can be a button of the potentiometer that regulates the magnitude of the reaction. In the last two cases, the on/off switch (S) is located on the regulation potentiometer. The outside antenna and ground hubs are located at the rear panel of the box. The wires connecting the hubs with the PCB should be isolated, flexible and long enough to be able to open the panel and put it at upright position.
If the receiver is power supplied from the outside net, a green LED should also be added, as the power indicator. The good place for it is just above the variable capacitor’s button, instead of the triangle-shaped marker.
Pic.5.18 shows the parts for the first box from pic.5.17. For the front and rear side two pieces of 5 cm thick plywood, measuring 22 cm x 15 cm are needed; for the side panels, two pieces of 10 cm thick plywood, 15 cm x 9 cm, and for the bottom side - one piece of 10 cm thick plywood, measuring 13 cm x 9 cm. The best way to cut these parts is to be done by the carpenter on the special machine, since only then will they be of strictly rectangular shape, and bottom and side panels will have exactly the same width, which is very important during assembling. On the front side, the circle and the arc are drawn with the aid of the sector, and the cutting is done with the carving saw. The part that is cut from the back panel will serve as a closure. When it is cut it isn’t necessary to treat it with emery, since it will fit nicely in the hole on the rear panel even if it isn’t cut evenly. On the inner side of the rear panel two plywood lattices measuring about 2 cm x 13 cm should be nailed. Four wood screws will be screwed in them later (the holes are shown as four dots), which will serve to tighten the closure. Connecting of the pieces is done with the wood glue and small nails. Before you start hammering, it is very useful to drill a few holes for the nails in the front and rear panel with 1 mm drill. The nails are partially hammered into the panels, the edges are then covered with glue, and the nailing can then be done. When all this is finished, the box should look as the drawing at the right end art of the pic.5.18 *vertical stripes over the loudspeaker opening are not shown. They can be omitted, and you can nail in a few thin lattices, when the box is finished, as shown on the last drawing on the pic.5.17). The semicircle part is made of 5 mm x 5 mm lattices, or similar, which are put side by side on the upper edges of the front and rear panels, that are covered with glue (the picture shows only one of these lattices). When the last one is fitted, the space between them is filled with “putty” that is made by mixing the fine wooden chips with the wood glue, with the aid of a steel plate. After that, the lattices are tightened to the panels’ edges by two pieces of strong scotch tape, which are shown in dashed lines, and everything is left to dry well. When drying is, after about 10 hours, finished, all the edges and lattice parts that protrude are well flattened with emery. All the remaining holes are filled with the fast-drying putty, and everything is abraded once again, and the putty is applied again, and abraded again, etc., until the upper part is semicircle-shaped, all the sides smooth and the edges correct.

* Before the loudspeaker is attached with screws, a piece of decorating cloth should be placed between the panel and the loudspeaker, which will protect it and contribute to better looking box.

* Perhaps some of the readers will seem that there’s a lot of exaggeration in previous lines, and even too much pedantry. There’s a Latin proverb, that says: AGE QUOD AGIS - Do the things you do, which, in our case, can be interpreted as: You should either make the box properly or not making it at all.

* This box is relatively small, it is predicted for the loudspeaker that is about 12 cm wide. If you have bigger loudspeaker, and it will certainly play both louder and better, you should make a bigger box. The dimensions calculation is done by dividing the diameter of the bigger opening, that will suit bigger loudspeaker, in centimetres, by 11, and all the measures on pic.5.18 are multiplied with the number attained. E.g. if the diameter for the new, bigger hole is 15 cm, new dimensions are obtained by multiplying the old ones by 1.36.

5.7. Bimboard, Protoboard . . .
The readers that have carefully studied all the radio receiver projects that are described here, have possibly noted that the author referred to experimenting either with values of some components or with entire circuits, and all that was in order to practically find the optimal solution. When small changes are discussed, such as finding the optimum value for some resistor, that can be done on the previously made PCB. In case of bigger changes that of course is not convenient, and sometimes it is almost impossible. For all kind of electrical diagrams check-outs as well as various experimenting with all electronic devices, including radios, it is the best to use a special experimental board, which can be purchased under various trade names: protoboard, bimboard, matador, steckplatine, steckboard etc. All of them have in common that component connecting is done without soldering, by simply inserting the legs into the small holes on the plate.
As an example, pic.5.19 shows a full-scale experimental board that can be purchased in one of the Belgrade electronic shops. It has 630 vertically aligned holes, connected internally in 126 groups of 5 holes each, and another 100 holes placed in two topmost lines, connected in two horizontal groups by 50 holes each. The connections between the holes are inside the board and cannot be seen, they are shown on picture in dashed lines. The two topmost parts are used fo bring the supply voltage, and the battery or the adaptor is connected to them. One of them, most often the one that has minus pole connected to it (in all the devices described herein), also serves as the device Ground. The holes contain miniature metal hubs that are elastic, so when a leg is popped in, a reliable mechanical and electrical junction is accomplished. The distance between the adjacent holes is 2.54 mm (1/10 inch), which allows for connecting the vast majority of electronic components, which are being produced with inter-leg distance that is equal to a hole number multiplied with 2.54 mm (in the producers and sellers’ catalogues the 2.54 mm distance is marked as R, which stands for raster, and the components that have their legs horizontally and vertically distanced to 2.54 mm multiplied by some whole number are said to have their legs in raster).
The necessary electrical junctions between the hole groups are accomplished with connecting wires that can be bought at Conrad, but are more often self-made from plastic-isolated 0.5 mm or 0.6 mm copper wire. These pieces vary in their length and can be bent as the biggest piece in the lower left part of the pic.3.19, although it is better and nicer to use regular pieces, shaped as the cyrillic letter P.
Pic.5.19 also shows an example of practical usage of one such board. The radio-receiver from pic.3.15 is made on it. As can be seen, the coil ends are stuck into the holes whose coordinates are j,37; j,39; i,45; i,47, the diode in holes i,39 and i,45, the pin No.1 of the IC in e,54, etc. With the connecting wires the legs No.1 and 3 are connected, the ones that are connected with the potentiometer slider, and legs No.2 and 4 are connected to the ground by means of 4 connecting wires (the minus pole of the battery), etc.
It is now clear that experimenting is done in a very simple manner. E.g. if you are interested how does a capacitance of C2 affect the tone colour in the headphones, all you should do is remove it and insert a capacitor of greater or smaller capacitance, etc.

* The hubs on the board are elastic, so that conductors of various diameters can be easily inserted. No wires much thicker than 0.6 mm should be inserted, since the hubs will deform. The components whose legs are too thick as the variable capacitors, potentiometers, transformers and similar, are connected over pieces of wire that are soldered to them.

* It is useful for the connecting wires to be made with isolations of various colours, so that red ones could be used e.g. for connecting with the + battery pole, the black ones with Gnd, the yellow ones for the signal etc.

5.8. Universal PCB
Practical realization of simple radio receivers, as well as other simple electronic devices, can be done in many ways, as it was discussed in PE No.2. One of those is shown also in this number. That is construction of the detector receiver from pic.3.11, where some of the components are mounted onto the box walls (the variable capacitor, coil and the hubs), while other (the diode and two block-capacitors) are placed between them. With some skills, and by the aid of few smaller nails nailed from the inside of the front panel, a more complex device could be made, say, that from pic.3.12. But this solution would start looking as “the dead cockroach technique”, which will be discussed in the “Funniest Electronics”. The real solution is the PCB that can be made from the drawings that are given, or those you will draw yourself, together with the instructions given in chapter 5.1.
There’s another option for practical realization. It is a universal PCB, that can be bought in the electronic components’ stores. There are more sorts of these PCB’s, and all of them have in common that the holes on them are drilled on the distance of 1/10 inch (R=2.54 mm).
One of the universal PCB’s is shown on pic.5.27. It consists from a huge number of round copper isles, with hole in the middle. The components are being soldered first (resistors, diodes, IC’s, capacitors...), and then the component pins are connected by pieces of isolated copper wire, on the soldering side.
As an example, pic.5.28 contains the photograph of the receiver from pic.3.21-a that is made with the universal PCB from pic.5.27. It can be placed into a box as on pic.3.21-c, except the box should be bigger, in order for the loudspeaker to.

5.9. A Modern Oldtimer
The receiver on pic.5.22 is designed for the readers that wish to make a semi-conductor model of a complete direct radio receiver that was being produced many years ago, with electronic tubes. It had a total of 3 tubes, one of the contained the HF pentode (utilized in the HF amplifier) and the diode (used in detector), the other one had a triode (pre-amplifier) and powerful pentode (power amplifier), whilst the third one contained the duo-diode (the rectifier).

5.5.2. Electronic Tuning

5.5.2. Electronic Tuning
Instead of the capacitor CR, that was used for fine tuning in the previous project, a capacitive (varicap) diode can be used. It’s a special HF diode which is polarized by exposing it to DC voltage in order to be non-permeable (+ to the anode, - to cathode). By changing the voltage diode’s capacitance also changes, which allows for it to be utilized as variable capacitor. If, acc. to pic.5.13-a, the DC voltage between the cathode and anode (UAK) varies from U1 to U2, diode’s capacitance goes from Cmax till Cmin.
The electronic diagram for the electronic fine tuning circuitry is given on pic.5.13-b. Diode capacitance is changed by moving the slider of the P1 potentiometer. By means of trimmer TP the necessary Cmax is set, and when this is done TP can be replaced by an ordinary resistor. All the components are mounted on the PCB, together with other parts of the receiver, except the P1. It is mounted on the front panel, and connected to the PCB with 3 ordinary wires.
* The variable capacitors that were used for tuning in all the receivers described so far are solid, lasting, reliable

components. Their mishap is they are hard to purchase, they are quite robust (compared to other device components), and their mounting isn’t simple because the shaft for the knob must go through the front plate of the device box. That is why varicap diodes are also replacing them. With the diode that has Cmax/Cmin ratio that is big enough, say, Cmax/Cmin>15, the circuit form pic.5.13 can be used as the variable capacitor (C is simply omitted). In that case, some bigger knob with an arrow is mounted on the P1 handle, and numbers from 1 to 10 are written on the panel, as shown on pic.5.13. This scale allows the listeners to see what station is the receiver tuned at. Of course, for the MW band, the numbers as those on pic.3.7 can also be written.
* In case of SW band, the P2 potentiometer is added for fine tuning.
The optical indication of the tuning, with and knob with arrow is the simplest solution possible. More prettier one is using a small movable-coil instrument (V), such as those used as battery indicators in industrial devices, or for tuning indication and similar. The connecting is done acc. to the diagram on the left part of the pic.5.13-c. In series with the instrument, the TP potentiometer is attached. Its resistance depends on the maximum instrument current, and can be found experimentally. For start, you may use a 1 MOhm linear trimmer, with its slider at lowest position (so that its resistance is maximum). Put the P1 slider also at the lowest position. Turn on the receiver. Start moving the P1slider upwards, and observe the instrument needle. if it soon goes to the end, you’ll have to take a trimmer with greater resistance or to add another resistor in series with it, so that when the P1 slider gets to its rightmost position, the needle goes somewhere around the middle of the full scale. If the needle, with P1 in topmost position, moves too little, you’ll need a smaller resistance trimmer. When you succeed in having the needle in the middle of the scale with P1 in topmost position, start moving the TP slider until the needle reaches the end of scale. The circuit is well adjusted if the needle goes from zero to full scale while P1 slider is moved from bottommost to topmost position. The instrument can have any shape, but the most appropriate (and cheapest) is square, like the one on the picture.

5.5.3. Suppressing the Signal of the Local Transmitter
From all the signals in the reception antenna, the one that is created by the local transmitter is by far the strongest one, due to the fact that it is hundreds, sometimes even thousands times closer than other radio transmitters. That signal can be so strong that it can jam normal reception of other stations. In case of simpler receivers its programme is heard,more or less, in all the positions of the variable capacitor. The solution for this problem is the so-called seal circuit, which serves to weaken the signal of the local transmitter, so that it doesn’t interfere (but is still strong enough for normal reception, when the receiver is tuned at it).

The seal circuit is a parallel oscillatory circuit which comprises the coil L1 and capacitor C1, as shown on pic.5.14-a. By means of C1 the resonance frequency of the circuit is set so that it corresponds to the carrier frequency of the local station. On that frequency, this circuit behaves as a huge resistor (see pic.3.2-b) and decreases the current that is created by the local transmitter signal. For other signals it has very small resistance and practically has no effect on them. The setup is done by tuning the receiver on the local station, and the reception is weakened enough by turning the C1. If the decay is too strong, a resistor should be added in parallel to C1.

Using a variable capacitor in the seal circuit (pic.5.14-a) isn’t an economical solution. It is much better, considering both economy and space, the solution given on pic.5.14-b. A block capacitor C1 and a variable inductance coil are used in the seal circuit. As shown on the framed part of picture, the coil is wound on the plastic body, with ferrite core. The number of quirks is found experimentally about couple of hundreds of quirks made with as thin copper wire as possible). The capacitance for C1 is also found experimentally (couple of hundreds of pF). The earlier mentioned IF transformer can also be used as a coil. With labelling acc. to pic.4.3-a, legs No.2 and 3 are used, the others are “hanging” (they are not soldered). C1 capacitance is also found experimentally. It is also possible to wind the coil on a piece of ferrite rod, as shown on pic.5.14-b, and setup to be done with trimmer Ct
5.5.4. Dual Tuning
The author of this book, as great radio techniqe lover (amateur, in French), owns great collection of over 150 pieces of various old-timer radio receivers. There is one among them that is over 60 years old, at which the tuning is being done by two knobs. With first one the receiver is set roughly to the desired station, which is usually barely heard at that moment. The second knob is then turned until the optimum reception is achieved, which is significantly better than before, and in case of weak stations - extremely better.
The selectivity of simple receivers that were described in previous chapters can be significantly increased by using the aforementioned dual tuning. The electronic diagram is shown on pic.5.15-a. Another oscillatory circuit, made of L1 and C1 connected in series, is inserted between the antenna connector and input circuit of the receiver (it can be any of the earlier described AM receivers). As with the earlier mentioned parallel oscillatory circuit, the resonance frequency of the serial circuit is given by the Thompson pattern:




The serial oscillatory circuit has very small impedance (compared to the parallel circuit whose impedance is very big on the resonance frequency). The dependance of the impedance (”resistance”) of the serial oscillatory circuit from the frequency is shown on the diagram on pic.5.15. As you can see, the serial circuit acts as a resistor of very small impedance only for the station that it’s tuned at. For all other stations, it behaves as a huge resistor (impedance). All in all, from all the signals in the antenna, the biggest current, and therefore the biggest voltage on the input circuit is created by the transmitter that both serial and parallel oscillatory circuits are set to. The tuning is done as it has already been described, first with C (so-so), then with C1 (much better).

* Between the coils L1 and L a magnetic coupling should be prevented. This is accomplished by mounting the coils to be as far from each other as possible, and to position their axes mutually perpendicular.

* Greater experimenting opportunities with dual tuning provides the diagram on pic.5.15-b. Once again, it’s the serial resonance (in circuit L1, C1), and parallel resonance (in circuit L, C), that are being used. The coils are placed side-by-side, in order to generate magnetic coupling between them. The tuning is done as previously explained, but now we also have a possibility of changing the amount of magnetic coupling between the coils by moving them closer or farther, which affects the antenna’s influence on the L, C oscillatory circuit, therefore changing its selectivity and sensitivity.
5.5.5. Separation of Stages - Preventing the Oscillation
On of the significant problems that occur at devices that comprise more cascade-linked amplifying stages is the occurrence of the feedback over the conductors that connect those stages with the positive pole of the battery, or the power supply. By the way, the feedback is a phenomenon when part of the signal exiting an amplifier gets on its input. Under certain conditions, this feedback causes the oscillation of the stage, which in devices that have the loudspeaker on output, manifests itself as strong whistling, squeaking and similar.
On of the ways to prevent this feedback is given on pic.5.16, where a block-diagram of a radio receiver that has four amplifying stages with active components (transistors or IC’s) that require the battery supply is shown. Separation of stages for the AC current (preventing the feedback) is accomplished by the LF filters with resistors and capacitors. Resistors are from couple of hundreds of Ohms to 1 kOhm. Capacitances of C1 and C2 are from couple of tenths till couple of hundreds of nF, and of C3 from couple of hundreds of nF to about 100 mF. The stage PCBs should be designed in such way to make the contact where right end of the capacitor is soldered as close to the contact where the positive end of the power supply voltage is brought (e.g. on pic.5.9, the right contact for C6 should be as close as possible to the contact where pin 8 of NE612 is soldered).
In the devices supplied from the battery, the C5 capacitor, which has capacitance of couple of hundreds of micro Farads, serves to take the role of the battery when it gets emptied a little bit, and strong tones have to be reproduced at the loudspeaker (in simple terms, C5 acts as a small accumulator that helps the worn-out battery to give enough power to the power amplifier, when necessary. When its help isn’t needed, the capacitor is refilled). This capacitor is not needed when the receiver is supplied from the adaptor that already has an electrolytic capacitor on its output, and when the wires that connect the adaptor to the receiver are not longer than about 15 cm.

5.3. NE612

5.3. NE612
5.3.1. Synchrodyne AM Receiver
If the author remembers well an article that he read in a professional magazine many years ago, the synchrodyne receiver is the ancestor of the superheterodyne receiver. Sometime at the beginning of the 20th century this device was called the Heterodyne receiver, and it was first constructed by Levvy. Armstrong improved it and gave new name to the new device, by adding the prefix SUPER to the old name.
The electronic diagram of this device is given on Pic.5.7. This receiver, as well as that on pic.4.2 has got the local oscillator with oscillatory circuit connected between pins 6 and 7. However, frequency of this oscillator is not greater for the value of fm, but is in fact equal to the frequency of the station we wish to listen to: fm=fS. Because of that, the important design difference compared to the diagram from pic.4.2 is that on pic.5.7 capacitors Co and Cto are not used, but the capacitor C which is obtained by connecting the legs O and A, acc. to pic.3.7. Its capacitance can be changed from 12 pF till 218 pF, so that the oscillator frequency, in case of MW reception, goes between 500 kHz to 1500 kHz. The oscillator voltage is emanated in the mixer by the signals from all stations coming from the antenna. The result of emanation with signal of the station whose frequency is equal to the oscillator frequency is the LF signal (speech, music, Morse Code etc.) that serves for performing the modulation in the transmitter. This signal is obtained on pin No.4, from which it is then led, over the 1 ìF capacitor, to the volume potentiometer and audio amplifier. Products of mixing the oscillator voltage with other stations’ signals are also obtained on that pin.

They are being suppressed by the LF filter that comprises the R* resistor and C* capacitor. The device we were testing did not, however, contain R*. It is to be installed if some disturbances occur (whistling or similar), and its optimum value is to be found experimentally. If necessary, greater capacitance of C* is also to be tried out.

* As mentioned earlier, it is very important for the supply voltage of the NE612 to be stable. This values even more for the synchrodyne then the superheterodyne receiver. The voltage control is done by the stabilizer, made with 78L06 IC. It is being placed in the low-power transistor package, either metal (as for BC107) or plastic (as for BC547), and its maximum current is about 100 mA (pic.5.7-b). A simpler stabilizer, made with the Zener diode, can be used instead, as on pic.5.9.

* Instead of factory-made coil LO, the self-made one can also be used. The simplest solution is to use the one from pic.3.6, in which case the mid leg is not used. Over this coil, the feedback coil should be winded, acc. to pic.5.7-c (its ends are marked with 4 and 1). When connecting with capacitor C and pins 1 and 7 of NE612, care should be taken to join properly: coil ends 1 and 3 with ground, 2 with capacitors C and 560 pF, and 4 with 1 nF capacitor. It is, of course, possible to use smaller coil, wound on a smaller body, with more quirks of thinner wire. Its inductance should be about 350 mH, and the number of quirks required is to be found by testing. The feedback coil (4-1) has app. 3x fewer quirks than the oscillatory circuit coil (2-3).

* On the pin 5 of the NE612 the LF signal is also obtained. It is the same as the one on pin 4, but has a 180° phase shift compared to it (in simple words, while one signal increases, the other one decreases, and vice versa). That gives us the opportunity to use the dual audio amplifier in the LF part, that has two amplifiers, with inverting and non-inverting inputs. As shown on pic.5.8, the counter-phase LF signals from NE612 are led onto the same inputs. The output signal has 2x greater amplitude, therefore making the output power 4x greater than when only one input is used (as on pic.5.79).
5.3.2. AM Receiver with Synchro Detector
In previous project, the NE612 was in fact used as the AM signal detector. The LF signal exiting the mixer is product of the simultaneous (synchronous) action of the station signal and voltage from the local oscillator upon it. That is how the term “Synchro Detector” emerged. There’s also a possibility to use a station carrier instead of local oscillator’s voltage, so that the station signal gets beaten by itself, however strange this may sound. Electronic diagram of one such device is given on pic.5.9.
The station signal, which the input circuit (C, L) is tuned at, is led to the regulating Gate of the BF960 MOSFET. Under the effect of this voltage, the AC current that creates voltage drops on resistors R2 and R3 runs through the transistor. These two voltages, taken between the S and ground and D and ground, are mutually shifted in phase for 180°, and are being led over the coupling capacitors C2 and C3 to pins 1 and 2 of the NE612, i.e. on one input of the mixer. On the other mixer input the Drain signal is brought, over C4, and beating occurs in the mixer, the result of which is the LF signal on pin 4. This signal is, over C8, being led onto the volume regulation potentiometer and the audio amplifier.
* The unwanted (and parasite) products of mixing, that are manifested as whistling, squeaking etc. are being suppressed by the C7 capacitor. If the obstructions still exist, the capacitance of C7 is to be increased and/or the R* resistor added.
* The voltage stabilization of the DC voltage on pin 8 is performed by the ZPD6.2V Zener diode and resistor R5. A diode with smaller voltage is also possible to be used, say, 6.2 V and similar. If the supply voltage is less than 12 V, the resistance of R5 should be decreased.

5.3.3. Input Circuits for Receivers with the NE612 IC
All the receivers with NE612 that are described here work better, especially considering suppressing the noise in case of the symmetrical station, if the proper input circuitry is added to them. Pic.5.10 shows two examples of the MW receivers that use the ferrite antenna. In both cases, the antenna taken from an old commercial radio is being used.
5.4. The Universal Audio Amplifier
We already spoke about the universal amplifier in the text connected with pic.3.22. Pic.5.11 contains the diagram of another such device, where the transistor amplifier with BC547 is used as the pre-amplifier, instead of that with TLO71 IC. It can be used for practical check of all the earlier mentioned radio receivers. The LF signal is being taken from the detector in the HF part of the receiver to the hubs marked as In and Gnd (if the links aren’t too long the ordinary wires are used, otherwise - the microphone cable). On the third hub the DC voltage is outputted, which is used in some HF circuits for their operation (such as e.g. those on pics.3.24, 3.25, 3.29 etc.).
* The LED (and the appropriate resistor) are added if the amplifier is being supplied from the adapter connected to the household voltage installation. It can also be used if the amplifier is power supplied from the battery, but this is not recommendable, since its power consumption is fairly big, which significantly shortens the battery life.
* The amplifier can be put in a box of any kind, one of the possible solutions shown on pic.5.11-b.
* A very useful solution can be to place the adapter also in the box, with the ability to control its output voltage from few volts to 12 V. In that case, you have both the amplifier and adapter in the same box, which can be used for power supply and check-out of various electronic devices, and not just radio receivers.
5.5. Additional Circuitry
5.5.1. Fine Tuning
During the tuning of the receiver to some station at the SW band with the variable capacitor, a problem occurs. In simple terms, the station frequencies are too close to each other, so the capacitor’s shaft should be turned for an extremely small angle in order to change station, which is practically impossible. It would certainly be useful if we could somehow stretch (a popular term for this) the part of the band near the frequency to which the receiver is tuned at. For the direct type (TRF) receivers that were described in the previous chapters, this can be accomplished if, acc. to pic.5.12, another variable

capacitor (CR) is added in parallel to the variable capacitor at the input circuit. Its capacitance should vary at substantially smaller scale than that of C, meaning from a few pF til about 20 pF. The tuning is accomplished by setting the receiver, by means of C, approx. at the middle of the band we are interested in, then tuning by means of CR to some station in that area. E.g. if the stations we want to receive are located in the part of the SW band from 6.1 MHz till 6.2 MHz (it’s a well-known 49-metre band), first we tune ourselves with C to approx. 6.15 MHz, and then we pick with CR some of the stations located in that area. The same applies for the famous Magic Band (at about 50 MHz).
The CR capacitor is mounted close to C in order for their knobs to be near each other at the front plate.
* As CR, some air-type trimmer capacitor can be utilized, with adjustment knob mounted on its shaft. Also, one of the sections of the variable capacitor from pic.3.8 can be used, as shown on the right part of pic.5.12.
* The problem of the station “adjacency” at the SW band also exists at the superheterodyne receivers. It is being solved by adding the CR in parallel to the variable capacitor in the local oscillator circuit. The reason for this is that, at supereterodynes, the station is chosen over the local oscillator. The important thing for the oscillator is to have the exact frequency, that is greater from the station frequency for the amount of the interfrequency. If the resonance frequency of the input circuit isn’t equal to the station frequency, it won’t significantly affect the reception. Because of all this, in the receiver on pic.4.5, CR is attached between the pins 2 and 3 of the LO circuit.

4.2.2.1 Mini FM Receiver

4.2.2.1 Mini FM Receiver

The electronic diagram of the monophonic FM receiver made with TDA7088T is shown on Pic.4.12. If built with SMD components it can be placed in a matchbox, altogether with two button-type batteries. The operating principle of this device is given in the previous chapter. The only thing new is a very simple audio amplifier made with BC547 transistor, which is loaded by cheap 16-Ohm headphones. The telescopic antenna is used, as on Pic.4.8.
Small mishap of this receiver is that it has no indication of station tuning. This problem can be solved by adding a small voltmeter in parallel to the BB909, whose scale is graduated in MHz, as described in the Appendix. This solution is not appropriate for the miniature receiver, since the voltmeter that has the scale that is big enough takes too much space. It is in this case better using a manual tuning instead of automatic. Such solution is given on Pic.4.13.
The tuning is done via the variable capacitor C with numbers written on its button, similar to that on Pic.3.11. It is most simple to use numbers from 1 to 10. The variable capacitor is like the one on Pic.4.8. Some experimenting is to be done with capacitances of Cx and Cy, in order to cover the entire reception bandwidth, from 88 till 108 MHz.

The AFC (Automatic Frequency Control) of the local oscillator is accomplished with BA483 diode, obtaining that station’s position on scale does not “walk” over the scale.
The complete radio receiver should still have a loudspeaker. Electronic diagram of such receiver made with TDA7088T is given on Pic.4.14. As one can see, that is a receiver from Pic.4.12 with an audio receiver made with LM386 IC.
Maximum value of the DC supply voltage for the TDA7088T is 5V, therefore if using a 4.5 V battery the LM386 will work with reduced output power, the D2 diode and C15 capacitor should be omitted, and R4 should be short-circuited.

If higher voltage battery is used, the voltage stabilizer, comprised by the aforementioned components, has to be activated. D2 is a Zener diode with 3 V Zener voltage. The optimum value of R4 is found experimentally: in order to make the power consumption as low as possible it should have the resistance as big as possible, while simultaneously keeping the voltage on Pin 4 about 3 V and the device working well within the entire reception bandwidth (One should start with, say, R4=1.5 kOhm, and if the receiver operates well bigger resistance should be tried out, and if not smaller one, until the optimum value is found).

Pic.4.15. shows the PCB for the HF part of the receiver with TDA7088T, that is realized with ordinary components, instead of the SMD’s. Pic.4.15-a shows the board layout from the soldering (copper) side. All the components apart from TDA7088T are mounted on the opposite side of the board, their pins are put through the holes and soldered through the holes. The TDA is soldered on the copper side, directly onto the copper contacts. That is why it is being drawn in dashed line on pic.4.15-b, where the board layout on the component side is given.

* Pic.4.16-a shows 3x enlarged picture of the IC and the surrounding lines. The soldering procedure for SMD is as follows:
A thin tin layer is applied on the copper contacts where IC legs are to be soldered to. The firs legs to be soldered are the diagonally opposite ones, in this case No.1 and 9. A small cushion-shaped amount of tin (not profuse) is applied on the contacts where these pins are to be soldered (pic.4.17-a). The IC is placed in its position, with all the pins properly laid. Pin No.1 is pressed against the tin pillow with a top of a bodkin, with iron head simultaneously touching both the tin and the pin end. The tin gets melted, and the pin lies down on its place with the aid of the bodkin, and gets soldered.
It is now time to check out the positioning of the chip. If it needs to be corrected, the tin surrounding the pin No.1 is melted with iron tip and the chip position is quickly and carefully adjusted, in order not to overheat the pin. Soldering the pin No.9 is shown on Pic.4.17-b. First, the iron tip is simultaneously put on the top of the leg and the copper below it, so that both of them are heated. After app. half a second, the iron is slightly removed from the leg but remains on the copper contact. The tip of the tinol wire is then approached to touch the iron, the pin top and the copper contact at the same time. The wire gets melted and adheres to the copper and the pin, so it has to be constantly moved downwards. When enough tin is applied, the tinol wire is removed first, then the iron, and the pin No.9 is soldered. Once again you have to check whether all the pins are properly placed, and then they too are soldered as it was just described. The solders are OK if they look app. as on Pic.4.17-c.

* Pic.4.15-b contains the PCB component side layout. The pushbuttons we used here are Siemens, type BO2AMAP-2. The common housing contains, as one can see, two button switches, one of which is being used by this device. Any other pushbutton switches can also be used. In that case small modifications on the PCB lines would probably be necessary. The board is mounted fairly close to the box edge, so that the switch shafts are passing through the panel, and that the buttons can be mounted on the outside. The panel-mount switches can also be used, in which case they are connected to the board by wires (pic.4.15-e).

* Any audio amplifier described so far can be used, e.g. the one with LM386, as on pic.4.8.
* Instead of the antenna, a 20 cm piece of wire can also be utilized.
4.2.2.2. Stereophonic Receiver Built with TDA7088T
Stereophonic radio broadcast is performed in the ultra short waveband, from 88 MHz till 108 MHz. All radio transmitters operating in this range are stereophonic, but their signal is designed so that monophonic receivers can also read it, performing the compatibility. The readers that wish to get acquainted in more details with the stereophonic broadcast basics can refer to the “Radio Receivers” textbook, for the IV grade of the Electrotechnical Highschool.

Making an introduction to this part, a operating principle of the stereophonic radio receiver shall be considered, its block diagram shown on pic.4.18. Comparing this diagram with the one of the monophonic receiver given on pic.4.6, one may notice that they are identical, up to the block called "The Decoder". It means that, as already described, exiting the FM detector the LF signal is obtained, i.e. the information that was used to perform the frequency modulation in the transmitter. However, this is not an ordinary LF signal, but the one, called the "composed" (KS) or "multiplexed" (Mpx) signal. Besides the full-scale LF signal used by the monophonic receiver,

it also contains the so-called auxiliary signal which allows the separation of left (L) and right (R) channels in the stereophonic receiver. E.g. if a direct broadcast of some band music is performed, the left part of performers is being recorded with one microphone (the signal marked as L), whilst the right side is recorded with the other one (it’s a R signal). These two signals are being led in the FM transmitter in the stage called “the coder”. Exiting the coder we have the multiplexed signal Mpx which contains, in an indirect manner, both left (L) and right (R) signal. Frequency modulation of the transmitter is being performed with the Mpx signal. In the receiver, Mpx signal is obtained on the output of the FM Detector and is then led to the decoder. This stage plays a role complementary to the one of the coder in the transmitter, therefore two signals are exiting it, the L and D signal. They are being amplified over two identical audio amplifiers, then reproduced over two same loudspeakers. The listener can now hear the left half of the performers from the loudspeaker placed on its left, and the right half from the loudspeaker that is placed on its right. The performers that are situated in the middle of the orchestra are being equally reproduced from both loudspeakers, making an impression to the listener as if there’s a third loudspeaker, located in the middle, between the left and right one. Based on all this, the listener has a picture about the layout of the performers in space, which significantly improves the total musical impression.
Electronic circuit of a portable stereophonic radio receiver with headphones reproduction, made with TDA7088T is shown on pic.4.19. It is a receiver whose practical realization was described in the previous project, with decoder with TDA7040T and dual audio amplifier with TDA7050T blocks added, the latter was discussed in PE5.
* L3, L4 and L5 are HF chokes that allow for the headphones cable to be used as a reception antenna. This is accomplished by connecting one of the headphones’ contacts from the plug-in, over the 10 pF capacitor, to the point where, acc. to pic.4.14, the outside antenna is connected. The coils represent big resistance to the station signals, preventing them to “go to ground” over the 47 mF capacitor or over the TDA7050T output. Each coil has 3 quirks of the 0.2 mm CuL wire, threaded through ferrite pearls, as shown on detail in the right corner of the pic.4.19. If telescopic antenna is to be used, these coils should be omitted.

4.1.2. The Fully (not exactly 100%) Superheterodyne AM Receiver No.1

4.1.2. The Fully (not exactly 100%) Superheterodyne AM Receiver No.1
Its electrical diagram is given on Pic.4.4. It is easily being noticed that this is the receiver from Pic.4.2 with inter-frequency (IF) amplifier with ZN415E added.
By adding ZN415 IC multiple enhancements are performed. Thanks to its huge input resistance, the MFT's oscillatory circuit is not choked, resulting in better selectivity. The sensitivity of the device is extremely increased since this IC has big amplification and the AAR (automatic amplification regulation) is also accomplished, making the usage of this device easier and more comfortable.
* It is very important to obtain the necessary value of the DC voltage in pin 6 of the ZN415 for its proper operation. Acc. to the table on Pic.3.36 it has to be about 1.3 V, and its setting is done via the TP1 trimmer. The receiver is set to some weaker station, the sound volume is made very low with potentiometer P, and the slider of the TP1 is carefully moved until the best reception is made. If that doesn't work, one should try changing the value of R5 resistor; this is to be done also if the supply voltage being used is other than 12 V. In case of voltage on the pin being much bigger than 1.3 V, and cannot be reduced on the trimmer, short-circuit one of the diodes.
* The voltage stabilizer with 78L06 isn't needed if the receiver is supplied from the 6 V battery.
* The receiver from Pic.4.2 needs input circuit to be 100% complete. That can be an independent input circuit from Pic.4.3-i, or input circuit and the HF amplifier that are described in the Appendix (Pic.5.10). If the former circuit is used, station tuning is being accomplished with 2 knobs, as explained in the previous chapter.

4.1.3. Fully (not exactly 100%) Superheterodyne AM Receiver No.2
All the receivers we made with NE612 IC were tested in our lab, except the one from the previous project, since we didn't have ZN415 "with us". We found, however, a ZN414 IC, so we tested the receiver from Pic.4.5 with it. The receiver was working great, from the amateur's point of view. He played us for long time, until we didn't require the board to test one of the receivers from previous projects afterwards, when we regretfully had to disassemble it.
* The diagram is very similar to that on Pic.4.4, so most of the things said about that receivers stands for this one, too.
* DC voltage setting on pin 1 of ZN414 is done with the trimmer TP. Its slider is put in mid position, the receiver is tuned to some weaker station close to the upper bound of the bandwidth. While making the reproduction very quiet (slider of P as low as possible), the trimmer slider is moved until reaching optimum reception. After that the trimmer is disconnected, its resistance measured and the ordinary resistor of similar value is put into circuit.
* The device operates nicely with the outside antenna made of a piece of wire measuring only half metres in length.
* The reception would certainly become better if an input circuit would be added, which we spoke about in the previous project.
* The receivers from pics. 4.4 and 4.5 can, with appropriate coils in the oscillator, accomplish the reception of AM stations from all the bandwidths from 70 kHz till 200 MHz.
4.2. Superheterodyne FM Receivers
The FM receivers being described in chapter 3.15 are the amateur solutions. These are extremely simple devices, that cannot perform the noiseless tuning, automatic oscillator frequency regulation and other features that ensure very high quality of the reproduction, being expected from an UHF FM receiver. The true solution is the superheterodyne FM receiver, whose block-diagram is given on Pic.4.6.
Station signals are taken from the dipole antenna and led through the appropriate cable into the input circuit (UK). Inside it, the signal selection is performed, of station whose frequency is fS, this signal is then amplified in the HF amplifier and led into the mixer. As in the case of earlier described AM receiver, the inter-frequency signal is obtained at the mixer output, whose carrier frequency is fm=10.7 MHz (this is the standard value, used in all radio-broadcast FM receivers). The IF signal is being amplified in the IF amplifier and led on the amplitude limiter (Ogr.). In this stage the signal whose amplitude exceeds certain level is being cut off, accomplishing with this the elimination of the parasite amplitude modulation, which is performed by various noise sources during the transmission (atmospheric charges, various electrical devices etc.), which significantly improves the signal quality. The signal then goes to the FM signal detector, where the information being modulated in the transmitter is extrapolated from the signal, followed by the LF part of the receiver. With AFC the circuit that performs the automatic frequency regulation of the local oscillator is labelled.

4.2.1. FM Receiver with TDA7000
The face that FM receivers operate on pretty high frequencies makes their practical realization somewhat difficult, but most of the problems, as in many other amateur builds, originates from building the coils, except the self-bearing, small-inductance coils (without the coil body), which are easy to make, especially if there aren't many of them in the device and if no special instruments are required for setting up their proper inductance value. The coils used in this FM receiver are just like this, and there are only two of them, making the practical realization much easier.
The basic data about the famous Philips' IC used in this project, TDA7000, are given in the following table.

Electronic diagram of the HF part of the device (from antenna to the LF output) built with TDA7000 is shown on Pic.4.7. As one can see, it is a simple device, made with relatively small number of components. The IC contains all the stages of the superheterodine receiver: the mixer, the oscillator, the IF amplifier, the amplitude limiter, the FM detector and few others. More about them will be told in the next project which contains the description for a receiver with TDA7088T IC, which is the improved version of TDA7000.
The station signal is from the (telescopic) antenna led to the input circuit that consists of L2, C13, C12 and C14. It is a parallel oscillatory circuit damped with R3 resistor, which has the reception bandwidth from 88 MHz till 108 MHz (it admits all the UHF signals on the pin 13, and weakens te signals outside the reception bandwidth). Inside the IC the signals are led into the mixer, where they are being given new carrier frequencies. The IF amplifier then follows, amplifying only one of those signals, the one whose frequency is equal to the inter-frequency, followed by the limiter, the FM detector, mute circuit and LF pre-amplifier. The output from the last stage is on the pin 2 (R2 is the collector load of the last transistor in the LF pre-amplifier). The oscillatory circuit of the local oscillator (L1, Cp, Cs, C and C5) is connected between pins 5 and 6.
Pic.4.8-a shows the PCB of the device from Pic.4.7, while Pic.4.8-b contains the component layout (on the PCB). The complete device can be seen on Pic.4.8-c. The variable capacitor from Pic.3.8 is used as the only variable capacitor here since the input circuit is aperiodic, the legs marked with FO and G. This capacitor serves us to tune the receiver to stations. In the LF part of the receiver, the amplifier made with LM386 from Pic.3.19 is utilized (the components left from the potentiometer are omitted).
* L1 and L2 are the self-bearing coils (without the core). They have few quirks and are made of relatively thick wire, therefore they don’t need a body of any kind, that is why they are called “self-bearing”. Their appearance is shown on Pic.4.9, and the calculus for them is done acc. to the table from Pic.3.5. They both have 6 quirks of the CuL wire, 0.6 mm in diameter, being spooled on the flat part of the 3 mm drill. In order to be able to solder the coil onto the PCB, the couple of mm of isolation has to be removed from the wire ends with sharp knife, and they have to be tinned afterwards. There must be a small gap between the adjacent quirks. The inductance of the coil is set by its shrinking (the inductance increases) or stretching (the inductance decreases). Stretching can be nicely done by inserting the screwdriver between the quirks and then turning it along the coil.
* The TDA7000 also contains the mute circuit (for noiseless tuning). It is being active when the S2 switch is open. Pocket-type receivers usually do not have S2 and R1 elements.
* The part of the receiver that requires biggest care during build is the oscillatory circuit of the local oscillator, which is connected between the pins 5 and 6. When changing the capacitance of C, its resonance frequency must change from 88 MHz (C=Cmax) till 108 MHz (C=Cmin). If that cannot be accomplished (not all the stations can be heard) some experimenting is required with capacitances of Cp and Cs. For start, you should omit the Cp. If the problem persists,

capacitance of Cs should be reduced (to 15 pF, 10 pF etc.), or it should be short-circuited. You can also try compressing or stretching the L1 coil, etc. The setup of the oscillatory circuit is completed when with C=Cmax some station that operates on app. 88 MHz can be heard, and with C=Cmin the one that works on 108 MHz.
The input circuit setup (it is connected between pins 13 and 14), is performed by tuning the receiver to some mid-range station (about 98 MHz). Then, the best possible reception is searched, by changing capacitances C13 and C12 and inductance L2.

4.2.2. FM Receiver with TDA7088T IC
The receiver described in the last project has two IC’s, one variable capacitor, two small coils and fairly small few other components, so it can be put into some small box, by carefully placing the components. Further miniaturization can be accomplished by using the SMD components. These are the resistors, capacitors, transistors, IC’s and other components, whose dimensions are significantly smaller than these of “classical” components. They are mounted on the copper side of the PCB, therefore it isn’t necessary to drill the holes on the board. TDA7088T is also an SMD component. Its drawing is shown on Pic.4.10.
This IC is the successor of the famous TDA7000, i.e. it is an improved model of TDA7000, that allows to implement both monophonic and stereophonic FM receiver. The basic features of TDA7088T are given in the following table.

The electronic diagram of the HF part of the monophonic FM receiver made with TDA7088T IC is given on Pic.4.11. The IC contains all the parts of the classic superheterodyne receiver: the local oscillator, IF amplifier and FM detector, but also some other circuits that extend the possibilities and improve the features of this IC.
As far as practical use is concerned, the most significant novelty is the auto-tuning circuitry. No variable capacitor is necessary for tuning, as it was in all the previous projects, the BB910 varicap diode is used instead. Its capacitance is being changed by varying the DC voltage supplied to its anode over the 5k6 resistor. This is how the tuning is performed: When the user press and releases the pushbutton marked with “RUN”, the positive voltage impulse is released to the S(et) input of the SEARCH TUNING circuit. The 100 nF capacitor then starts chargingl and the voltage on the pin 16 increases. This voltage is then transferred, over the 5k6, to the anode of the BB910, causing its capacitance to decrease, which increases the frequency of the local oscillator (VCO). The VCO voltage is led into the mixer (MIXER) which also receives, over pin 11, the signals of all the other FM stations. The mixer outputs the FM signals whose frequencies are equal to the differences of the oscillator and the original station frequency. The only signal that can reach the demodulator (FM detector) is the one whose carrier frequency is equal to the inter-frequency, i.e. fm=73 kHz (selectivity is being accomplished by two active filters whose components are the capacitors connected to pins 6, 7, 8, 9 and 10). Therefore, the oscillator frequency increases until it gets the condition fO-fS=73 kHz is accomplished. When this happens, the charging of the capacitor is halted by the command that is sent into the SEARCH TUNING circuit by two detectors (diode-blocks) located in the MUTE circuit. The AFC (Automatic Frequency Control) circuit now gets its role and prevents the voltage on pin 16 to be changed, until the RUN button is pushed again (this voltage can vary from 0 V til 1.8 V during the tuning).
When the RESET button is pushed, the 100 nF capacitor is discharged, the voltage on pin 16 drops down to zero, and the receiver is set to the low end of the reception bandwidth, i.e. 88 MHz.
Let us get back to the mixer. On its output, the 73 kHz FM signal is obtained, and it is modulated by the programme of the first station that is found after the RUN button is pushed. This signal then passes the active filters, gets amplified in the IF amplifier (IF LIMITER) and passed onto the input of the demodulator. By connecting the demodulator exit, over the LOOP FILTER, the adder (+) and resistor, to the VCO, the so-called FFL (Frequency Feedback Loop) circuit is accomplished, reducing the deviations of the signal being received from ±75 kHz to ±15 kHz.
The LF (AF) signal is led from the demodulator, over the LOOP FILTER stage, the invertor (-1) and MUTE circuit onto the pin 2. The detectors (diode-blocks) control the operation of the MUTE circuit, preventing the LF (AF) signal to reach the output pin (2) until the tuning on the station that creates the signal in the antenna that is strong enough for quality reception is obtained.

Chapter 4 Superheterodyne Radio Receivers

Chapter 4 Superheterodyne Radio Receivers
As far as the professional manufacturers are concerned, the direct (TRF) receiver has "played out his tune". After half-century struggle on the market, it has been replaced by the superheterodyne receiver, that was patented in 1918 by Edwin Armstrong. In that time, commercially speaking, its main advantage was its substantially easier tuning to the station. It requires only one button for this, comparing to the TRF receiver that needs two buttons to be intermittently adjusted for optimal reception, and also it requires much of the knowledge, skill and patience, which the average buyer does not have. The superheterodyne receiver is, however, also more complex than the TRF, and setting of its stages during its production requires some special instruments, that the average radio amateur does not possess. Nevertheless, it is not impossible to build such device in the amateur environment, and when the operating principles are known, the necessary adjustments can be done "by hearing".
4.1. Superheterodyne AM Receivers
On Pic.4.1 you can see the block diagram of a radio-broadcast superheterodyne receiver The input circuit (UK) refines the signal of the tuned station from all the voltages created in the antenna (A) by various radio transmitters and sources of disturbances. In our example, it's an AM signal that has the carrier frequency fs, and is modulated by a single tone, as seen in the rectangle above its label. This signal is being led into the stage called the mixer. Another voltage is also led into it, the voltage from the local oscillator that has the frequency of f0, and a constant amplitude. Under the effect of these two signals, the phenomenon called the outbreak takes place in the mixer, and an AM signal appears on its output, its frequency being fm=455kHz. This signal is called the inter-frequency (IF) signal, and its frequency fm the interfrequency. The IF signal has the same envelope as the station signal entering the mixer. That means, that the information from the transmitter to the mixer is carried by the signal frequency fs, and in the mixer it is being assumed by a new carrier, that has the frequency fm. When transferring to another station, the user changes the capacitance of the variable capacitor C by turning the knob, setting up the resonance frequency of the input circuit to be equal to that station's one. Another variable capacitor, Co, is located on the same shaft as C, so its capacitance changes simultaneously to that of C. This capacitor is located in the local oscillator and that is how it gets the new oscillating frequency, having such value that the difference of the oscillator and station frequencies is again equal to the inter-frequency value.
Here's one numerical example. The interfrequency is being adopted by the constructor of the device, and it is mostly fm=455 kHz. When the receiver is set to the station that has the frequency of fm=684 kHz, the frequency of the local oscillator shall be fO=1139 kHz, therefore making there difference be
1139 kHz-684 kHz=455 kHz=fm.
When tuning to a station that operates on the frequency of fS=1008 kHz, the listener will change the capacitances of the two capacitors until the resonant frequency of the input circuit becomes fS=1008 kHz, and the oscillator frequency fO=1463 kHz, therefore yielding
1463 kHz-1008 kHz=455 kHz=fm.
If the receiver has more wavebands (LW, MW, SW1, SW2…) it is being constructed to have the same inter-frequency value for all of them.
What do we gain with this change of the carrier frequency? So far we haven't mentioned one very important thing, that is that the input circuit can never be selective enough, to extrapolate only the signal of the tuned station, from all the signals that exist in the antenna. On the output of this circuit, besides the station signal, also signals of strong and local transmitters are obtained, especially the signals from the neighbouring channels (their frequency being very close to the one of the tuned station). All these signals are receiving new signal carriers in the mixing stage, with their frequencies deviating fm as much as their carrying frequencies differ from fS. E.g., if the input circuit is set on the station whose frequency is 1008 kHz, another two signals from the neighbouring channels can also emerge on its exit.

Their frequencies would be 999 kHz and 1017 kHz. The ordinary TRF receiver would in this case be totally incapable of suppressing those signals, which is not the case with the superheterodyne receiver. These 3 signals are entering the mixer, which gets the 1463 kHz voltage from the oscillator. The outbreak occurs, and 3 AM signals are exiting the stage, their frequencies being 455 kHz, 464 kHz and 446 kHz. All 3 signals go to the IF amplifier (MFP), which has several amplifying stages with oscillatory circuits set to 455 kHz, making it very selective, so it amplifies only the 455 kHz signal and suppresses the others enough not to disturb the reception.
the signal exiting the IF amplifier is led onto the detector (Det.), the LF voltage amplifier (NFP) and the output stage (IS), the circuits we spoke about in the previous projects.
The ARP signifies the circuit that turns back the DC component of the detected signal into the IF amplifier, to obtain the automatic amplification regulation.
Above every block on the picture you can see the signal shape exiting that block, as seen on the oscilloscope, in case the modulation in the transmitter is done by the single, sinusoidally-shaped tone. The upper part of the picture contains the average voltage amplifications for each block, for the mass-production devices. Total voltage amplification, which is the ratio of the voltage on the loudspeaker to the voltage in the antenna is A=750000. The amplification in decibels is therefore: A(dB)=20logA=117.5
4.1.1. The Simplest Superheterodyne AM Receiver
The author presumes that most of the readers, especially those just entering the world of radio with this book, are somewhat scared by the block diagram from Pic.4.1. Their question probably is: Can an amateur build such a receiver? Yes, he can. The author has a friend that succeeded in this some 40 years ago, when all had been done with the electronic tubes, making the practical realization of a receiver much harder than it is today, with semiconductors (its radio amateur call sign is YT1FA, and those who doubt it may contact him). However, he was doing this in the premises of YU1EXY Radio Club, in the attic of the Electrotechnical Faculty in Belgrade, using the club (more less trophy) instruments and, more important, he had help of Sasa Piosijan, Radivoje Karakasevic and Kiro Stojcevski, who knew all about the radios, especially Sasa.
The main problem in making a superheterodyne device is not the circuitry complexity but its setup, which requires lot of practical experience and some special instruments, that our readers probably don't possess. But they are much better than the TRF receivers, both regarding the sensitivity and selectivity, so we made simpler devices that are simple to set, with no instruments necessary than your ears. They are realized around the NE612 IC, whose pin description, block diagram and main features are given on Pics.4.2-a & b.
This IC comprises the critical stages of an AM superheterodyne receiver, the mixer and local oscillator. the station signal is led either on pin 1 or on pin 2 (or on both of them, in case of symmetrical coupling with the previous stage), and the IF signal is obtained on the pin 4 or 5 (or on both of them, in case of symmetrical coupling with the next stage). An oscillatory circuit, that determines the frequency of the local oscillator and the positive feedback circuit are connected between the pins No.6 and 7. Pin 3 is connected to Gnd, i.e. the minus pole of the DC supply voltage. Pin 8 receives a positive DC supply voltage which can, acc. to the table given on Pic.4.2, vary between 4.5 V to 8 V. The value of this voltage is not critical, but it is extremely important for normal operation of the receiver that this voltage is stable, therefore urging for it to be separately stabilized (with special care), as seen in some projects in this chapter and in the Appendix, that involve the NE612.
In the text that follows 3 simple superheterodyne receivers made with NE612 will be described.

More will be discussed in the chapter dedicated to NE612 IC, and the reader should pick one of these, or make the receiver that suits him best by combining these diagrams with earlier described HF amplifiers and input circuits.
The electronic diagram of the simplest superheterodyne AM receiver in the world, with reproduction over the loudspeaker, is shown on Pic.4.2-c. The device has got only one oscillatory circuit in the IF amplifier (being marked as MFT), whose frequency does not need to be set to some specific value (meaning the receiver will work OK even if its frequency is bigger or smaller than standard 455 kHz). Further simplification was done by omitting the input circuit, therefore avoiding the problems with quite complex adjustments between the input circuit and the local oscillator. All these simplifications do have their price: this device is less sensitive and selective than the complete superheterodyne, and is also more prone to disturbances. Even so, it has better both the selectivity and sensitivity than the TRF.
Signals of all the stations are being led directly from the antenna onto the pin no.1, i.e. the mixer. On the other hand, the mixer also receives the HF voltage from the local oscillator, whose frequency is equal to the resonance frequency of the parallel oscillatory circuit made of CO, CtO, and LO. This frequency, if neglecting the parasite capacitances, is:





On the mixer exit the signals from all the stations are obtained, but now they have new carrier frequencies, that are equal to the difference of the oscillator frequency and their original one. Nevertheless, only one of these signals will have the frequency that is equal to the resonance frequency of the MFT, and it will be the only one to appear on the ends of this oscillatory circuit. Here's a numerical example.
Let us assume that we have (only) 3 MW signals in the antenna, having the frequencies of fS1=711 kHz (Nis), fS2=855 kHz (Bucharest) and fS3=1008 kHz (Belgrade 2). The IF transformer frequency could be fm=455 kHz. If we set the frequency of our oscillator on fm=1166 kHz (with CO), the following signals, modulated by the radio stations' programs, will exit the mixer:

fm1=f0-fS1=1166-711=455 kHz,
fm2=f0-fS2=1166-855=311 kHz and
fm3=f0-fS3=1166-1008=158 kHz.

Since the oscillatory circuit on the mixer exit (MFT) is set to 455 kHz, we will have Radio Nis's signal from it, others will be suppressed. If we wish to hear Bucharest, the oscillator frequency should be set to 1310 kHz, and for Belgrade 2 1463 kHz. Of course that the listener doesn't need to know all these frequencies, he will just turn the knob of CO until hearing the desired station's broadcast.
The IF signal is led from the pin 3 to the detector with AA121 diode. The LF signal is taken from the R1 resistor and over the capacitor C4 it is led to the volume potentiometer P and the audio amplifier.

* the MFT is also being called the inter-frequency transformer. It is a special type component that is hard to find in the ordinary electronic shops, therefore the radio amateurs are usually obtaining them from disused factory-made devices. The IF transformer is shown on Pics.4.3-a,b,c & d. As you can see it on 4.3-a, the MFT is, in fact, a parallel oscillatory circuit with a leg on its coil. The coil body has a ferrite core (symbolically shown with single upward straight dashed line) that can be moved (with screwdriver), which allows for the setting of the resonance frequency of the circuit, being mostly fm=455 kHz. The same body contains another coil, with less quirks in it. Together with the bigger one it comprises the HF transformer that takes the signal from the oscillatory circuit into the next stage of the receiver. Both the coil and the capacitor C are placed in the square-shaped metal housing that measures 10x10x11 mm (Pic.4.3-b). From the bottom side of the housing you can see 5 pins emerging from the plastic stopper, that link the MFT to the PCB, being connected inside the MFT as on Pic.4.3-a. Besides them, there are also two noses located on the bottom side, that are to be soldered and connected with the device ground. Japanese MFT's have the capacitor C placed in the cavity of the plastic stopper, as shown on Pic.4.3-c. The part of the core that can be moved with the screwdriver can be seen through the eye on the top side of the housing, Pic.4.3-d. This part is coloured in order to distinguish the MFT's between themselves, since there are usually at least 3 of them in an AM receiver. The colours are white, yellow and black (the coil of the local oscillator is also being placed in such housing, but is being painted in red, to distinguish it from the MFT).
Un-soldering the MFT isn't that simple and is to be performed very carefully. The iron is not to be kept leaned too long on the pins, since there's danger of melting the plastic stopper. All the tin from the pins and noses has to be removed first, by the aid of the iron and the vacuum pump (or a piece of wire stripped from the antenna coaxial cable). You can then safely remove the MFT from its original PCB.

* Pics.4.3-a, b, c & d almost fully apply for the oscillator coil as well (LO). The only difference is that LO doesn't have the capacitor C. looking from the outside, LO and MFT can be distinguished only by the marking colour, until they're lifted from the PCB.

LO's have red colour, while MFT's (IFT’s) are white, black or yellow. During the PCB design, absolute care must be taken that pins 1 & 4, as well as 2 & 3, do not permute. If that would happen, the feedback would be negative (instead of positive) and the oscillator wouldn't function. However, if you conclude during the design phase that it would be more convenient to connect pin 4 to Gnd (instead of pin 1), do have in mind that it can be done only if you connect also pin 2 to Gnd (instead of pin 3).

* Fine tuning (if necessary) of the LO's and MFT's inductance values is done by adjusting the position of the ferrite core with screwdriver.

* With CO and CtO, variable capacitor and the trimmer capacitor in the oscillator are labelled. Acc. to Pic.4.3-e & f, which shows the capacitor we spoke about in the connection with Pic.3.7, the abovementioned capacitors are connected with the circuitry over the legs O and G (Ca and Cta are not used), with G connected to Gnd.

* The receiver from Pic.4.2 can be utilized for the reception of AM stations in the SW waveband. All there is to be done is to make a new oscillator coil, acc. to Pic.4.3-g & h. It is being made of 0.4 mm CuL wire (a thicker one can also be used), on the 32 mm diam. carton body, the same one used for making coils on Pics.3.6 & 3.28. Number of quirks on the picture is 9, but other combinations should also be tried, say, 12 quirks, or somewhat less than 9. The feedback coil has 3 quirks and is spooled along the oscillator coil (as shown on picture), or over it. If you have already accomplished the reception of SW stations with some of the previously described TRF devices, you will be surprised with much bigger selectivity of the receiver from Pic.4.2. in the evening hours you'll be able to perform the receipt of huge number of stations on the radio-broadcast, professional and amateur wavebands.
For the reception of SW stations smaller capacitances for C1 should also be tested, say, C1=33 pF and similar, since it affects the oscillator frequency.

* In the previous numerical example we saw that tuning is done by setting up the frequency of the local oscillator and that fm=455 kHz, Radio Nis will be heard when the oscillator frequency is fO=1166 kHz. The story is not over, though: What will happen if there is a station that operates on 1621 kHz? Mixing its signal with the voltage from the local oscillator the modified signal is made, its frequency being
1621 kHz-1166 kHz=455 kHz.
We now have two signals on the MFT. They both have the same carrier frequency (455 kHz), one of them is program of Radio Nis, and the other comprehends the program of the station transmitting on 1621 kHz. Both of them are being heard in the loudspeaker, the interference occurs. Speaking in expert language, the obstruction because of the symmetrical station occurred. That is a station whose frequency fSS iz greater than fm for the value of the oscillator frequency:
fSS=fO+fm
Suppressing the symmetrical station signal must be done before the mixing stage. In the radio-broadcast receivers this is being done over the input circuit, and in the professional devices, by input circuitry and the HF amplifier. If you have experienced disturbances while using the receiver from Pic.4.2 (mixing of stations or, more common, whistling or squeaking tone) try changing the MFT's oscillation frequency (by turning the ferrite coil), then re-tune the receiver.

* If the receiver from Pic.4.2 is power-supplied from the battery (or adaptor) whose voltage is over 6 V, a voltage stabilizer should be inserted in the plus (+) line of the power supply for NE612, as it was done with the receivers on Pics. 4.4, 5.7 and 5.9.
If you cannot receive the signal of some station transmitting on 1500 kHz, not even with the capacitor CO knob in the rightmost position, start reducing the CtO capacitance (turning the trimmer with screwdriver) until you hear the signal. similarly, if you can't hear some station you're fond of, that transmits on 500 kHz (e.g. Radio Budapest), try increasing the LO inductance (by turning the core towards inside with screwdriver). If this doesn't succeed, change a little the MFT frequency, then try again.

* The reception can be significantly improved if input circuit (UK) is added to the receiver. In order to avoid problems with attuning the UK and the LO, the UK with special variable capacitor can be used, as on Pic.4.3-i. It is "our" capacitor from Pic.3.7, with all the capacitors connected in parallel, and "our" coil from Pic.3.6. Station tuning is now being done with two buttons, which isn't "a job for everyone". The receiver is first roughly tuned to the station using these two buttons, and then the optimum reception is carefully searched.

* If you omit the amplifier with 386 IC on the Pic.4.2, and connect high-resistance headphones instead of R1, it is the truly the simplest superheterodyne receiver in the world.

3.15. Direct (TRF) FM Receivers

3.15. Direct (TRF) FM Receivers

Frequency modulation is used in radio broadcast in the bandwidth range from 88 MHz til 108 MHz. This range is being marked as “FM” on the band scales of the radio receivers, and the devices that are able to receive such signals are called the FM receivers.
Radio broadcast transmitters are using the amplitude modulation on LW, MW and SW bandwidths. According to international treaties, each of the transmitters has a 9 kHz wide broadcasting channel, therefore making maximum frequency of the information being transferred fNFmax=4.5 kHz, according to the characteristics of the AM signal. To put it more simple, the highest frequency of the sound that can be heard from the loudspeaker of an AM receiver is 4.5 kHz, all above it will be simply truncated in the circuitry. Considering the speech itself, this isn’t so important since the most important components are located below these 4.5 kHz (during the telephone transfer, all the components above 3.2 kHz are being cut, and nobody is complaining). Things stand different, however, for the transfer of music. Music has much more sound components, with their frequencies spreading up to 15 kHz, so truncating them above 4.5 kHz does deteriorate the transmission quality.
The radio-broadcast FM transmitter has a 250 kHz wide channel on its disposal, therefore allowing for the maximum frequency of the information (acc. to the characteristics of the FM signal) to be fNFmax=15 kHz. That means that music is being fully transferred and its quality is significantly better than in the case of the AM transfer. The FM transfer has some other advantages, perhaps the most significant of them being the possibility of eliminating various disturbances that are manifesting themselves as snapping, squeaking etc. The main disadvantage, however, is not the result of the frequency modulation itself, but rather of the fact that this method is being used on high frequencies, and that high-frequency electromagnetic waves behave themself as light, spreading themselves in straight line, not reflecting from the ionosphere etc. This is why obtaining this kind of radio-link requires optical visibility between the transmission and reception antennas, which is not the case for the links obtained on frequencies which are less than 40 MHz. In practical terms, it is possible to receive the SW signal from anywhere on Earth, whilst the range of an UHF link is limited to the horizon. Or, as Hamlet would say: “The quality or the range, that is the question!”
Can we have it both, somehow? Yes we can, and it is already being done, over the satellite links, using the same equipment as for the TV signal receipt and an audio amplifier connected to the audio output of the satellite receiver. For now, in the earthly conditions, those that are interested in the worldwide news will make and use the AM receivers, and music lovers will stick to the FM’s. And what can those interested in both do? Well, they make AM-FM receivers :)
The direct-type (TRF) FM receivers have never been produced, the industry started right away with the superheterodynes, made acc. to the block diagram on Pic.4.6, which will later be discussed. In amateur life, however, the direct FM receivers do exist, having very simple electronic diagrams and being easy to manufacture. These receivers have very strong positive feedback, making the intermittent oscillations in it, and are therefore being called the super-reaction receivers.

3.15.1. The Simplest FM Receiver

On Pic.3.43 you can se the electronic circuit of an extremely simple direct FM receiver. The T2 transistor together with the R1 resistor, the coil L the variable capacitor C and internal capacitances of the T1 transistor, comprises the so-called Kolpitz oscillator. The resonance frequency of this oscillator is being set by C to correspond to the one of the station that we wish to hear (meaning it has to be altered between 88 and 108 MHz). The signal, i.e. the information being used in the transmitter to perform the modulation, is extracted on the R1 resistor, and being led from it to the high-resistance headphones, over the coupling capacitor C1.

* The capacitance of the variable capacitor should be able to change from a couple of pF (Cmin) to app. 20 pF. During the testing off this device, we were using the capacitor from Pic.3.8. The legs marked as FO and G were used, the G leg being connected to the ground. When all the trimmers from the circuit on the Pic.3.8 are set to minimum capacitance, the capacitance between the FO and G legs should be adjustable between 7 and 27 pF.
* The coil L has 4 quirks of lacquer-isolated copper wire (CuL), bended to have a 4 mm internal diameter. The practical realization of this coil is explained in text connected with Pic.3.45. During the setup of the bandwidth, the inductance of the coil can be altered by changing the distance between the quirks. If the coil is stretched the inductance decreases, and vice versa. If this cannot give the desired results, new coil must be made.
* The telescopic antenna taken from a disused device can be used. If you can’t find one, you obtain very good results with a piece of isolated copper wire, about 60 cm long (the optimum length to be found experimentally).

3.15.2. The Simplest FM Receiver with Audio Amplifier

The radio-broadcast FM transmitters operate with output power that is much smaller than that of the AM transmitters. That is why the LF signal coming from the device on Pic.3.43 is rather small, urging the use of very sensitive headphones. They are much more expensive than the “ordinary” ones, making it better to use the cheap headphones in connection with audio amplifier. One such solution where TDA7050 IC is used is given on the Pic.3.44. The R3 resistor and capacitors C5 and C6 are to be added only if the operation of the device is unstable. There optimum values are to be found experimentally, starting with those shown in the picture.
For loudspeaker reproduction any of the previously described amplifiers can be used, e.g. that from Pic.3.21 (which we have been using, very successfully), or one of the devices described in P.E.4 and P.E.5. Since in these amplifiers a battery with voltage bigger than 3 V is used, using of R3 and C5 is obligatory. The R3 is counted from the formula




where UBAT is battery voltage, and 0.235 mA is the current through R1, that supplies T1 and T2. E.g. if UBAT=9 V, it is then and the nearest existing resistor is used.

Capacitors C5 and C6 comprise, together with R3, a pass-filter for very low frequencies, which is used to separate the HF and LF parts of the receiver.

The battery itself acts as a short-circuit for the AC currents. But when it ages its resistance increases and there is no more short-circuit. That is why C3 and C4 are added, to accomplish it.

3.15.3. FM Receiver with one Transistor and Audio Amplifier

We have made this receiver on the experimental plate, and it was playing for days in our lab. Its electronic diagram is given on Pic.3.46. Regretfully we had to disassemble it, since we needed the plate for one of the devices described later in this book. This, too, is a reaction-type receiver, where the BF256 transistor, coil L and capacitors C, C* and C2 form the Hartley oscillator. Its frequency is being adjusted by means of the variable capacitor C to be equal to the frequency of the station that we wish to listen to. The LF signal is being taken from the R1 resistor, and led into the audio amplifier.

* The coil L is self-supporting (doesn't have the body), made of 5 quirks of CuL wire, its diameter being from 0.8 to 1 mm. It is spooled on some cylindrical object (pencil, pen etc., the best thing is the round part of a 9 mm drill), in one layer, quirks put tight to each other, as shown in the left, framed part of the picture. When the coil is finished, it is taken off the cylinder and stretched a little, so that the quirks do not touch each other. Its final length should be about 10 mm. The mid coil leg, which is to be connected to the left end of the C3 capacitor, is made by taking off couple of millimetres of the lacquer from the wire, approximately in the middle of the coil. This place is then tinned and a piece of thin wire is soldered to it. The other end of this wire is soldered onto the PCB, on its place, to be connected to the left end of C2.

* For the variable capacitor C the one from the Pic.3.8 (legs FO and G, G goes to Gnd). If you are using some other capacitor, that has bigger capacitance, and you cannot achieve the reception of the full FM bandwidth (88 til 108 MHz), try changing the value of the C*. Its capacitance is to be determined experimentally, usually being about a dozen pF.

* HFC is the high-frequency choke. Together with C2, it makes a filter that prevents the HF current to flow through the R1, simultaneously allowing for DC and LF current to go through. The muffler is, in fact, a coil that has 16 quirks of 0.6 mm CuL wire, spooled on a round part of a 3 mm drill.

* This receiver works well even without the external antenna. It can, of course, be connected to it, as shown in dashed line. Instead of antenna, a 50 mm piece of wire can also be used.

3.15.4. FM Receiver with (just) one Transistor

On the left side of the Pic.3.46 you can see the diagram of another very simple FM receiver, that has only one transistor as the active element. That is, as one can see, the HF part of the receiver from Pic.3.45, where the reproduction is being accomplished over the high-resistance headphones. But, as previously noticed, they are pretty expensive, therefore making it better to use the "regular" headphones and a simple amplifier, as shown on the right side of the Pic.3.46.