The receiver from Pic.3.25 can be made portable (without an external antenna) if ferrite antenna is used instead of the carton-body coil, right? Well, not exactly. The ferrite antenna is a directional - type antenna, meaning that it does matter where are the waves made by transmitter coming from. The voltage being induced in it under the effect of EM field is the biggest if the ferrite rod is in horizontal position, aimed in such a manner that its axis is perpendicular to the direction of spread of the waves (i.e. perpendicular to the direction towards the transmitter). If the rod is being turned (in horizontal plane), the voltage decreases and gets to minimum when the rod is turned for 90 degrees, i.e. When the rod axis is parallel to the direction pointing towards the transmitter. That means that when the receiver from Pic.3.25 (with the ferrite antenna) is tuned to some station, the reception can be additionally improved by simply rotating it. The directional feature of the antenna is useful here. This, however, can be bad if the receiver is used in motion, since the volume would be constantly changing, and the receiver should be constantly turned towards the transmitter. It would clearly prove itself useful to have an electronic circuit in the receiver that would be automatically changing the amplification of the receiver, in such a manner that it would be increased when the signal in the antenna weakens and vice versa. This would even be convenient in the stationary - type receivers, since it would provide the constant sound strength in the loudspeaker during the reception of signals of various power. Such circuit does exist in modern receivers, and is called the Automatic Amplification Regulation circuit (AAR).
AAR circuit could be added to some of the receivers described in previous projects. The mishap is that electrical diagrams would then become significantly more complex, which would reflect itself both on the price and the realizability. The solution, however, does exist, and is rather simple: it’s the ZN414 IC, being promoted as an “AM Receiver”. Its block diagram is given on the Pic.3.30. The signal from the input
One end of the oscillatory circuit is connected to the ground over the C1 capacitor (for AC voltages), since input into the amplifier Z is between pin 2 and the ground, where the input signal is to be brought.
The automatic amplification regulation (control) is achieved by returning the DC component of the detected signal from the output to the input of the IC, over the R1 resistor. This DC voltage is being created on R2 resistor. It is substantial for the correct operation of ZN414, its resistance must be such that DC voltage on the pin 1 (to the ground) when no station is being received, is equal to 0.9 V. While calculating the R2 one must have in mind that the idle current of the IC goes through it, its typical value being 0.3 mA, and maximum 0.5 mA (more data about ZN414 can be found in table on Pic.3.36).
The electrical diagram of a small portable MW radio receiver, reproduction being done over the headphones, is given on Pic.3.31. The LF signal is led from the ZN414 output over the coupling capacitor C3 to simple amplifier made with BC547 transistor (or similar), which we discussed about before. This can even be done without the LF amplifier. If you have high-resistance headphones that are sensitive enough connect them between the right end of C3 and the ground, and omit the transistor, R3 R4 and C4.
The 1.5V battery is being used in this device, therefore the necessary 0.6V voltage drop is done with R2=1.5 kOhms. You should, just in case, connect first a 5 kOhms trimmer instead of R2, put its slider in the mid position, turn on the receiver and set it on an empty place on the scale, where no signal can be heard. Connect the voltmeter between the pin1 and ground, and carefully move the slider until the instrument shows 0.9V. If you have no instrument, tune the receiver to some station and move the slider carefully until you reach an optimum receipt. Then turn the variable capacitor’s knob across the entire scale, to ensure that receiver is working well throughout the entire reception range. If everything is OK turn off the receiver, disconnect the trimmer, measure its resistance and solder an appropriate resistor on the PCB. While experimenting with R2 please have in mind that its resistance should be in any case no less than 600 Ohms.
On Pic.3.32 the PCB, ferrite antenna and look of entire device are shown.
If you plan to make a different PCB, since the device works on high frequencies, you have to obey certain rules in order to have a reliable and stable operation:
a. The splitting capacitor C2 has to be mounted as close as possible to the pin 1 of the ZN414. Its capacitance affects both the amplification (which increases with increase of C2) and the limit frequency of the LF signal (which decreases with increase of C2), so the compromise has to be found. You may put for start C2=82 nF (or even 100 nF), and if the reproduction quality pleases you - everything is in order. You could try with smaller capacitance, the amplification will decrease but the reproduction will be better, etc.
b. All the connections, especially those near the ZN414, must be kept as short as possible.
c. The ferrite antenna and variable capacitor should be placed as far away from the battery, loudspeaker (if existing) and the cables connecting them to the PCB.
d. The rotor (G- leg) of the variable capacitor must be connected with the junction of R1 and C1.
Regarding the ferrite antenna, the best thing would be using some that is retrieved from some disused conventional receiver, more on this was told in the project No.3.8. If you can’t find one, or it is unsuitable for some reason, you can make it according to Pic.3.32-c. The length of the ferrite rod is 42 mm. If you have a longer rod, cut it down to size. This cannot be done with the saw, but a groove must be made with the rasp all around, after which the rod can be simply broken in two. The coil body is, again, made of paper tape that is spooled and glued onto the rod. Before you start with spooling, several pieces of 0.5 mm wire (3 on the picture) should be inserted between the rod and the paper. The coil has got 80 quirks of lacquer -isolated copper wire, its diameter being app. 0.2 mm. The beginning and the end of the coil are fixed with the scotch tape (the starting quirks are pressed on the coil body and fixed with several reels of 3 mm tape. The same is done with the ending ones). When the body is finished, the wires are removed. It is thus achieved that the coil body doesn’t lay firmly on the rod, which can now be moved side-to-side, changing thereby the inductance of the coil, so that its optimum value can be established. If you still decide not to use the ferrite antenna, you can use our coil from Pic.3.6. In that case, the leg should be kept “in air”, i.e. it is not used.
* The battery can be connected to the PCB with two pieces of wire that are soldered to it. This solution is fine if you are skilled in soldering and can easily un-solder the old battery and attach the new one. But if you intend to give the receiver to someone, and he/she is not a soldering-lover, you’ll have to find another solution. The simplest thing to do is take the battery housing from an old receiver, do the necessary adjustments and connect it with the PCB with two pieces of flexible (litz) wire. If you cannot do the former, make two battery platforms of brass, as shown on Pic.3.33 and solder them on two copper areas on PCB that are big enough to support them. If you accept this solution, your PCB must be bigger (The additional part is shown in dashed line, on Pic.3.32-a). The board now also contains the holes for the screws, which are fixing it onto the device box. On the platform that supports negative (-) battery pole, a small spring can be attached, to provide a good contact. If you don’t have such a spring, bend the platforms inwards a little, to keep the battery firmly in place.
If you are using a power source whose voltage is greater than 1.5 V, the R2 resistance should be increased. The exact value for it is best to find as previously described, by using the 50 kOhms trimmer. Even better solution is using one of the circuits from the Pic.3.34. Which one should it be? The one on the Pic.3.34-d gives the best operating performance. The setting is done with the TP trimmer. The slider is put in the lowest position, and then is slowly moved upwards until the voltage on the pin 1 doesn’t reach the level required. However, this circuit applies a big load onto the battery, surging from it the current I=(9V-3V)/680Ohm=8.8 mA.
The author was taking notes in his lab while testing the receivers being described herein, and used them later to write this book. One remark about the receiver whose electrical diagram is given on Pic.3.35 was: “Works EXCELLENT”. The reader will probably ask himself: Isn’t this the aforementioned “The Best Receiver”? No, it isn’t, just remember: DE GUSTIBUS... If, however, you consider it to be “The One”, please send your vote on E-mail: tesla@drenik.net;
This device is very similar to the one being described in the previous project. The most important difference is that the LF signal exiting the ZN414 does not go to the transistor amplifier but to the power amplifier built around the LM386, which was used in some previous projects. Any other audio amplifier can be used instead, e.g. those on Pics. 3.15, 3.21 and 3.22.
With this receiver, special care should be taken regarding the voltage on pin No.1. As you did in previous project, put the potentiometer’s slider in mid-position, turn the receiver on and tune it to some station. Move the slider carefully, until you reach the optimum reception. Start changing the capacitance of the variable capacitor, covering its entire scope, to make sure that receiver works well in its entire operating range. If a problem occurs, re-position the slider again. When everything gets OK, turn the receiver off, disconnect the potentiometer, measure its resistance, and solder the resistor of such resistance on the board. The R2 resistance must be no less than 600 Ohms.
