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.
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.
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,
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.
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.
