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
* 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
* 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
* 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
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
* 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.