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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. 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. 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.
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.
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