VHF FM receiver on one transistor. Scheme, description

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VHF FM receiver on one transistor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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The simplest VHF FM receiver, available for repetition by a beginner radio amateur, can be assembled according to the scheme of a single-transistor synchronous-phase detector (L1). A schematic diagram of such a receiver is shown in the figure.

VHF FM receiver on one transistor

The signal is received by antenna W1, the role of which can be played by a piece of mounting wire. This signal is fed to the input oscillatory circuit L1C2, by adjusting the capacitor C2, the circuit can be tuned within the VHF FM range of 64-73 MHz. The signal voltage allocated by this circuit from the tap of the coil L1 is fed through the capacitors C3 and C4 to the base of the transistor VT1. This transistor stage performs several functions simultaneously: the functions of a phase detector, a tunable local oscillator (tuning with C5), a control device, a low-pass filter, a DC amplifier, and a low-frequency amplifier.

Phase detection occurs at the pn junctions of the transistor, equivalent to the junctions of diodes, to which the voltages of the input signal and local oscillator are applied. The heterodyne circuit L2C5 is included in the collector circuit of the transistor, and the positive feedback voltage necessary for the local oscillator to generate is supplied through the capacitor C6 to the emitter of the transistor. One of the features of this receiver is that the local oscillator frequency is half the frequency of the input signal. This happens because two diodes work in the detector - emitter and collector junctions, one of which is controlled by a positive half-wave of the heterodyne voltage, and the second by a negative one. As a result, the frequency of the local oscillator doubles, as it were. But this makes it possible to eliminate the parasitic influence of the heterodyne circuit on the input, since their frequencies differ significantly.

The essence of synchronous detection is that at the output of a diode mixer made on transistor junctions, a certain alternating voltage is obtained, which is equal in frequency to the difference between the frequency of the input signal and the doubled frequency of the local oscillator. With fine tuning, this frequency is minimal. Then, from this voltage, a control voltage is obtained, which is applied to the local oscillator and changes its setting in such a way that the difference between the frequencies of the input signal and the doubled frequency of the local oscillator is minimal. Since frequency modulation is used on the VHF FM band and the frequency of the input signal changes in time with the sound, this control voltage also changes in the same way as the voltage coming from the microphone at the transmitting station. Then this low-frequency voltage is fed back to this single transistor, which amplifies it already as a simple amplifier, and then it is already amplified supplied to the BF1 headphones.

All these complex processes occur in a cascade on just one transistor.

You can assemble the receiver by volumetric installation, right on the desktop, like other designs from the "radio school" section, or you can develop a printed circuit board based on a circuit diagram, and arrange the parts on it in the same order as in the diagram. Coils L1 and L2 do not have frames; for winding, a drill shank with a diameter of 7 mm is taken and coils are wound on it with a PEV wire of 0,4 ... 0,5 mm. Coil L1 has 14 turns and coil L2 has 25 turns. Coil L1 has a tap from the middle (from the 7th turn), you need to strip the middle turn of the coil and solder wires to it, which will go to capacitors C3 and C4. After winding, the drill is removed from the coils (it only serves as a winding mandrel).

Transistor GT311Zh can be replaced with GT311I. Headphones - any small-sized. Capacitors C2 and C5 of the PDA type are ceramic, for 6 ... 25r, 5 ... 20r or 4 ... 15r, they are adjusted by turning the screw located in the middle. The remaining capacitors of any type, their capacities may differ from those indicated in the diagram within 30%.

As a power source, you can use a battery from a microcalculator or one "round battery" of 1,5 V. Any switch, for example, a toggle switch.

The setup is relatively simple. You need to connect headphones, power and antenna - a piece of mounting wire, the longer the better. It is desirable to hang the antenna out the window or hang it on the window frame. Now you need to put on headphones (they should have a slight hiss) and alternately rotate the rotors of capacitors C2 and C5 to try to catch one station. If this does not work, you need to slightly stretch the turns of both coils and repeat.

If the station is heard with distortion and whistling, you need to slightly change the resistance R1 (you can temporarily put a variable resistor).

Good results cannot be achieved from such a simple receiver, but it can receive one or two stations in the VHF FM band. Experiment with the resistance of the resistor R1, the length and location of the antenna, the supply voltage. Instead of headphones, you can connect a 0,5 ... 1 kOhm resistor and, from the connection point of this resistor and capacitor C7, apply an AF voltage to some amplifier, then you can listen to the speakers.

Literature

Zakharov A. "VHF FM receivers with PLL". Radio 1985-12 from 28-30

See other articles Section radio reception.

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Comments on the article:

Alexander
Is it possible to replace this bipolar transistor with a field brand, say, KP307?

Alexander
I am interested in whether it is possible to make the input capacitor C1 and the coil L1 from two segments of a two-core symmetrical telephone cable of the "noodle" type with a linear capacitance Cx \u120d 1 pF / m. One wire will be used as a coil, the second will be connected to the antenna. Several cases are possible: 2) connect the segments formed when connecting, with a tap to an asymmetric antenna; 3) having insulated, leave the outlet alone, and connect the ends to a symmetrical antenna; 1) It is also possible to use both antennas at once, which will be discussed below. Regarding cable laying: 2) Wind the cable around the frame and shield it, and connect the antenna to the middle of the second wire; 1) Wrap the cable around the receiver body. The BF3 loudspeaker should be placed in a separate shielded enclosure to avoid mic effect. 1) It is also possible to use both antennas at once - following the example of a "fox hunting" receiver. In this case, several solutions are also possible: 2) Switching antennas manually; 3) Automatic antenna switching; XNUMX) Coordination of both antennas.

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