How to improve the selectivity of the FM radio receiver
Radio, 1991, 12
The FM radio receiver with PLL designed by A.Zaharov  still attracts the attention of radio amateurs. As noted in a publication of the magazine "Radio" [2, 3], one of the major drawbacks of this type of detectors is the low adjacent channel selectivity. The reason for this is the direct detection of strong signals by base-emitter junction of transistors connected across a source of AC signal voltage.
To prevent direct AM demodulation of the signal it is recommended to use anti-parallel connection of diodes in the mixer circuit. The diode mixer can be connected in parallel to the signal source (Fig. 1) or in series with it (Fig. 2). To prevent direct demodulation of the signal with this method, in the circuit of A.Zaharov it takes to connect a diode in reverse direction across the base-emitter junction of the transistor VT1 (Fig. 3). But we can not do it, because in this case, the diode will be closed by the voltage of the base - emitter junction of the transistor. To overcome this difficulty it takes to connect the diode in series with the base - emitter junction to DC, and in anti-parallel to AC (Fig. 4).
VT1 - KT315B (a silicon transistor, hfe = 50...350 at Uke=10 V and Ik=1 mA; ft = 250MHz); VD1 - KD521A (a silicon diode);
C1 - 12pF, C2* - 27pF, C3, C4 - 50μF x 6 V, C5 - 18pF, C6* - 10nF (mono) of 2nF (stereo), C7* - 12 pF, C8 - 10μF x 6 V;
R1* - 100K, R2 - 100, R3 - 2.2K, R4 - 400...680, R5 - 0.5...2 (see text);
L1C2 is tuned to 65.8...73 MHz, L2C7 is tuned to 32.9...36.5 MHz;
Asterisk * means tweak the value.
The locking range of this modified receiver is wider then the locking range of previously described receivers. It is useful to reduce the Q factor of the resonant tank circuit L2C7. To do this, wound the coil L2 with thinner wire (0.2...0.3 mm in diameter (AWG 32..29)), or add into the tank circuit L2C7 an additional resistor R5 (0.5...2 ohms).
In the modified receiver the coils are used similar to those used in circuit of the FM receivers with PLL. The coil L1 is formerless with the diameter of 5 mm, the winding step is 1 mm and the number of turns is 6 of the wire 0.5 mm (AWG 24). The coil L2 is wound on a frame with the diameter of 6.5 mm, it has 9 turns of the wire with diameter of 0.27 mm (AWG 30). The winding step is 1 mm. This coil L2 has a slug made of aluminium pipe with the length of 20 mm and diameter of 5 mm.
Tweak the capacitors C2 and C7 to tune the receiver to the VHF band (65.8...73 MHz). Tweak the value of the resistor R1 to get maximum sensitivity of the receiver. To prevent self oscillation at audio frequency, the capacitance of the diode VD1 should be considerably smaller than the emitter junction capacitance of the transistor VT1. This condition is fully satisfied by using diodes KD521 or KD522 (any of this series). The value of the capacitor C6 depends on the operating mode of the receiver (mono or stereo). In the schematic diagram shown in Fig. 4, the shown capacity is intended for a mono version. For the stereo version, it should be five times less.
The FM receivers with PLL has been tested in city of Minsk, Belarus. The receiver was able to receive all four local FM radio stations with no interference between them, but in the pauses were heard interference of the television signal of the first and third TV channels. After modification the receiver the interference are gone. The anti-parallel connection of the diode VD1 and the base-emitter junction of the transistor VT1 work as signal limiter circuit for the signals of power radio stations.
In the modified receiver can be used an electronic tuning circuit (Fig. 5), in which a transistor KT315 (with any letter index) is used instead of the varicap. This tuning circuit powered by 3 volt can overlap whole the standard VHF FM band(65.8...73 MHz).
- A.Zakharov. FM receivers with PLL.- Radio, 1985, № 12.
- S. Chekcheev. Detectors for PLL receivers.- Radio, 1987, № 5.
- A.Zakharov. Ring stereo decoder in VHF FM radio receivers.- Radio, 1987, № 10.
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