How to improve the selectivity of the FM radio receiver

Posted on Feb 6, 2014

The FM radio receiver with PLL designed by A. Zaharov [1] 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 t

How to improve the selectivity of the FM radio receiver
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ransistors 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). 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...

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