FM Receiver circuit

Posted on Apr 19, 2012

Based on Valvo FD-1A module, TBA120, and MC1310. Operation of the receiver is based on the phase locked loop principle. The FD-1A contains an RF amplifier, oscillator and mixer (made with BF324, BF451 and BF324 transistors, respectively). Tuning is performed with four varicaps (BB104). There is an additional AFC diode which allows voltage control of the oscillator frequency over a relatively narrow range (see Fig.2). The IF amplifier starts with a 30 dB gain stage using a single transistor (T1 in fig.3). Additional IF gain is provided by the TBA120. A phase detector (which is also part of the TBA 120) compares the 10.7 MHz IF signal with the signal of a 10.7 MHz oscillator built with two RF transistors (T2 and T3 in Fig.3) and using a ceramic filter (SFE10.7MA) in stead of a 10.7 MHz crystal.

FM Receiver circuit
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Via a feedback loop, the phase detector continuously adjusts the frequency of the oscillator in the FD-1A in such a way that there is 90 degrees phase difference between the FD-1A oscillator and the external oscillator (T2/T3). Since the received signal is frequency-modulated, the output of the phase detector is an exact replica of that modulation. Alignment of the receiver involves the trim potentiometers P1 and P2 in the IF amplifier, besides P3 in the stereo decoder. P1 is initially turned completely counterclockwise and P2 completely clockwise. Some stations can now be heard. If P1 is turned clockwise, the audio volume increases but at a certain point distortion will occur. The right setting of the pot is just before the onset of distortion. Checkpoint 10 (or H) is then about 0.5 Volts positive with respect to checkpoint 9 (also marked J, see Fig.3). P2 is now turned counter-clockwise. The signal-to-noise level improves but at a certain setting of the pot a crackling” kind of distortion sets in. The right setting is again: just before the onset of distortion. P2 adjusts the bandwith of the IF amplifier. P1 presents a DC bias voltage to the AFC diode in the FD-1A, this should ensure that the diode works in the linear part of its range. Alignment should be performed during reception of a stereo broadcast. The alignment should be repeated a few times since there is some interaction between the two trimpots. Once aligned...

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