SUBCARRIER_ADAPTER_FOR_FM_TUNERS

Published:2009/6/18 21:32:00 Author:May | From:SeekIC

Op amp U1 and its associated components comprise the 67-kHz bandpass filter. A twin-T net-work, comprised offour 1100-Ω resistors and four 0.0022-pF capacitors, is connected in the feed-back network of the op amp. That gives some gain at 67 kHz and heavy attenuation for frequencies above and below that frequency.An additional passive filter at the input to the twin-T network (containing a 220-pF capacitor and a 10,000-Ω resistor) provides some additional roll-off for frequencies below 67 kHz.In practice, the bandpass-filter action covers a frequency range of about 10 kHz above and below the 67-kHz center frequency. Resistor R18 sets the gain of the bandpass-filter stage.Integrated-circuit U2 is a National LM565 phase-locked loop that modulates the 67-kHz fre-quency-modulated (FM) signal from U1. The LM565 PLL consists of a voltage-controlled oscillator (VCO) set to 67 kHz, and a comparator that compares the incoming frequency-modulated 67-kHz signal at pin 2 with the VCO signal that is fed into pin 5.The output of the comparator represents the phase difference between the incoming signal and the VCO signal. Therefore, the output is the audio modulated by the subcarrier. A treble deemphasis of 150 ps is provided by a 0.033-μF capacitor (at pin 7).The free-running VCO frequency is determined by the 0.001-μF capacitor at pin 9 and by the re-sistance between the positive rail and pin 8 (100 Ω in series with R19). Variable-resistor R19 adjusts the oscillator frequency (also known as the centerfrequency) so that the incoming signal is within the lock range of the PLL.



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