Subcarrier Adapter For Fm Tuners Circuit

  
Op amp Ul and its associated components comprise the 67-kHz bandpass filter. A twin-T network, comprised of four 1100-12 resistors and four 0.0022- capacitors, is connected in the feedback 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- 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.
Subcarrier Adapter For Fm Tuners Circuit - schematic

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 Ul. 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 is provided by a 0.033- capacitor (at pin 7). The free-miming VCO frequency is determined by the 0.001- capacitor at pin 9 and by the resistance between the positive rail and pin 8 (100 in series with R19). Variable-resistor R19 adjusts the oscillator frequency (also known as the center frequency`) so that the incoming signal is within the lock range of the PLL.



 

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