I've built a TV tuner radio using a tuner with voltage synthesis tuning and a CD2003 FM demodulator. A similar schematic can be found here. The reception quality is very good, but it is difficult to tune in a station even if I use a multiturn potentiometer. The frequency tends to drift away. So I've added an AFC (automatic frequency control) function.
This works great. Frequency is direct proportional to tuning voltage. When tuned on a station, if frequency drifts, audio signal level drops and modifies the varicap voltage.
However, I believe that the DC tuning voltage may affect my demodulator IC. Here are the voltages measured on
No signal Tuned on a station Without AFC 1.10 V 1.50 V With AFC 1.98 V 2.25 V
To tune the entire FM band I need a tuning voltage of 10 to 14 V (the tuner covers 45 to 160 MHz, I only need 87 to 108 MHz). The voltage with AFC was measured when tuning voltage was closest to 14 V (maximum).
This is the schematic (without power supply and audio amplifier):
This is the tuner pinout:
This page shows how to add AFC to a LC oscillator. But things are different in case of a varicap oscillator.
Is there any chance this voltage imposed by AFC will affect my demodulator? If yes, how should I do it correctly?