# How to add automatic frequency control to varicap voltage?

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.

simulate this circuit – Schematic created using CircuitLab

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 Test Point:

                   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?

• I can't find any circuit on either of the two links that covers what you are trying to do - I couldn't see varactor tuning on either and TBH I can't see the relevance of the drawing you have done. If you have a circuit to show, then show it. – Andy aka Jun 6 '14 at 10:39
• @Andyaka I uploaded the schematic. The varactors are inside the tuner. V_TUN pin is directly connected to varactors. I don't have a schematic of the tuner. – Cornelius Jun 6 '14 at 12:39
• Why would the audio signal contain any information about the precise value of the IF frequency that you could use to nudge the local oscillator in the "TUNER1" thing? – Andy aka Jun 6 '14 at 12:59
• I know I could better use the AGC pin of CD2003. But the DC component of the FM detector seems to be proportional to the amplitude of IF signal. – Cornelius Jun 6 '14 at 13:04
• The detector's DC component can be used providing there is a constant modulation as per continuous data on a fixed carrier frequency link. You have neither of these situations - A DC output of zero from the FM detector can mean other things i.e. no modulation or not in lock at all. – Andy aka Jun 6 '14 at 13:17