[old_tv_nut]

C108, R167, and C109 in combination with the output impedance of the phase detector (two 1 Megohm resistors in parallel) form a classic phase lock loop filter. At frequencies below 48 Hz (the combination of R167 and C109), the frequency response is rolling off from 1.0 at DC to 1/15 above 48 Hz (33k/500k). The response is flat up to about 480 Hz (the combination of R167 and C108) and rolls off again for frequencies above 480 Hz.

The bandwidth and transient response of the closed loop (and the amount of phase error due to frequency drift with temperature) is also very much dependent on the loop gain, which involves three factors: the gain of the phase detector (volts per radian); the gain of the control tube (picofarads per volt); and the gain of the oscillator response (radians per second per picofarad). [radians per second = 2 times pi times Hz]

To design this circuit, you would measure/design the gains for each of these stages individually, and calculate the RC filter values to give the desired frequency and transient response (damping factor).