Low pass filter in PLL


Joined Apr 20, 2004

The low pass filter helps keep the VCO from hunting. The filter supplies the control voltage to the VCO, and it helps insure the voltage is reasonably clean DC. Otherwise, you can imagine that an unfiltered control input could cause the PLL to become an oscillator, with the VCO always chasing the signal frequency, but constantly under-and overshooting the stable point. It also places an upper limit on the PLL's response, so you don't get it trying to lock on radar frequencies.