The phase detector checks if the VCO feedback signal is too fast or too slow. The output of the phase detector drives the charge pump. If the VCO feedback signal is too fast, the charge pump outputs ‘down’ pulses to tell the VCO to slow down. If it’s too slow, the charge pump outputs ‘up’ pulses to tell the VCO to speed up. The charge pump outputs a current signal. The loop filter converts this current into a voltage to drive the VCO tuning input.
