Building Blocks of a PLL
The PLL consists of a pre-divider counter (N counter), a phase-frequency detector (PFD) circuit, a charge pump, loop filter, a VCO, a feedback multiplier counter (M counter), and post-divider counters (K and V counters).
The PFD detects the differences in phase and frequency between its reference signal (fREF) and feedback signal (Feedback), controls the charge pump, and controls a loop filter that converts the phase difference to a control voltage. This voltage controls the VCO.
Based on the control voltage, the VCO oscillates at a higher or lower frequency, which affects the phase and frequency of the Feedback signal. After the fREF signal and the Feedback signal have the same phase and frequency, the PLL is said to be phase-locked.
Inserting the M counter in the feedback path causes the VCO to oscillate at a frequency that is M times the frequency of the fREF signal. The fREF signal is equal to the input clock (fIN) divided by the pre-scale counter (N).
The reference frequency is described by the equation fREF = fIN/N. The VCO output frequency is fVCO = fIN × M/N, and the output frequency of the PLL is described by the equation fOUT = (fIN × M)/(N × K) for the signals.