During operation, the reference frequency generated by the crystal oscillator is divided by the reference divider. The divider value is referred to as the modulus. The PFD controls the frequency of the VCO via the charge pump, which produces an output current that is proportional to the phase error between the divided reference and feedback clocks. The output of the VCO is fed back to the PFD through the feedback divider to close the loop. The PFD will drive the VCO up or down in frequency until the divided reference frequency and the divided VCO frequency at the inputs of the PFD are equal.
Dividers are the key Each divider has a different function, resulting in a different divider length.
In the case of an 8-bit divider, the reference divider can be programmed for any modulus from 1 to 255 by programming the equivalent binary value. The dual-modulus technique ensures reliable operation at any speed the VCO can achieve and reduces the overall power consumption of the divider. This lets the divider operate at a high speed while allowing the programmable portion to operate at low speed. Frequency accuracy Reference frequency accuracy depends mainly on the frequency characteristics of the crystal. As described above, the PLL changes the reference frequency to another frequency by the ratio of the dividers used.