Building Blocks of a PLL
The main blocks of the I/O PLL are the phase frequency detector (PFD), charge pump, loop filter, VCO, and counters, such as a feedback counter (M), a pre-scale counter (N), and post-scale counters (C). The PLL architecture depends on the device you use in your design.
The following terms are commonly used to describe the behavior of a PLL:
- PLL lock time—also known as the PLL acquisition time. PLL lock time is the time for the PLL to attain the target frequency and phase relationship after power-up, after a programmed output frequency change, or after a PLL reset.
Note: Simulation software does not model a realistic PLL lock time. Simulation shows an unrealistically fast lock time. For the actual lock time specification, refer to the device datasheet.
- PLL resolution—the minimum frequency increment value of a PLL VCO. The number of bits in the M and N counters determine the PLL resolution value.
- PLL sample rate—the FREF sampling frequency required to perform the phase and frequency correction in the PLL. The PLL sample rate is fREF /N.