1.1 General Operation

The following figure shows the general operation of PolarFire Jitter Attenuator.

PLL's integer and fractional feedback division bits control the JAPLL interfaces with Transmit PLL (PF_TXPLL). The PF_TXPLL automatically configures as a fractional PLL when Jitter cleaning mode is selected in the PF_TXPLL configuration tool in Libero. The output clock from the TXPLL is tied to the Feedback clock (FFB) input of the JAPLL to compare to the JAPLL’s noisy reference input clock (FIN). The JAPLL has three phases of acquisition: frequency tracking, coarse phase tracking, and fine phase tracking. Each phase of acquisition utilizes programmable gains to tune the performance of respective stages.

Frequency gain control and frequency tolerance control modify the response time of PLL when in frequency tracking mode. This allows the tracking or filtering of a reference, tunable to the expected noise on the reference. Coarse phase gain control modifies the response while in phase tracking mode, ensuring the PLL can pull-in regardless of how large the frequency error may be when frequency lock is established. Fine phase gain control permits the PLL to have very low bandwidth, effectively averaging out higher frequency noise that may be due to spread spectrum or other noisy input references.