JAJSER5A February 2018 – April 2018 LMK05028
PRODUCTION DATA.
The APLL has a 40-bit fractional-N divider to support high-resolution frequency synthesis, wide output frequency range, very low phase noise and jitter, and the ability to tune its VCO frequency through sigma-delta modulator (SDM) control.
The APLL XO doubler doubles the XO input frequency into the phase frequency detector (PFD) input. The APLL multiplies the PFD frequency by the total N divider value to generate the VCO clock. The desired VCO output to PFD input frequency ratio is the total value of N (INT + NUM/DEN) applied to the SDM to tune the VCO frequency.
In free-run mode, the APLL uses a low-jitter XO input as a initial reference clock to lock the internal voltage controlled oscillator (VCO). The PFD compares the fractional-N divided clock with the XO doubler frequency and generates a control signal. The control signal is filtered by the APLL loop filter to generate the VCO’s control voltage that sets its output frequency. The SDM modulates the N divider ratio to get the desired fractional ratio between the PFD input and the VCO output.
In 2-loop or 3-loop mode, the APLL's SDM is controlled by one of the DPLL loops to pull the VCO frequency into lock with the DPLL reference input.