ZHCSHN9A February 2018 – April 2018 LMK05028
PRODUCTION DATA.
The principle of operation for 2-loop TCXO-DPLL mode is similar to 3-loop mode except the REF-DPLL stage is bypassed. After power-on reset and initialization, the APLL locks the VCO to the external XO input clock and operates in free-run mode. Once the external TCXO/OCXO input clock is detected, the TCXO-DPLL begins lock acquisition. The TCXO TDC compares the phase of the TCXO/OCXO clock and the TCXO FB divider clock (from the VCO) and generates a digital correction word corresponding to the phase error. The correction word is filtered by the TCXO DLF, and its output controls the APLL N divider SDM to pull the VCO frequency until it is locked to the TCXO/OCXO clock.
If DCO mode is enabled on the TCXO-DPLL, a frequency deviation step value (FDEV) can be programmed and used to adjust (increment or decrement) the TCXO FB divider SDM, where the frequency adjustment effectively propagates through the two nested loops to the VCO output.
In 2-loop mode, the TCXO-DPLL loop bandwidth (BWTCXO-DPLL) must less fTCXO/50 and less than the maximum bandwidth of 4 kHz.
When operating in 2-loop mode and the TCXO input is lost, the PLL channel will operate similar to 1-loop APLL only mode.