ZHCSLM0A May 2020 – January 2021 LMK5B12204
PRODUCTION DATA
In APLL-only mode, the external XO input source determines the free-run frequency stability and accuracy of the output clocks. The BAW VCO1 determines the APLL1 output clock phase noise and jitter performance over the 12-kHz to 20-MHz integration band, regardless of the frequency and jitter of the XO input.
The principle of operation for APLL-only mode after power-on reset and initialization is as follows. If APLL2 is in Cascaded mode as shown in Figure 9-5, VCO1 is held at the nominal center frequency of 2.5 GHz while APLL2 locks. Then APLL1 locks the VCO1 frequency to the external XO input and operates in free-run mode. The DPLL blocks are not used and do not affect the APLLs. VCO2 will track the VCO1 domain. Cascading APLL2 provides a high-frequency, ultra-low-jitter reference clock from VCO1 to minimize the APLL2 in-band phase noise/jitter impact that would occur otherwise if the reference of the APLL2 is from a XO/TCXO/OCXO with low frequency, high phase noise floor, or both.
If APLL2 is not cascaded as shown in Figure 9-4, VCO2 will lock to the XO input after initialization and operate independent of the DPLL/APLL1 domain.