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AD9552 cannot lock with 2 reference inputs

Hi colleagues,

We are using AD9552 as the clock generator for the system. Our system requires a redundant reference clock for higher system stability. We notice that AD9552 has the ability to switch the reference clock source automatically according to the result of the reference monitor.

Because of the special requirement of the system we use directly the output of a clock oscillator to feed in the XTAL pin of AD9552 instead of equipping it with an crystal. The reference input pin is connected of course directly to the output of another clock oscillator. Therefore, the two clock input sources of AD9552 are from clock oscillators, not like in the datasheet or in the design of the EVB, i.e., the reference input comes from an external clock generator (such as an oscillator) and the other clock source comes from the internal oscillator circuit with an external crystal.

We set AD9552 to automatically select the external reference path if an external reference signal is present. However, we found that under the hardware setup as I mentioned above, AD9552 can not get locked when two reference clockes are both present. It seems that only when XTAL pins are equipped with a crystal, the input clock switch in the block diagram of AD9552 can work correctly. Is it the case? Is there any other thing I should take care of or is there any chance for our usage?