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ADF4350 lock detect (LD pin)

We are experiencing a weird thing that the ADF4350 LD pin is always HIGH, or the locked LED is ON even if the output frequency is wrong. 


For example, we have 10MHz REF IN and set ADF4350 to have 1600MHz output.

However the output is 2200MHz instead and LD is still HIGH (LED is ON). 

The LD pin is still HIGH even if the 10MHz REF was removed. 


The 2200MHz is free run output from the VCO, right? 

Thank you.