What happens to the AD9510/11 clock outputs if the Reference Input (REFIN) signal goes away?
When the reference input is lost, the PLL will go out of lock. However, as long as power remains to the VCO, it will continue to drive the CLK2 input. Therefore, the clock outputs will have a signal but the frequency will not be fixed. It is possible for the user to monitor the reference externally and put the charge pump in tri-state upon loss of reference. This is a form of holdover which allows the control voltage to float. However, the PLL frequency will drift slowly. The drift in frequency is set by the range of the oscillator, so a VCXO will have much less drift range than a VCO.
