FAQ: Does the AD9523, AD9524, or AD9523-1 have glitchless switchover?

Document created by MFelmlee Employee on Jun 15, 2011Last modified by AndyR on Jan 31, 2012
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When a reference is switched from REFA to REFB in the AD9523/24/23-1 will there be glitch of the output clock signal?




The AD9523/24/-1 does not have a “phase build out” feature.  This means that the phase of REFA and REFB are not aligned prior to the phase detector.  When switched from REFA to REFB there will most likely be a phase difference that the PLL will need to phase lock.  The first PLL (AD9523/24 are dual loop PLLs) typically is setup to have a bandwidth of 30Hz for this reason the PLL will reacquire phase lock gradually at a rate approximately equal to the time constant of the narrow loop bandwidth.  Plus, most VCXOs only have a tuning range of 30ppm.  For the above reasons I would say the AD9524 is glitch free.