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Struggling to achieve a stable Lock Detect signal with the AD9517-1

Question asked by moysis on Sep 22, 2012
Latest reply on Oct 4, 2012 by pkern



I am facing a serious issue with the Lock Detect signal of the AD9517-1 device. I am not using Analog

Lock Detect or Current Source Lock Detect. Only the normal Digital Lock Detect (DLD).


The SPI communications work ok and I am getting the frequencies I need in the outputs.The issue

that I am trying to solve is why on the LD pin of the device I am receiving a square pulse. The duty

cycle is ~20%. That means the PLL locks for a short time and after unlocks!


My basic settings are present here:

- I am using REF2 as clock source input with a clock 9.216 MHz

- PLL is ON and the VCO frequency is 2359.296 MHz. The VCO Divider is 3.

- The counters have the values: R=1, B=8, A=0, Prescaler (Divide-by-32)

   That means in the PFD inputs the compared frequency is 9.216 MHz.

-- For the Lock Detect my settings are : Antibacklash width 2.9ns, High Range

    and 5 cycles.


   I am using a custom designed board so I don't have the opportunity of changing the loop filter's

   characteristics. With ADIsimCLK inserting my boards values it gives a 85.8KHz bandwidth and

    60.4 degrees phase margin.


   It is important to stabilize the Lock signal since I need to synchronize my peripheral hardware

    according to that.


   If someone has something to suggest I will be glad.


   Thanks a lot in advance.