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AD9901 Phase Detector

Hello, I am planing to build an all digital PLL using AD9901 as a phase detector. As far as I am informed, AD9901 has an internal phase/frequency detector to compute the error signal. When two square waves with the same frequency are applied to the two input (REF_IN and VCO_IN),the output swing is approximately +3.2V to +5.0V ,it really did a good job so far.However,question comes up when I change phase difference of the two inputs,the output suddenly became +3.2V dc,or sometimes became +5.0V dc, and sometimes it came back to right swint +3.2V to +5.0V.How strange it was since the inputs are right all along,the only diffrerence is the phase,but the output is unstable.

Another condition is,two inputs remain unchanged , when I touch the REF_IN or VCO_IN with a metal bar,the output will change between the 3 states mentioned before.

Does it mean that the output is unstable or it is sentitive to be affected by the other factors. maybe there are some more hints I should take into account. Thank you. The following figure is my schematic diagram.



  • Hi,

    The AD9901 was designed for analog PLL applications, and I'm not aware that anyone has ever been successful using it in a digital loop.  I believe the sensitivity of the PFD causes it to transition into the Frequency mode, which is why you see the dc voltage.  Our clock and signal synthesis team recommends you consider the AD9557 as an alternative. 



  • Thanks for your attention.

    I'm sorry that maybe I didn't describe clearly.

    The AD9901 is just applied to generate an error signal, and then pass a loop filter, sampled by an ADC. Now at the beginning of my experiment,The inputs are two square wave in the same frequency,which are generated by a signal generator,I just want to see whether the output is right or not. as the AD9901 datasheet illustrates below,I don't know why it would transition into the Frequency mode?And even if it transition into the Frequency mode maybe because of interference,will it transition back into the Phase mode while the two inputs are in the same frequency all the time?

    Looking forward to your reply.



  • Hi,

    I think in your application that the 2 frequencies are not exactly locked, as there is not a closed loop.  That is why the PFD goes back into frequency mode, and why no one else has been successful getting this device to work in this application. 



  • Thanks,

    But I still can't understand why it goes back into frequency since the two inputs are in almost the same frequency with a fixed phase difference. Do you mean that the AD9901 works successful only when there is a closed loop?

    So can you show me an example of successful application with this device?

  • Hi,

    The  AD9901 works very well as a stand alone phase detector.

    It doesn't care a whit about feedback loops.

    I use it as a phase detector, which drives an ADC.

    My microcontroller can then do whatever it likes with that information. I've driven VCOs with DACs. I've fed this info to DDSes. Easy peasey.

    Can you monitor the differential outputs to verify they are behaving?

    I test my AD9901 by driving it with two signals: 80 MHz and 80.0000001 MHz. The you can plainly see the phase difference vs time on a scope.



  • Hi, 

    I am planning to design frequency discriminator, center frequency 30MHz and bandwidth is ±240Hz. Should be capable to produce output voltage as per frequency difference.

    Is it possible to use AD9901 for this application. Let me know if any alternative method or part is available for this application.

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