AD9557 issue - frequency output is different from expected

I have run into an unexpected issue with AD9557 evaluation board. As you can see in the screenshot from the settings, I provide XOA sine wave input of 50 MHz, the reference clock frequency is 80 MHz, and I expect to get 80 MHz on the output as given by the evaluation board software. However, the resulting output frequency is 77 MHz. The image of the scope screen shows all 3 signals and the measured frequencies (yellow XOA 50 MHz, red-RefA 80 MHz, blue output A - 77MHz instead of expected 80 MHz.

Also, I'm never able to achieve phase and frequency lock in the PFD that is connected to RefA.

Could you advise how to resolve this?

Thank you!

  • 0
    •  Analog Employees 
    on Aug 27, 2017 1:35 AM

    Hello Dmitri,

    This is strange. Something isn't configured correctly.

    Did you use the frequency planning wizard to specify the input/output frequencies, and allow it to configure the PLL, or did you attempt to enter the settings yourself?

    There are two easy checks to do: The first is to put the AD9557 into freerun mode, sync the outputs, and verify that the frequency is ~80 MHz. If it is, we know that the frequency shift is in the DPLL settings.

    -Paul Kern

  • I noticed a mechanical problem with the board, the balun next to XOA input is damaged. I have to fix it first, and may be it will be the reason for this strange behavior. I'll get back to it once I find the replacement.

    Thank you for your suggestions!


  • 0
    •  Analog Employees 
    on Aug 30, 2017 6:42 PM

    Hi Dmitrit,

    OK. I'm glad to hear that we might have an explanation, as the behavior that you're seeing is very strange. One way to separate a DPLL configuration from system clock issue is the put the chip into freerun mode. In this mode, the DPLL isn't active, and if you see the wrong output frequency, it's either the a system clock frequency error, or a mis-configuration of the output drivers or freerun tuning word.