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LMX2594: LMX2594 Spurs when toggling output

Joseph Petersen
Prodigy 10 points
Part Number: LMX2594

I am using the LMX2594EVM right now and I am trying to replicate frequency hopping, however I have noticed that when I change frequencies (N, NUM, DEN) I need to disable the output otherwise I get weird products while the registers are changing. But when I toggle the output port I am getting random spurs, any ideas on how to fix this?

Note) In this example I am just setting the frequency to 50 MHz and toggling OUTA_PD repeatedly, and the oscilloscope is on max hold.




  • 0
    TI__Genius 16141 points

    Hi Joseph, 
    Are you utilizing full assist or ramping or neither? 
    Are you triggering a VCO calibration? 

    Regards, 

    Vicente

  • 0 in reply to vicente flores prado
    Prodigy 10 points

    I am using no assist, and ramping is not enabled.
    I am not triggering a VCO calibration right now because the frequency is not changing.
    The only thing I am doing is toggling OUTA_PD repeatedly which is causing spurs

  • 0 in reply to Joseph Petersen
    TI__Genius 16141 points

    Hi Joseph, 
    Why do you have have signal analyzer on max hold? 

    I would expect to see something weird due to the settling time after enabling output channel. 

    It takes some time for output to stabilize - many times the immediate pulse seen isn't correct as the output hasn't settled. 

    Given you have signal analyzer set to hold max it make sense it's recording some weird spurs. 

    Now if you toggle the output and let the output stabilize (so measure in real time or avg but not hold maximum) do you still see the spurs? 

    Regards, 

    Vicente 

  • 0 in reply to vicente flores prado
    Prodigy 10 points

    The output stabilizes eventually, but even when I was testing, I dont understand why keeping the frequency the same and only toggling OUTA_PD would cause the spurs to happen?

  • 0 in reply to Joseph Petersen
    TI__Genius 16141 points

    Just to make sure I am on the same page -  if you don't have signal analyzer set to max hold you no longer see the spurs correct after enabling OUTA_PD?  

    Regards, 

    Vicente 

  • 0 in reply to vicente flores prado
    Prodigy 10 points

    I can still see the spurs momentarily when it is in real time (not max hold), it settles very quickly to the correct frequency. They do not happen every time, but when I continuously toggle OUTA_PD the spurs happen randomly every once in a while.

    To clarify, when it settles, its just the frequency I set it to, no spurs around it. but when I toggle the output, thats when random spurs happen.

  • +1 in reply to Joseph Petersen
    TI__Genius 16141 points

    That is more along the lines of what I expect. 
    When you disable an output and reenable the output, it will still take a small porting to settle to the correct frequency as you report. Like in the plots i drew, the overshoot before the output settles and or the pulse(s) that can be incorrect are the cause for the spurs you see. That's why in max hold they stay displayed but in real time or avg you only see them momentarily (just immediately after the toggle) but they quickly disappear. 

    If you have equipment capable of time versus frequency measurements you can view the frequency response I plotted where the frequency does not immediately settle to the intended RFout. This portion anytime before the frequency settles are the cause for spurs you see but only when your signal analyzer is set to max hold. 

    Now if the spurs were still present after the output has settled - that would be a different story. 

    In other words what I am trying to say is the method you're using to measure the spurs is not ideal due to what I just explained - please measure the spurs after the output has settled and not using the max hold feature.

    Regards, 

    Vicente