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LMX2594EVM: Outputting SysRef Signal

Kristene Aguinaldo
Prodigy 20 points

Part Number: LMX2594EVM
Other Parts Discussed in Thread: LMX2594

I am using the LMX2594 to output a SysRef signal through TICs Pro. I have successfully output my desired clock signal through output A, but the SysRef signal is not showing on a spectrum analyzer.

I followed the user guide step-by-step with no success. I set VCO_PHASE_SYNC, SYSREF_EN, SysRefReq to high, while changing the appropriate PLL parameters and powering up output B's mux. I am outputting the SysRef signal single-ended with a termination on the unused port.

I am unsure of several steps in the process:

  1. User guide and data sheet require that I put the board into SYNC mode (category 1). To do this, I simply set VCO_PHASE_SYNC = 1 and follow the instructions for IN_PIN_IGNORE. (I have tried this both with and without a 10 MHz SYNC input but no success.)
  2. Data sheets says to "Apply the SYNC." It does not give specific instructions on how to do this. I toggled the SYNC pin but it also did not work.
  3. I am using output B's mux to output the SysRef signal; however, there is a SYSREF port on the LMX2594, which I leave unused but am weary.

Are there any additional steps needed to output the SysRef signal that are not explicitly stated on the user guide/data sheet?

  • 0
    TI__Mastermind 28031 points
    Here are a few things.
    1. Ensure INPIN_IGNORE=0
    2. Apply SYNC can be done by toggling SYNC pin or VCO_PHASE_SYNC bit from 0 to 1
    3. SYSREF_EN has to be enabled and you can't use INPIN_FMT = LVDS mode in master mode.
    4. Remember that EVM board is AC coupled outputs for SysRef out OUTB

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 20 points
    A couple of more questions:

    1. When making determinations of the SYNC category, we have to decide if "FOUT%FOSC = 0." Does "FOUT" refer to the output frequency of OUTA or the SysRef frequency of OUTB?
    2. How would outputting the SysRef signal change if our SYNC category was category 3? (Do we need to use the SYNC input on the board?)
    3. As for your last comment, do you mean that an internal SysRef signal is AC coupled with OUTB? If we were using a spectrum analyzer to measure the SysRef signal, how would this AC coupled output affect the spectrum analyzer measurement?
  • 0 in reply to Kristene Aguinaldo
    TI__Mastermind 28031 points

    1.   When using SYSREF, I would consider Fout the output of OUTA.

    2.  If you have multiple LMX2594's, then you would need the SYNC input for category 3 to ensure that the output of OUTA to OSCin was the same for all of them.  However, if it was just one LMX2594, you could get away without the pin.

    3.  If you are using a Spectrum analyzer to measure sysref, then the AC coupling effect is actually what you want as you don't want DC to a spectrum analyzer.  Also ensure the folowing that INPIN_IGNORE=0, SysRefReq pin is high, INPIN_FMT is CMOS.

    Regards,

    Dean