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LMX2594: Frequency output from lmx 2594 appears unstable?

user5788058
Intellectual 410 points
Part Number: LMX2594

Hi there,

Using TICS Pro, we set

Fosc = 260 MHz,

Doubler = X2,

PreR = 10,

Multiplier = Bypass,

R = 1 and Fpd = 52 MHz.


We also set CPG = 15 mA,

N _ Divider = 50,

Channel _ Divider = 2 (SEG1 _ EN),

Sysref _ DivPre=2,

SYSREF _ DIV = 20

so RFOutA = 5200 MHz (Via Channel _ Divider),

RFOutB = 32.5 MHz (Sysref), and FVCO = 10400 MHz.


However, when I looked at the output with a spectrum analyzer, I saw a signal of about 5100 MHz and other miscellaneous signal components, so the output appeared to be unstable.
Are there any other parameters to set?

  • 0
    TI__Guru** 106870 points

    Hi There,

    Couple things to try:

    1. the max. OsCin frequency when doubler is enabled is 200MHz. You should disable x2 and set Pre-R to 5 in order to get 52MHz fpd.

    2. make sure you have set FCAL_HPFD_ADJ to 0x00 (fpd <= 100MHz)

    3. Set CAL_CLK_DIV to 0x02 to reduce the state machine clock frequency.

  • 0 in reply to Noel Fung
    Intellectual 410 points

    Thank you for answering my question.

    I set three parameters you taught me and observed the waveform.
    However, the problem has not been solved.
    When I checked the waveform of the CPout, it was discharged before charging was completed.

    This is expected to cause an unstable clock output.
    I think there may be a problem with the configuration of the analog filter outside the IC.
    I would like to present a circuit diagram here and ask a question, but my boss didn't allow it.
    I am sorry that I can only provide a little information, but I would like you to tell me if you know the reason why CPout is strange.

  • 0 in reply to user5788058
    TI__Guru** 106870 points

    Hi There,

    Did you see problem at 5200MHz only or you have problem for any frequency?

    Could you provide a screen shot of TICS Pro having your configuration?

  • 0 in reply to Noel Fung
    Intellectual 410 points

    Hi there,

    The setting of TICSPro is as shown in the attached image.
    When 1040 MHz is set to be output from RFoutA, stable output of 1040 MHz is obtained compared to 5200 MHz.
    But even then, the 32.5 MHz SYSREF was unstable with a lot of jitter.

  • 0 in reply to user5788058
    TI__Guru** 106870 points

    Hi There,

    The configuration looks fine. One thing you can try is to disable the QUICK_RECAL_EN bit. 

  • 0 in reply to Noel Fung
    Intellectual 410 points

    Thank you for your help.

    It turned out that SYSREFREQ was not stably supplied with High.
    I fixed the SYSREFREQ pin to High on the jumper wire and confirmed that the output is stable.