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LMX2594EVM: spurious

akihiko yokouchi
Intellectual 360 points

Part Number: LMX2594EVM
Other Parts Discussed in Thread: LMX2594, LMX2572, LMX2582

Tool/software:

Could you please give me some advias?
I have checked Fpd and Fout in Figure 1 (Figure 2) with LMX2594.
As a result, we confirmed the waveforms in Figure 3 (yellow).
Can you guess what the Δ109kHz spurious is generated from?

Regards,
Akihiko Yokouchi

FIG.1

FIG.2

FIG.3

  • 0
    TI__Guru** 119790 points

    Hi Yokouchi-san,

    Based on your configuration, you should have spurs at multiple of 500kHz and 41.6667kHz. 109kHz spurs should not be generated by the synthesizer. Likely this spurs came from power supply or the reference clock.

  • 0 in reply to Noel Fung
    Intellectual 360 points

    Hi Noel

    Thanks for the advice.

    The spurious Δ109kHz was caused by Fosc(SG).
    We are currently considering a PLL that needs to be locked in 250 kHz steps or 500 kHz steps in the 5.6G band.
    Is there any other suitable device besides the LMX2594?
    Or, if we use LMX2594, is there any frequency where we can suppress spurious by setting Fosc and Fpd like this?
    By the way, we are considering a loop bandwidth of about 100 kHz and a phase noise integral value of -40 dBc/Hz(100 Hz to 200 kHz).

    Regards,
    Akihiko Yokouchi

  • 0 in reply to akihiko yokouchi
    TI__Guru** 119790 points

    Hi Yokouchi-san,

    We have LMX2572, it can support up to 6.4GHz, it is a low power synthesizer but phase noise is not as good as LMX2594. 

    Whenever it is a fractional channel, there is chance that fractional spurs will appear. Spurs frequencies are predictable using our simulation tool PLL Sim (https://www.ti.com/tool/PLLATINUMSIM-SW).

    fractional spurs can be reduced if we use a narrow loop bandwidth. However, your design goal is best jitter, then you have to use wide loop bandwidth. 

    Small charge pump current can reduce spurs, but again smaller charge pump current will limit the maximum possible loop bandwidth. 

    Another trick we can try is use an irrational number for PLL_DEN. For example, instead of using 240, we can try 2400001. Spurs should go lower but phase noise may increase. 

    you can check LMX2582 datasheet section 8.1.1. for spurs theory. 

  • +1 in reply to Noel Fung
    Intellectual 360 points

    Hi Noel

    Thanks for the advice.