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LMX2594: Fractional spur

Toshihiro Watanabe
Genius 10235 points
Part Number: LMX2594

Hello,

We measured the phase noise. When (output freq) / (ref freq) = integer, we didn't see much spur but (output freq) / (ref freq) is not integer, some spur appears.

For example, 10MHz ref clock, 15GHz output is ok but 10MHz ref clock, 15.001MHz is not ok.

The spur freq are 1kHz, 5kHz, 5MHz, 10MHz for example.

Can we remove or reduce these spur?

Best regards,

Toshihiro Watanabe

  • 0
    TI__Mastermind 28011 points
    Toshihiro-san,

    This sounds like the integer boundary spur. It is modeled by the PLLatinum Sim tool and this tool can give insights into spurs. It is not a simple topic, but spurs are discussed in my PLL book at ti.com/tool/pll_book.

    A few things I would try:

    1. As the input frequency is low, the slew rate may matter. If there is a way to make this input a square wave (Like LVDS), then this would be better. 10 MHz from a signal generator will yield bad spurs.

    2. You can try also changing the modulator order (FRAC_ORDER).


    Regards,
    Dean