LMX2594: Reference clock low frequency spur rejection

Toshihiro Watanabe

Part Number: LMX2594

Hello,

The customer would like to know if there is a way to remove reference clock spur at near fundamental clock.

I think PLL can't remove spur which is inband of loop filter but if there is a method to remove the low frequency spur, please let us know.

Best regards,

Toshihiro Watanabe

over 7 years ago

0 Noel Fung over 7 years ago

TI__Guru** 113400 points

Hi Watanabe-san,

Reference spurs is usually outside the loop bandwidth. For example, if the reference clock is 20MHz, then we may see some spurs at carrier frequency +/- 20MHz.
I assume you were talking about Integer Boundary Spurs (IBS), which is an in-band spurs. You can try with the MULT to avoid IBS. Theory of the MULT is here: www.ti.com/.../snaa289.
Even if the MULT can, in theory, avoid IBS; the IBS due to crosstalk is something we cannot avoid or able to reduce.

0 Toshihiro Watanabe over 7 years ago in reply to Noel Fung

TI__Genius 10235 points

Hello Noel-san,

The reference clock is via clock distribution and longer trace. If the noise is coupled with the trace which frequency is around fundamental, I think we can't remove the spur because of in-band of loopfilter.

We will pay attention not to occur this but in case it happens, we would like to know how to remove it.

Best regards,

Toshihiro Watanabe

0 Noel Fung over 7 years ago in reply to Toshihiro Watanabe

TI__Guru** 113400 points

Hi Watanabe-san,

Right, the long trace may increase crosstalk, so a good layout may be helpful.
Unfortunately, the crosstalk that happens inside the chip is something we can do nothing to reduce.

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LMX2594: Reference clock low frequency spur rejection