LMX2572: TI's phase-locked loop is LMX2572 the red letter marked is the local oscillator, the final output is 70M, the step is 25kHz, the clutter is a bit large, help me see what is the possible cause?

Hi Xiaofu,

Those are fractional spurs, they might appear in a fractional channel, spurs frequency depends on PLL configuration, spurs level depends on loop filter bandwidth, input clock format and level. 

For example, using your register configuration and a 20MHz sine wave 10dBm input clock, at 3030.025MHz, expected spurs frequency is 50kHz, we might see spurs at 50kHz offset, 100kHz offset, 150kHz offset, etc.

Same for 3030.05MHz.

At 3030.075MHz, expect spurs frequency is 75kHz, 150kHz, etc.

At 3030.1MHz, expect spurs frequency is 100kHz, 200kHz, etc.

At 3030.175MHz, expected spurs is 175kHz, 350kHz, etc.

At 3030.975MHz, expected spurs is 75kHz, 150kHz, etc.

Fractional spurs frequencies are predictable, see LMX2582 datasheet, section 8 for details.

https://www.ti.com/lit/gpn/lmx2582

Dear Noel:

    thank you for your share. Great!