LMX2594: phase noise of some frequency do not match the datasheet of LMX2594

user4783307

Part Number: LMX2594

Hi, i am using LMX2594 as a LO in my frequency synthesizer. I find that phase noise of some frequency do not match the datasheet of LMX2594.Here are my circuits and testing results.

LMX2594 works in the fractional mode. I want to get 8780~8789MHz, 8880~8889MHz, 8980~8989MHz and etc, with 1MHz step.

Why the phase noise of 8789MHz is worse than that of 9789MHz?

Thank you!

over 5 years ago

0 Noel Fung over 5 years ago

TI__Guru** 112040 points

Hi There,

What is your PLL configuration? How does your reference clock phase noise look like?

Could you provide a TICS Pro configuration?

0 user4783307 over 5 years ago in reply to Noel Fung

Prodigy 60 points

These are my register configuration.The rest registers are default. I use a OCXO as reference clock, and the phase noise of OCXO is -155dbc/hz@1kHz and -165dbc/Hz@10kHz.

I change the mash order from 4 to 3.The phase noise of 8781MHz is much better, but some unexpected spur occur, which rolls from one side to the other side of 8781MHz.

0 user4783307 over 5 years ago in reply to Noel Fung

Prodigy 60 points

Sorry, I do not have a TICS Pro.

0 Binayak Shrestha88 over 5 years ago in reply to user4783307

Expert 1560 points

Hi,

Just a thought:

I don't know the sensitivity of LMX2954 to power supply noise. If LMX2954 is sensitive to power supply noise, your phase noise plot will vary depending on the regulator you use to power your circuit. Your phase plot may not be same as in the datasheet as you most probably will be using a different power supply system compared to what was used to measure for the datasheet.

World-renowned Power integrity expert Steve Sandler has a very good presentation on this very same topic entitled, "Best Design Practices for Systems with PDN Noise-Sensitive Circuits,"

- https://www.picotest.com/blog/free-webinar-best-design-practices-for-systems-with-pdn-noise-sensitive-circuits/

This should clear many of your doubts.

Hope this helps.

--binayak

0 user4783307 over 5 years ago in reply to Binayak Shrestha88

Prodigy 60 points

Thank you for your reply.

I have also considered the power supply factor and have done some experiments. First of all, I think if it is the DC power supply or regulator‘ problem, then all frequencies will be affected. Secondly, I use the DC power supply to supply the lmx2594 directly, and the test results are basically the same. This DC power supply is used in many other projects，and there is no similar problem.