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LMX2571: LMX2571 output has many frequency

Simba Guan
Prodigy 140 points
Part Number: LMX2571
Other Parts Discussed in Thread: PLLATINUMSIM-SW

I design LMX2571 as (In Altium designer):

and PCB is 

I program it output as 147MHz , and test it with oscillograph 

it output like:

Frequency of output is right ,about 147.0MHz. It is the frequency I set.

But (I think) it is strange, I think it should be a sine wave.

so I use signal analyzer (Agilent N9030A) and get :

so frequency is 147.000 006MHz. And phase noise might be OK. ( I estimate 1kHZ offset is -60dBm)

But above 1GHz, there are many other frequency output：

Why? What wrong with my design?

And how can I get a sine wave output?

Thanks!

  • 0
    TI__Mastermind 28001 points
    Simba,

    For the picture of the spectrum analyzer, this looks like harmonics. So if you design for 147 MHz, you should expect even harmonics at 2*n*147 and higher harmonics at 3*n*147, which looks to be what you see. The 3rd harmonic looks about 10 dB down as theoretically expected for a square wave. We don't have some simple "sine wave" option, but we do have an option to use an open collector output for lower harmonics, but this is set by the pull-up component(s), which can be better tuned to 147 MHz.

    In other words, if you don't like the harmonics, it will either playing with the pull-up component in open collector mode, or putting some sort of filter.

    Now as for the 1 kHz spurs, these could be fractional spurs, but you don't give enough information about your settings. For instance, if your input frequency and phase detector frequency is exactly 20 MHz and your output frequency is exactly 147.000006 MHz, this implies you are running in fractional mode and you would expect fractional spurs. We do have hte programmable multiplier to mitigate this. Also our PLLatinum Sim tool at (ti.com/tool/PLLATINUMSIM-SW) can simulate these spurs.

    So based on what I see, it looks like you are doing nothing wrong and the issue is more with harmonics and fractional spurs.

    Regards,

    Dean