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LMX2492EVM: Phase Noise in X-band LFM signal

Amit kumar
Prodigy 90 points
Part Number: LMX2492EVM
Other Parts Discussed in Thread: LMX2492

Dear TI teams,

I am using LMX2492EVM hardware for our application. We are using this hardware as a X-band LFM signal source. And we are mixing the same signal with some delay in Mixer. According to principle one single tone signal will come at output of mixer after filtering high frequency signal (X-band). We are getting a single tone frequency at the centre and a 100 KHz fixed frequency signal mirror to centre frequency. We are suspecting, Phase noise of LMX2492EVM signal is the reason behind this behaviour. Please let us know your analysis on the same. I am attaching some figure which is output of Mixer.

Thanks.

  • 0
    TI__Guru* 86810 points

    Hi Amit,

    To measure the phase noise of a signal, we use phase noise analyzer or at least a spectrum analyzer. From your plots, we cannot tell anything about the phase noise of the synthesizer.

    FYI, below is the phase noise of the EVM at default EVM configuration.

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Hi Noel Fung,

       We have gone through with above plot you have mentioned, but that graph is for EVM board only. In above thread, attached second figure is spectrum analyser plot (Attached below).  Below graph is for the system where we are using EVM as a signal synthesizer.

    In datasheet, it is mentioned that phase noise is not excellent for this EVM board. Can you suggest other latest board with same functionality and less phase noise?

     

  • +1 in reply to Amit kumar
    TI__Guru* 86810 points

    Hi Amit,

    I don't think your "spectrum analyzer" has the resolution to tell the phase noise of the signal.

    Anyway, LMX2492 is just a PLL, it will contribute to the in-band noise, out-of-band noise will be determined by the VCO. 

    If you want to improve the phase noise, you also need a higher quality VCO. 

    Reference clock will also affect in-band noise, you need a better quality reference clock as well. The onboard XO being used in the EVM is not a good quality reference clock. 

    You can use PLL Sim (www.ti.com/.../PLLATINUMSIM-SW) to design the loop filter and simulate the phase noise.