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LMX2595: Rise in noise floor of LMX 2595 after few hrs

Vikas Kumar40
Intellectual 650 points
Part Number: LMX2595

Hi

I am usign LMX2595 for generation of  3 GHz from 50 MHz, After configuring the device, the device locks to the required frequency and lock status comes as logic 1. Phase noise specs also meets. But sometimes, once in a while , may be after 30 min or 1 hr, noise floor in close span raises up which affects the phase noise at 10 Hz, 100Hz and 1 KHz, but the lock status shows as logic 1 only. During this time, reference has been measured and there no change in noise floor observed.

What can be the reason.

Regards

Vikas Kumar

  • 0
    TI__Guru** 107300 points

    Hi Vikas,

    At these offset frequencies, the phase noises come from the reference clock. Was the problem happen once for a very short time? What kind of reference clock you are using?

  • 0 in reply to Noel Fung
    Intellectual 650 points

    Hi Noel

    I am using Wenzel clock at 10 MHz. Following this, I have a multiplier circuit to get 50 MHz and amplifier to give 8 dBm power to PLL. In normal scenario , if I see the PLL output in close span ie 200 KHz, spectrum looks very clean. But after few hrs, in the same span, I can see lot of noise which has raised above the noise floor. As the span is 200 KHz, any addition of noise adds phase noise.

    I monitored the VCO tune voltage during the issue, there had not been any difference. Even the Lock detect status shows high indicating lock.

    If the problem comes, I need to switch off the module , then programme it and the issue vanishes.

    Regards

    Vikas

  • 0 in reply to Vikas Kumar40
    TI__Guru** 107300 points

    Hi Vikas,

    Maybe you should do the same test without the synthesizer, see if you would observe the same issue from the amplifier.

  • 0 in reply to Noel Fung
    Intellectual 650 points

    Hi Noel

    I had tried with RF chain when the issue comes in Synth side, could not see any noise floor lift up.

     

    Recently I have updated the code with the simple fraction to get output frequency, I observed the module for 8 hrs for two days. The issues have not shown again.

    Initially I was using the large fraction to get output frequency.

    Any thing related to VCO calibration could be issue?

    Regards

    Vikas

  • +1 in reply to Vikas Kumar40
    TI__Guru** 107300 points

    Hi Vikas,

    Calibration does not care the fraction.

    Your fpd is 50MHz while VCO is 12GHz, it is an integer channel. No matter what is the denominator value, the numerator is zero, you don't need large fraction. Large fraction is used in fractional channels when we want to randomize the spurs. At integer channel, large fractional may create spurs in some cases.

  • 0 in reply to Noel Fung
    Intellectual 650 points

    Hi Noel

    Thats what I believed, there could be some issues with selecting the large fraction.

    Thanks for the confirmation.

    Regards

    Vikas