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LMX2594: LMX2594 - using external VCO?

AnBer
Mastermind 31205 points

Part Number: LMX2594

Team,

The datasheet suggests that it is not possible to use an external VCO.
Can you please confirm?

Are there plan for a comparable device that would allow to use an external VCO?

Thanks in advance,

Anthony

  • 0
    TI__Expert 5370 points
    Dear Anber,

    thank you for reaching out and ask. That is true, this device does not have external input for feedback. This request has been made numerous times and a external VCO feature should be considered in the future. I do not have timeline.

    Regards, Simon.
  • 0 in reply to Simon Damphousse
    TI__Mastermind 31205 points

    Hi Simon,

    Thanks for the answer. Some feedback I got:

    With internal VCO we cannot achieve the noise performance required.

    To hide the VCO noise the cut off frequency of the loop filter could be increased but there is a limitation related to C3 (1500pF minimum).
    In my case with 1500pF the cut off frequency is 100kHz. I would need 250 kHz or 500kHz.
    With an external VCO then C3 could be decreased as much as necessary and improve the Phase noise in the 10kHz and 100Khz range.

    Is there ways to still improve the noise performance and phase noise in the 10kHz and 100Khz range? Any advises?

    Thanks in advance,
    A.

  • 0 in reply to AnBer
    TI__Expert 5370 points
    Dear Anber,

    Feedback noted. thank you. A few points of interest:

    1) raise the CP gain to max so you get the largest BW for the cap limitation of 1500pF. You likely have done that already buy just in case.

    2) Use the highest PFD frequency you can. We support up to 400 MHz.

    3) Enable the input doubler whcih is really low noise and improve inband by not quite 3 dB but more like 2 dB. You could input 100 MHz for example and double to 200 MHz.

    4) try to change the filter order from 3rd to 2nd order. Use Pllatinum Sim for simulation.

    I hope this helps,
    Regards, Simon.