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LMX2594: What is the Modulation Bandwidth of the LMX2594

Layne Jakahi
Intellectual 820 points
Part Number: LMX2594

Hello

What is the modulation bandwidth of the LMX2594 by definition?  

The Modulation Bandwidth is defined as the maximum rate of change in the output frequency that can be attained using the Control Voltage of a VCXO.

I have seen the response to the other LMX2594 modulation bandwidth post where the "Calibration Free Ramping.pdf" was given.  That document describes the maximum and minimum frequency (ramping) that the VCO can be tuned to at a specific lock frequency.  Im actually looking for the "maximum rate of change in the output frequency".  Most VCXO have this spec listed but I couldnt find it in the datasheet.

Please assist.

Thanks

Layne

  • 0
    TI__Mastermind 27661 points

    Layne,

    This is a VCO with many bands, so this spec might not make as much sense.  For instance, if I tune the VCO to 7.5 GHz, I can make it go to 15 GHz in < 5 us, using full assist mode, and this 5 us is almost all dominated by the wrote speed, not some slewing.  So by this logic it would be >>1.5 GHz/us, but I'm not sure if this is what you mean.

    If you run the VCO calibration, this will dominate switching time, and this calibration time is modeled with our PLLatinum Sim tool.

    So I think that perhaps it only makes sense to talk about this spec if you are referring to the VCO working in it's calibration free range.  But for this we have no data.

    Also, if you are thinking of an ultra-wide loop bandwidth, be aware that if the capacitance closest to the VCO is <3.3 nF, you will start to see the VCO phase noise degrade.  PLLatinum Sim says the input capacitance of the LMX2594 VCO is 70 pF, but more recent measurements suggest it is closer to 250 pF.  So in terms of loop filter bandwidth, this constraint could be limiting.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Intellectual 820 points
    Hi Dean
    We were looking at your response and this is what we needed to know but it did bring up 2 questions.
    1) In regards to your statement about "the capacitance closest to the VCO is <3.3 nF". Can you tell us where this limit is from? From the demo board we see a capacitance of 1800pF but we couldnt find any statements about this 3.3nF in any document nor datasheet.

    2) You mention that the "VCO PHase noise will degrade if the capacitance closest to the VCO is <3.3nF". Can you tell us HOW will the phase noise degrade? We werent clear as to your meaning of degrade. Are you saying that the use a wide loop bandwidth, will cause the phase noise to suffer because the phase noise of the phase detector will dominate over the phase noise of the VCO? Is this what you are referring to?

    Thanks dean for your help.

    Layne
  • 0 in reply to Layne Jakahi
    Intellectual 820 points

    Hi Dean

    Any clarification on your comment??

    Thanks 

    Layne

  • 0 in reply to Layne Jakahi
    Intellectual 820 points
    hello
    is there anyone on this forum that can clarify Dean's comment about "VCO PHase noise will degrade if the capacitance closest to the VCO is <3.3nF"?

    please assist.

    thanks
    Layne
  • 0 in reply to Layne Jakahi
    TI__Mastermind 27661 points

    3730.LMX2594 Minimum High Order Cap for Vtune.pdfLayne,

    Sorry for the delayed reply;  I was traveling.

    The VCO phase noise is degraded if the capacitor at the Vtune pin is too small.  This document gives more details.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Intellectual 820 points

    hi dean

    thanks for the document...this is the info i was hoping for.  will be going over it.

    thanks again

    layne