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LMX2571: Max Modulation Frequency

Ryan.Bishop
Expert 3900 points

Part Number: LMX2571

Hello,

What is the maximum modulation frequency this device supports?

Is it possible to get a loop frequency more than 40 MHz and a modulation on FSK of 1.2 Mbps?

Thanks,
Ryan B.

  • 0
    TI__Guru** 120880 points
    Hi Ryan,

    The modulation is done in the PLL, that means the loop filter has a low-pass characteristic to the PLL. If you want 1.2Mbps, then the loop filter bandwidth has to be greater than 600kHz. Otherwise, you will get some distortion.
    600KHz is quite a wide bandwidth but I think it is still possible with LMX2571.
  • 0 in reply to Noel Fung
    TI__Expert 3900 points
    Thanks Noel,

    For the LMX2571 can this actually support modulation at 1.2Mbps if the filter is opened wide enough. What is the maximum loop frequency on this we can run?

    Thanks again,
    Ryan B.
  • 0 in reply to Ryan.Bishop
    TI__Mastermind 28031 points
    Ryan,

    Using the PLLatinum Sim tool, this has a partially integrated loop filter with C3=50pf and C4=100 pF. If i assume a 100 MHz phase detector frequency and use the maximum charge pump gain, the max bandwidth I can get is 1.23 MHz. So maybe possible, but not a whole lot of margin.

    If you have a lower phase detector frequency , then this will reduce this maximum bandwidth.

    Regards,
    Dean