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LMX2592: On-the-fly frequency change

Carmen Mendo
Prodigy 10 points
Part Number: LMX2592

Hello,

I'm considering a design where LMX2592 would be used to generate a sampling clock; and the sampling frequency must be adjusted dynamically.

So my question is, can I change the fractional divider setting on-the-fly, without a reset/calibration? Is this a valid use of the circuit, or is there some drawback?

Thank you,

Carmen

  • 0
    TI__Guru** 108130 points

    Hi Carmen,

    Depends on how wide you want to adjust the frequency with the fraction, this could be possible. There are several VCOs inside the chip and each VCO is divided into a lot of sub-bands, so a calibration is needed. After calibration, Vtune is around 1.3V. if you change the fraction afterward without a calibration, Vtune will change to accommodate the required frequency change. When the required Vtune is outside the possible voltage range, the PLL will not be able to lock. 

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hi Noel,

    Thank you very much for the info.

    In the short term, I expect small frequency variation (say in the range of few tens of ppms), which from your reply I assume would be OK without calibration. So the procedure would go: 1) initialize to nominal settings 2) calibrate 3) operate with small adjustments only to the FRACN divider (without re-calibration).

    Looking forward, it would be good to try a wider span, say up to +/-100ppm around the initial/calibrated frequency (up to +/-500ppm would be nice to have). Do you think this would still be feasible?

    BR

  • +1 in reply to Carmen Mendo
    TI__Guru** 108130 points

    Hi Carmen,

    100ppm at 5GHz VCO will be 500kHz, this is a small amount of change for the VCO, I expect this could be handled solely with the Vtune without a calibration.