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LMX2595: Switching Speed

Stephanie Fang
Prodigy 40 points

Part Number: LMX2595
Other Parts Discussed in Thread: PLLATINUMSIM-SW

1. What is the switching speed between to toggle between frequency to another?

2. What setting or approach will aid in reducing the switching speed? i.e. VCO setting, etc.

  • 0
    TI__Mastermind 28031 points

    Stephanie,

    This switching time is the sum of the VCO calibration time and the analog settling time, which both depend on the application.  The VCO Calibration time is on the order of 50 us, but can be sped up and depends on the OSCin frequency.  The analog settle time depends on the loop bandwidth.  To speed up the VCO calibration time, we offer partial assist and full assist modes, and these are described in the datasheet.

    I also encourage you to download our PLLatinum Sim tool at ti.com/tool/PLLATINUMSIM-SW

    This tool simulates both VCO calibration time and analog settling time.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 40 points

    Hi Dean,

    Will switching frequency within the same VCO core assist in reducing the re-calibration time?

    For instance, VCO1 is selected and calibrated for 7.6GHz.  What would be the switching speed to toggle to 8GHz, within the same VCO1 core? Or will the VCO have to be re-calibrated every time a frequency is changed?

    Regards

    Stephanie

  • 0 in reply to Stephanie Fang
    TI__Mastermind 28031 points
    Stephanie,

    This depends on how you set up the calibration.
    If you use close frequency assist and you are at 7.6 GHz and go to 8 GHz, then you will start at VCO core 1 and it probably will be a little faster, but not all that much. The calibration includes (1) Find the Right VCO core, (2) Find the right VCO_CAPCTRL, (3) find the right VCO_DACISET. Finding the VCO_DACISET might take some time.

    In your example, you are at the lower end of VCO1, but you have to go through all the VCO capcodes and the VCO_DACISET will be much different at the bottom of VCO1 and the top at VCO1.

    Now if you switch from 7.6 to 7.65 GHz, close frequency calibration could help.

    If your switch is greater than about 100 MHz, then you could use partial assist and use the recommend values from the datasheet (also calculated by TICSPro). Or the ultimate is to use full assist, but this requires you to lock every device at every frequency and then save a table of calibration settings.

    Regards,
    Dean