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LMX2595: Ramp setup

Feiran LIu
Prodigy 90 points
Part Number: LMX2595
Other Parts Discussed in Thread: LMX2594, LMX2492, LMX2491

Hi,

      Can you please help me to set up the ramp mode of LMX2595? So I am trying to sweep frequency from 300 MHz to 500 MHz without calibration. Frequency range can be vary. Right now I set up my start frequency to 316.7MHz and stop frequency to 329 MHz. However, I cannot get a chirp signal. The frequency looks like fixed. I attached my settings below. Please let me know if I did something wrong.

Thanks

  • 0
    TI__Mastermind 28011 points
    Feiran,

    If the VCO frequency changes more than a certain amount (based on frequency, but on the order of 100 MHz), then the VCO will calibrate and take the time it needs (on the order of 20 us, but varies). As this changes with part and freqeuncy, this creates an erratic ramp with random glitches. The LMX2594 allows the user to specify a calibration interval less than what is needed and a calibration time more than what is needed so that the ramp will calibrate at predictable intervals for a predicted time. In other words, the ramp can not go from 7600 to 7900 MHz without calibrating.

    Looking at your settings, they seem reasonable, athough the RST box for RAMP0 should be checked and you should probably set the ramp limits slightly wider than the actual range of the ramp (maybe this is the issue). Did you toggle the RAMP_EN bit to initiate the ramp?

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 90 points
    Hi Dean,

    I do toggle the RAMP_EN, but the frequency doesn't change. I try to get the same waveform of figure 12 in LMX2595 datasheet. I think that's in calibration free mode. Can you please help me to set up that? Cause I cannot get rid of calibration now.

    Thanks,
    Feiran
  • 0 in reply to Feiran LIu
    TI__Guru** 108510 points
    Hi Feiran,

    Calibration free is not possible in your application.
    Your target output frequencies are 300MHz to 500MHz. That means, the VCO has to sweep between 7200MHz and 12000MHz. Without calibration, the VCO is not able to move from 7200MHz to 12000MHz.
    As long as the VCO sweep range is more than 100MHz, you have to be caution, calibration free ramping may not be possible.
  • 0 in reply to Noel Fung
    Prodigy 90 points
    Hi Noel,

    Suppose I want to do calibration free ramping with 1 or 2 GHz bandwidth, is this practical by using any other TI PLL? For our design, time is very critical, and the calibration spends too much time.

    Thanks
  • 0 in reply to Feiran LIu
    TI__Mastermind 28011 points

    Feiran,

    If you want wide bandwidth and calibration free, then you need to use a device with external VCO.

    This would be the LMX2491 or LMX2492.

    Regards,

    Dean