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Original question:

LMX2594: I'm looking for maximum ramp without calibration

WEBENCH® Tools/LMX2594: low frequency signal shape and the minimum ramp start frequency

Joe Da
Prodigy 70 points
Part Number: LMX2594

Tool/software: WEBENCH® Design Tools

Hi,

We have a LMX2594 EVM and we are learning how to use it.

First, we found when we set the output frequency to 100-500 MHz, the output waveform is near a square wave. Is there any reason why it is not sinusoid? 

In addition, we want to use LMX2594 to generate a frequency ramp. What's is the minimum starting frequency in the "Automatic Ramping" mode? For example, if we want to start from 1GHz  and go to 2GHz, and then come back to 1 GHz and repeat this process continuously, is it possible? Or is it possible to have any sample code to do it？

What's more, in the "Manual Ramping" mode, we set the "Step Frequency (MHz)" to 1000 MHz and pressed "Clock Ramp" button. However, the final frequency we obtained is much less than 500MHz (about 100 MHz). Is there any reason why this? Our starting frequency is 1000 MHz.  

Thanks a lot.

  • 0
    TI__Mastermind 18070 points

    Hello Joe,

    My coworker will get back to you next week.

    Regards,

    Hao

  • 0 in reply to Hao Z
    TI__Guru** 107780 points

    Hi Joe,

    Output from the VCO is a sine wave signal. When this signal gets divided down, it becomes a square wave signal.

    During ramping, only the freq of the VCO will be changing, CHDIV is fixed. If the desired start-end ramp freq requires a change in CHDIV, them the ramping function is not able to support. In your case, ramping between 1GHz and 2GHz requires change CHDIV value, so this is not possible.

    In your manual ramp example, the ramp range is very big, a VCO calibration is required. Make sure RAMP_TRIG_CAL is set to 1, so that a VCO calibration will be initiated every time you click the Clock Ramp button.