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LMX2595: SAWTOOTH GENERATION

henry lincoln
Prodigy 40 points
Part Number: LMX2595
Other Parts Discussed in Thread: LMX2492

Hello,

       Can you show me an example about how to generate sawtooth with LMX2595? The user guide only talked about triangular wave. Please let me know what should I set in GUI.

Thanks,

Henry

  • 0
    TI__Mastermind 28001 points

    Henry,

    Sawtooth is very easy, you only need one ramp.

    Looking on the ramping setup example on page 22, all you need to do is change RAMP0_NEXT from "RAMP1" to "RAMP0".  Because RAMP0_RST is checked, it will automatically start back at the beginning frequency.

    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 40 points
    Thank you, Dean.
    How can I get a sawtooth without calibration? I just tried to sweep 100 MHz in 100 us only using ramp0, but the ramp is like a triangular wave.
  • 0 in reply to Dean Banerjee
    Prodigy 40 points

    Dean,

    Look at the graph. The waveform is so ugly. Can I get a smoother one? Like no calibration? 

    Henry

  • 0 in reply to henry lincoln
    TI__Mastermind 28001 points


    To get to the point ... No.  Calibration free ramping is only possible if the VCO frequency changes a small amount.  If we take the frequencies that you have at the output and map it back to the VCO.Henry,

    Based on one device over temperature, here's some data:

    Below is how far I measured one device can go up in frequency "calibration free" in MHz

    Fvco Ta=-40 Ta=25 Ta=85
    7.5 GHz 76 43 20
    10 GHz 108 59 20
    12 GHz 191 124 64
    15 GHz 231 115 55

    And here is how far it could go down "calibration free" in MHz

    Fvco Ta=-40 Ta=25 Ta=85
    7.5 GHz 36 70 86
    10 GHz 53 103 123
    12 GHz 64 131 175
    15 GHz 100 216 260

    Now this is just one device and pushing the VCO to  the absolute limit.  Calibrations are a reality of ramping and integrated VCOs.  The only way not to have them for a large ramp is to use a part with an external VCO, like the LMX2492.


    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 40 points

    That really helps a lot. Thank you so much.

    Did you generate a Sawtooth? Cause I got a triangular wave even I tried to get a sawtooth.

    Regards,

    Henry

  • 0 in reply to henry lincoln
    TI__Genius 9920 points

    Hi Henry,

    Datasheet shows triangle wave example, but it is easy to become a sawtooth wave. As Figure 30, when RAMP0_NEXT still is RAMP0, then it is a sawtooth wave.

    TICS Pro GUI could help you check which register could be changed for ramp settings.

    Regards,

    Shawn