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LMX2594: Manual ramp mode not calibrating

Christer Andersson
Intellectual 290 points
Part Number: LMX2594

We are not able to get manual ramp mode to automatically recalibrate the VCO as we're manually stepping through our bandwidth. Our start frequency is roughly 10.35 GHz and we're trying to step to 10.55 GHz in steps of about 1 MHz. We're initially doing this slowly to be able to observe the frequency on a spectrum analyzer. We can see that after a bit more than 100 MHz of total stepping the VCO control voltage hits the rails and the frequency becomes irratic. We expect that it's possible to get the VCO to recalibrate in manual mode (not only in automatic ramping mode), either on every step or after every N steps.

We've tried setting RAMP_THRESH to reasonable values, e.g. less than 10 MHz. We've also tried both enabling and disabling RAMP_TRIG_CAL, but apparently no change of results.

Unfortunately our evalboard recently broke. Can you confirm that manual ramping is compatible with some sort of VCO autocalibration and if so, can you suggest what we might be doing wrong?

Thanks!

  • +1
    TI__Guru** 108200 points

    Hi Christer,

    We should setup RAMP_THRESH smaller than 100, I personally like to make it even smaller to account for part-2-part variation and temperature variation. When this accumulated ramp frequency hits this threshold, the part will calibrate automatically so that it could continue to support further ramping.

    Alternatively, we can setup RAMP_THRESH to be much bigger than the chip can support but enable the RAMP_TRIG_CAL. This way, the chip will calibrate on every ramp. 

    I tried with the following configuration, it works.

  • 0
    Intellectual 290 points

    Thanks for the help. Turned out the PLL on one of our boards was damaged and didn't calibrate properly. Changing the board we can get the PLL to calibrate in manual ramp mode after a deterministic number of frequency steps.