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LMX2492: Simultaneous Ramping of Two Devices

David Langan
Intellectual 270 points
Part Number: LMX2492

I have an application using two LMX2492 devices.  One is at roughly 8.1 GHz and the second is at 9 GHz.  I'm measuring the beat frequency (900 MHz) between the two as I engage the ramping functionality.  Both are configured to perform identical ramps (same length and increment) and this has been confirmed.  I notice that as the ramp bandwidth increases beyond roughly 190 MHz (inc ~ 1550) the higher frequency synthesizer (9 GHz) is delayed relative to the low frequency synthesizer and the beat frequency is progressively pushed lower until it wraps around.  Is there a frequency dependent functionality in the PLL that would produce such a delay?

It seems quite odd because they are configured for the same ramp length, are operating on the same clock and produce identical ramp bandwidths.

Thanks all.

  • 0
    TI__Expert 5370 points
    Dear David,

    thank you for reaching out; this is an interesting question.

    The ramp function operates on the phase detector frequency and add increment to the fraction for some many steps to create the ramp. Is the phase detector frequency the same in both instance of the PLL?

    are you also able to trigger the ramp at ther same time?

    Otherwise, I am not sure why the delay increase with more steps in the configuration.

    Regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 270 points

    Simon,

    The behavior was unexpected and a bit nightmarish to uncover which is part of why I'm posting this.  Others had asked questions regarding the coherence recently. To answer your questions:

    1) Both devices are driven off of the same reference so their phase detectors are synchronized.  

    2) I can confirm that the ramps are initiated simultaneously.   I'm using a simple sawtooth ramp and would expect a constant frequency offset if that were not the case.  By capturing the beat frequency waveform I can observe a constant frequency offset indicating zero delay in triggering for the cases where the ramp increment is below the magic value.  Once the ramp increment goes beyond the magic value I observe the two waveforms diverging immediately once ramping is triggered.  

    My assessment is that there is some additional delay added at the start or end of each ramp, that is not accounted for by the ramp length parameter.  Thus the relative frequency between the two devices increments by some value after each ramp cycle. Clearly there is some behavior that alters the determinism when using different N divider values between two devices.  

    Truthfully, I've taken an excellent device and attempted to perform a more sophisticated operation than it was likely intended for. It still performs very well as a synthesizer and as a modulator.


    David