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LMX2594: Bode Errors in Pllatinum Sim

DEP
Prodigy 240 points
Part Number: LMX2594

I've just started a few very simple simulations of LMX2594 PLL. I keep finding the quoted Loop bandwidth does not match the Bode plot and is out by over a order of magnitude.

I specified a loop bandwidth of 100 kHz and phase margin of 70 Deg. Clicked Calculate Loop Filter, there were no errors like VCOCap/MinHighCap Restrict Bandwidth, new filter values appeared and phase noise changed. The summary at the bottom has a realistic loop bandwidth - 110 kHz vs 100 kHz specified, but the calculation trimmed to realistic values for R & C so this error seems reasonable.

The Bode window shows a markedly different gain in the chart and the marker data. The 0dB point is maybe 4.3MHz. The phase margin hits ~70deg at 110kHz - so that looks correct. Its like the calculated Bode gain is maybe 30dB higher than expected.

I have tried also with 3rd and 4th order scenarios and had the same issue.

Below is a screen shot of the Bode window.

Regards David

  • 0
    TI__Mastermind 28001 points

    David,

    The difference is a factor of 20*log(N) for the open loop gain.  So instead of checking where the open loop gain crosses 0 dB, check where it crosses 20*log(N) and you should get the results you want.  I can't read the feedback valuefor 20*log(N), but it's at the level the the closed loop gain is at low offset frequency.

    Regards,

    dean