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WEBENCH® Tools/PLLATINUMSIM-SW: Loop Bandwidth and Phase Margin


Tool/software: WEBENCH® Design Tools

Why does the PLLatinumSIM tool appear to report the closed-loop bandwidth and the phase margin at that frequency. Stability is related to the open-loop bandwidth. Looking at the open-loop unity gain frequency, it appears to be MUCH higher than the reported Loop Bandwidth. The Bode Plots make it very difficult to determine stability - the open loop gain appears to be too high.

  • Paul,

    PLLatinum sim gives closed loop bandwidth, but this is defined at the point where the open loop bandwidth has magnitude of 1, so it is related.

    The tool also gives gain margin and uses Routh's stability criteria to report stability on the bode plot tab.

    For the "Open Loop Gain", I think that you are saying you would prefer to subtract  20*log(N) so that you can see the point where this crosses 0 dB.  Might not be obvious from the graph, but this is how PLLatinum Sim defines "Loop Bandwidth".



  • Understood - I trust the numeric bandwidth calculation because we verified it with open loop gain / phase margin measurements, but you're right about the 20*log(N) issue.

    I have done these calculations in Maple and don't understand why PLLatinumSIM would include the extra 20*log(N) factor in open loop gain - N is already factored into the open loop gain calculation.

    The unity gain frequency for the open loop gain is the important value when looking at the phase-margin, right? With the extra factor included, the unity gain frequency appears to be much higher.

    I do understand that the graph is scaled by the extra factor - it just doesn't make sense to me - why was it done this way - maybe there's some advantage to displaying it this way?

    Thanks for the dialog,