Electronic – Are phase angles in filter transfer functions primarily for stability

bode plotfilterphasetransfer function

I see transfer functions and bode plots (magnitude and phase) and it occurred to me, that I don't really use the phase plots at all. My career experience seems to have gotten away without having to use or calculate them except once where I had a stability issue with an opamp. But this just shows a gap in my experience / knowledge that I would like to resolve before I genuinely need it.

When we look at general filter topologies that show phase plots, why is that relevent to show. For instance, a single order LPF will have a phase shift of -45 deg @ fc – so what ? Why do we care about these numbers in filters ? Is it that when we cascade them, it becomes relevant when looking at phase margin ?

Best Answer

Different applications are sensitive to phase variation with frequency for different reasons.

  • It can impact stability margin if the filter is in a closed-loop system.
  • The total error between a source signal and a filtered signal is a function of phase (as an extreme example, if h(x) defines a filter with a gain of 1 and a phase shift of 180 degrees at some frequency, then at that frequency the error signal y = x - h(x) will have an amplitude twice that of x).
  • If the phase does not change linearly with frequency then the group delay of the filter won't be constant. This will causes pulses to be smeared out, which is a Bad Thing in communications systems.
  • If the group delay of a filter is high it will contribute to the overall delay in a communications system. This is much less likely to be a problem, but can still be an issue in some applications.
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