Hello,
I need to ask some basic questions which I am not able to find good answers for. When I look at filter programs and other information regarding filter responses I see phase and gain plots. Could someone tell me how phase affects the filter, or how it relates? Also what is a good phase response vs a bad phase response? How can I tell if the filter is unstable?
Thank you, Rob
"Rob"
** A phase plot shows how input and output sines waves will differ in phase at any frequency.Most fairly simple filters are "minimum phase" circuits - true when there are no time delays or "all pass filters" included in the filter network.
Such simple filters have the property that *phase follows gain* - which means that if the gain plot is known, the phase plot will correspond to it and cannot be varied without altering the shape of the gain plot.
** As in life, good and bad are subjective judgements - it depends entirely on the application.But for all "minimum phase" circuits, you are stuck with the phase response for any given gain curve.
** Unstable filters oscillate or are continually on the verge of oscillation.Simulation software should be able to warn when this is likely.
...... Phil
To expand slightly on Phil's good explanation, note that analog filters have a frequency-dependent phase shift. Because different frequency components are shifted by different amounts, the shape of the filtered waveform can change (besides the rounding or whatever that is due to removing high frequencies, etc). Typically you see this as overshoot and/or ringing in the step response.
That's not an issue with single pole (one RC section active) filters, but as you get to higher orders you have the option of trading different response features around. For a given order, you can have a sharper cutoff if you are willing to accept some ripple in the frequency response and ringing in the step response.
So-called "linear phase" designs have sacrifice frequency response sharpness to get nice waveforms. They esentially are just a delayed version of the input, with some high frequencies removed by the filter. They get their name from (you guessed it!) the phase plot is a more-or-less straight line in the region of interest. Linear phase is one of the virues of FIR digital filters, by the way. Just as in the analog world, this comes at a price of many more stages for the same cutoff sharpness, compared to simpler IIR filters.
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
"Bob Masta"
** Courtesy noted.... Phil :-)
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