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Ideally, for FM, the group delay should be constant across the band occupied by an FM signal's sidebands. Any variation translates into distortion of the demodulated audio.
Constant group delay is the same thing as a linear-phase characteristic in that same region. The slope of the phase/freq plot = group delay. so the phase slope can have ant value, but the plot must be a straight line. Deviation from straightness again makes audio distortion. I would have been clearer if I'd said straight rather than flat, sorry.
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Thanks. I have plotted the phase (Arg S21) and S21 for the 5th order Cheryshev bandpass and compared its with the phase and group delay from software. They look very similar and slopes of the phase are quite straight. But the group delay has two peaks and is far from flat. Bear in mind this is a wide band filter though (probably can be used for SDR FM reception with 50 ohms antenna input).
If slope of the phase/freq plot = group delay, it should have unit of degrees per unit Hz. In the rf-tools.com software, it uses time unit of ns. Dimensionally it is consistent as the inverse of frequency is time. NB. Note the large phase noise near the noise floor of S21 at -90-100db of the NanoVNA V2 plus 2. The plot has no sample averaging.
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In any other company the bean counters would have such a fit of the heebie-jeebies that it would take months for their golf scores to recover.
David
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