Quote:
Originally Posted by regenfreak
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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.
So posher ceramic filters will claim linear phase, but deliver something with less variation from true linearity than lower grade filters. Ceramic filters can be a bit off on centre frequency and you may need to match the things for best overall performance, also some seem to age.
That Revox tuner I have uses LC filtering in its IF, not a ceramic filter to be found. It also uses separate stages of IF limiter-amplifier. The LC filter can be adjusted to get the required bandwidth and still allow adjustment of phase linearity. The adjustment labour is a lot more expensive than ceramic filters, so this isn't a common approach. 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