Don;
Don't forget Bessel filters. These are maximally flat for group delay, but
have a much slower rolloff.
A 5-pole 500-Hz 0.5 dB Chebyshev will be down 80 dB at 1 kHz and will have a
group delay of 16 ms at band center and 44 ms at -3 dB. A 5-pole 500-Hz
Bessel filter will be down 40 dB at 1 kHz and have a constant group delay
across the passband of 10 ms.
You can have a flat group delay or a fast rolloff, but you can't have both.
-John
KI6WX
Brian and all,
The number of poles is not the primary deciding factor. It is the type of
filter (Cohn, Chebychev, Butterworth, Gaussian to 6 dB, Gaussian to 12 dB,
etc.) that will influence the group delay. See the discussion on filters
in Experimental Methods for RF Design for further information.
In general, those filters with a 'rounded nose' will have the best group
delay characteristics - but that is only a generalization, the details
will tell "the rest of the story".
73,
Don W3FPR
Brian Lloyd wrote:
A number of messages have gone back and forth here about roofing filters.
We did mention group delay but I wonder if Elecraft can provide group
delay characteristics for the various filters offered for the K3.
Seems to me that, in general, fewer poles tend to provide better group
delay in a filter at the expense of the skirts. OTOH, if the skirts in
the roofing filter are sufficient to attenuate a strong, undesired signal
so that it cannot cause desense, then it strikes me that the 5-pole
filters might actually provide superior performance for digital
communications.
73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com
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