John,
Thanks for that additional info - I had forgotten about the Bessel filter.
There is a distinct advantage with the K3, the 'roofing filter' is not
the major element in setting the rolloff of the total filtering and the
ultimate receiver selectivity because that aspect is handled by the DSP.
That makes the use of roofing filters with a slow rolloff (not steep
skirts) quite feasible.
I believe we should be looking at group delay filter characteristics
more closely for use with data modes, especially with the advent of
digital voice mode.
73,
Don W3FPR
John, KI6WX wrote:
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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