Posted by
John, KI6WX on
May 14, 2007; 7:18am
URL: http://elecraft.85.s1.nabble.com/K3-roofing-filter-group-delay-tp447425p447427.html
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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