Posted by
Don Wilhelm-3 on
May 14, 2007; 1:32pm
URL: http://elecraft.85.s1.nabble.com/K3-roofing-filter-group-delay-tp447425p447430.html
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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