Posted by
Jack Smith-6 on
May 14, 2007; 3:04pm
URL: http://elecraft.85.s1.nabble.com/K3-roofing-filter-group-delay-tp447425p447432.html
Don:
Another interesting classical design is the equal-ripple-delay filter.
It uses the Chebyshev concept of allowing ripple in the passband to
provide increased skirt selectivity, except that the ripple that is
allowed is in the delay function, not the amplitude. By allowing a
slight amount of ripple in group delay of an equaphase filter, the
skirts can be improved over similar order filter with a flat group delay.
Also -- since the K3's roofing filters are backed up with a DSP, could
not the DSP implement a delay equalizer, so that the net delay is flat?
I also don't know whether the roofing filters are repeatable enough to
permit a single equalization setting or if each filter will require
custom coefficients. (If so, I suppose each filter could be equipped
with a small EEPROM chip with its equalization parameters, to be read
when a new filter is installed into a K3.)
I don't know if the K3's DSP implements IIR or FIR filters, but an FIR
can be implemented with linear phase (flat group delay) Hence, if FIR
filters are used, the only group delay requiring equalization will be
the crystal roofing filter.
Jack K8ZOA
www.cliftonlaboratories.com
Don Wilhelm wrote:
> 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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