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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