[Elecraft] K3 roofing filter group delay

[Brian Lloyd]

On May 13, 2007, at 11:18 PM, 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.

Right. TANSTAAFL (there ain't no such thing as a free lunch). Given  
that we can get very steep skirts in DSP at the expense of  
introducing more overall latency (delay) in the filter (usually not a  
problem for what we are doing), then we can probably accept analog  
roofing filters that don't provide the same ultimate attenuation in  
the skirts.

Given the choice between the two filters I would probably opt for the  
Bessel filter myself.

So, Elecraft, will you be offering a group of roofing filters  
optimized for digital communications? How about your "brick wall"  
filters in DSP?

73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com