[Elecraft] K3 roofing filter group delay

[John, KI6WX]

You can cascade a pair of Bessel filters to get better frequency response 
while maintaining a flat group delay, but you do have to isolate them so 
that they both are seeing a resistive source and load impedance.  The 
cascaded filters would show a 6 dB rolloff at nominal cutoff frequency.  The 
3 dB point will occur at about 70% of bandwidth.  A pair of 5-pole Bessel 
filters cascaded with a 500 Hz 3 dB bandwidth will be down about 55 dB at 1 
kHz.

The better method is to build a 10-pole 500-Hz Bessel filter which will be 
down nearly 60 dB at 1 kHz.

There was also a question about a linear phase filter with equiripple error. 
A 0.5 degree error is about +/-5% variation in group delay.  A 5-pole 500-Hz 
0.5 degree equiripple filter will be down 50 dB at 1 kHz and have about 11 
ms of group delay.
-John
 KI6WX


>> Don't forget Bessel filters.  These are maximally flat for group delay, 
>> but
>> have a much slower rolloff.
>
> John,
>
> What penalty (if any) would occur in cascading identical Bessel filters to
> improve roll-off and stop-band attenuation?
>
> Would the overall response still be considered Bessel, or will a point be
> reached where the response begins to take on Chebyshev characteristics?
>