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?
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