Vasile Surducan wrote:
> I need a low pass filter with an abrupt shape between pass band and
> stop band (like Cauer or elliptical) but with a constant group delay
> in the passband, like Bessel. Does something like that really exist ?

I don't know if such a beast exists in the analog domain or not. 
However, it should be noted that an elliptical (or Cauer) can be 
designed to have a group delay that is much smaller than the deviation 
in the Bessel filter's group delay. In other words, if you look at a 
Bessel filter's group delay as the filter transitions from the pass band 
to the stop band you'll find that the group delay changes. An elliptic 
filter can be designed such that for the same (actually greater) 
attenuation the overall group delay is much smaller.

Another option is a Chebychev type II. This is monotonic in the passband 
(like Bessel and Butterworth) and thus probably has similar group delay 
characteristics.

Scott
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