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1. As another electroda member wrote you use linear filter , that will give you about 90% of bandwidth with equal delay. But those filters are quite complex.
2. Add delay equalizer-sfilsyn, filpro, eagleware, pcfilt can do it for you.
3. Another common method if possible , design a much wider filter that still meets your rejection. Thus you can use the center of the bandwidth which will have flat delay. If fizable this last approach is easy to realize.
suppose i have a bessel filter that is a maximally flat filter and have a flat group delay and have a bad magnitude response (relative to butterworth filter) and i want to improve the magnitude response and have that flat groupdelay.
or suppose i have a butterworth filter that have a good magnitude response but a bad groupdelay response (relative to flat groupdelay), so, iwant to improve the GD.
consequently, i want a filter with a "GD response similar to bessel and magnitude response similar to butterworth".