Re: design microwave filter
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.