Wideband, High Selectivity Diplexers Utilizing Digital-Elliptic Filters

The high selectivity and compact form of digital-eliptic filters [1-2] makes them attractive for use in wideband diplexers. The theory of diplexers has been described in several references [3-6] where it is shown that a perfect match at the input of a diplexer requires the component filters to be complementary. Filters with equal-ripple response in both pass band and stop band can be designed to be complementary; however, it has been pointed out [4] that this places an undesirable restriction on the isolatian characteristic. The use of "pseudo-complementary" filters [4-5] allows the achievement of equal-ripple designs with high isolation characteristics at the cost of a slight increase in input VSWR.