A diplexer can be either contiguous or non-contiguous. If the two portions of the spectrum are adjacent and are defined so as to include a region where the power would be split equally between the two spectral portions, (i.e. the channels have a “3 dB crossover”), then the diplexer channels are contiguous. If a guardband exists between the two channels, then the design is non-contiguous. If the diplexer is provided with a common port at which the two channels are both excited, and with separate ports for each of the two channels located opposite the common port, then the diplexer is a “true” three port network.
If the diplexer is provided with two common ports (i.e. both channels connected together at both ends) the resultant two-port device is called a “double-diplexer”. This latter device displays two passbands, stopping all other frequencies within the limits of the physical structure. The common-port interconnection methods differ, depending on the bandwidth and intrinsic selectivity of the two channels.
At RS Microwave, a multiplexer design philosophy is employed wherein the interconnection network is separated from the selectivity-generating channel filters. In essence, the interconnection networks consist of lowpass-highpass elements that provide at least 10 dB of isolation between the channels. The selectivity of the channels is determined by the topology of the channel filters. RS Microwave has recently incorporated cross-coupling into the filters comprising the channels. In this manner, extra tranmission zeros are inserted into the channel transfer functions, improving the close-in selectivity of the channel. In cases where the filters are basically inductively (capacitively) coupled, the cross-couplings are primarily capacitive (inductive).
Earlier articles in this series have discussed the intrinsic benefits of cross-coupling and the means for realizing these benefits, but a combination of network synthesis and E-M simulation are required to achieve success without exorbitant cost. Typically (but not exclusively) the cross-coupled filters are folded so as to allow for coupling between first and last resonators, second and next-to-last, etc. An example of such a three-port diplexer is RS Microwave P/N 80431AD, an S-band diplexer used in PCS applications. Use of cross-coupling allows for achievement of quasi-elliptic frequency response, which means that for the same insertion loss values, the selectivity is enhanced beyond what is experienced with Chebychev or Butterworth transfer functions.
An example of a double-diplexer has been presented in an earlier article concerning GPS diplexing, but P/N 63491A-1 is worth re-visiting in the light of the above explanation. Many combinations are possible and are under development, with realizations in L-C configurations from 1 MHz to about 2 GHz, evanescent mode, propagating waveguide, resonant cavities and dielectric resonator implementations up to at least 40 GHz. The use of cross-coupling in diplexer applications can lead to improved performance, smaller size and reduced cost.