Microstrip multiplexer with four channels for broadband system applications

A microstrip multiplexer for broadband system applications from 10 to 21 GHz has been developed. The multiplexer consists of four microstrip bandpass filters with center passband frequencies at 10, 12, 19, and 21 GHz. The multiplexer efficiently suppresses the parasitic passbands of the 10- and 12-GHz filters and balances the phase relationship among the multiplexer channels. A method for improving the parasitic passband rejection has been devised. Measured results show that parasitic passband rejection is more than 38 dB. Measured insertion losses are 1.81, 1.90, 2.88, and 2.51 dB at 10.3, 12.25, 19.4, and 21.285 GHz, respectively. Measured out-of-band isolation is more than 32 dB at all these frequencies. This multiplexer has applications in communication transceiver systems with dual-channel receiver (12 and 21 GHz) and dual-channel transmitter (10 and 19 GHz).