A 800MHz Coaxial Cavity Diplexer

Microwave diplexers are very a important part of modern communication system, and usually select and join channel. So they are generally used in microwave communication, satellite communication and remote sensing system. With the rapid development of microwave technology, the microwave communication frequency resources are becoming precious and crowded. The high performance microwave filters and diplexers have played more important roles in the microwave and wireless communication system. The high performance of high selectivity, small size, low inserts loss and low cost accelerate development of filter design and physical structure.The generalized Chebyshev filter which was composed by traditional Chebyshev filter and cross coupling technology can produce finite transmission zeros in filter stopband. This technology can increase stopband attenuation and reduce filter order.The issue expatiatory introduces theory about diplexers composed by Chebyshev filters, and research theory about Chebyshev filter, and also deduces the Chebyshev polynomial. The full coupling matrix of filter was calculated from equivalent circuit model of filter. The usual filter topology structure and the distribution of their transmission zeros were simply introduced. The procedure deduced the coupling matrix of filters which were prescribed in advance by using gradient-based Optimization technique. And all the steps were programmed by Matlab language. Using the above synthesis design a coaxial cavity diplexers working in 800MHz. Using 3D electromagnetism software HFSS simulates resonant frequency of diplexer's receive/transmit filter, coupling coefficients of each two resonator, and external quality factor. It is easy to make sure the diplexer physical structure and dimension form above steps. And the simulation result of equivalent circuit is given. Finally, the diplexer was manufactured and tested. The test data shows the diplexer has high selectivity, small size and low inserts loss. The example is reported which validates the proposed synthesis procedure. At last, the diplexer has been successfully used in a microwave wireless communication system.