High-Q micromachined resonant cavities in a K-band diplexer configuration

Microwave diplexers are often used in transmit/receive systems to isolate a power transmit stage from a sensitive receive stage sharing a common antenna. For efficient bandwidth usage and optimum antenna performance, the transmit/receive frequencies are typically placed close together in the frequency spectrum. With today's requirements for high performance, small-size wireless devices, diplexers exhibiting close channel spacing, low insertion loss and small channel bandwidths are increasingly necessary. Utilising two high-Q micromachined silicon cavity resonators, a planar K-band diplexer has been designed, fabricated and tested. This diplexer shows transmit/receive bandwidths of 1.11 and 1.53% and insertion losses of 1.4 and 1.0dB, respectively. Channel center frequencies of 18.64 and 20.47GHz provide a channel separation of approximately 9%, and channel-to-channel isolation greater than 26dB over the entire measured frequency range. The diplexer can be accessed via microstrip lines and can be easily integrated with other planar circuits and structures, such as mixers, amplifiers, oscillators and patch antennas. The proposed design is the first step towards a more mature multi-pole diplexer with enhanced overall performance.