Research On W-Band Quasi-Elliptic Waveguide Bandpass Filters

Quasi-elliptic bandpass filters can provide steep out-of-band suppression with a minimum number of resonators,minimize signal interference in adjacent channels,and reduce the size and quality of systems,which can better meet the requirements of high performance and small size of modern communication systems.With the development of the frequencies of wireless communication system towards higher frequency band,rectangular waveguide is an ideal transmission medium for quasi-elliptic bandpass filter in order to ensure low loss and high power processing performance.Based on the development trend of wireless communication systems,combined with two implementation methods of quasi-elliptic filter,the W-band quasielliptic waveguide bandpass filters have been designed and fabricated in this paper.Firstly,a W-band quasi-elliptic waveguide bandpass filter is designed based on the crosscoupling theory,which can generates a transmission zero on each side of the passband.The transmission zero can be positioned by adjusting the sizes of the coupling structures to improve the out-of-band suppression of the filter.The filter has a central frequency of 88 GHz,a 3 dB fractional bandwidth of 8.5%,an insertion loss of less than 0.2 dB,and a return loss of more than 18 dB.In 3 GHz out of passband,the out-of-band rejection is higher than 30 dB.Secondly,a W-band third-order and a fifth-order quasi-elliptic waveguide bandpass filter is designed based on the non-resonant nodes,respectively.The structure of the non-resonant node is realized by a T-junction waveguide.The positions of transmission zeros for both filters can be arbitrarily placed by adjusting the sizes of T-junction waveguide structures to improve the out-of-band suppression.The third-order filter has a transmission zero can be located at the low or high frequency band.The fifth-order filter has a center frequency of 100 GHz,an insertion loss of less than 0.2 dB,a return loss of more than 20 dB,and a transmission zero on each side of the passband.In 0.3 GHz out of passband,the out-of-band rejection is higher than 34 dB.Thirdly,two quasi-elliptic waveguide bandpass filters are fabricated by UV-LIGA process with high precision and low cost.SU-8 photoresist is used as structural material,and the process flow and key processing parameters are introduced in detail.Finally,the filters are tested.The test results are basically consistent with the simulation results.