Improved notch filter for microwave plasma diagnostics in 70 GHz range

摘要

A notch filter in 70 GHz range is developed to eliminate undesirable signals for plasma diagnostics using millimeter waves. We require a robust and narrow band characteristic for the notch filter. Since the previous notch filter which we reported contains the spurious notches, the plungers are improved to reduce the spurious notches. The lossy disks infill the gaps between the plungers and the cavities. The insert of the lossy disks minimizes the spurious notches around 76–78 GHz, while the insertion loss increases in the frequency range from 77.5 to 80 GHz. The plungers with differential screws are also used to make the tuning fine. For further performance increase, based on the same cavity mode, the design of coupling slits is modified to coupling holes. The simulated results in the case of coupling holes predict the characteristic of less than 50MHz at -3 dB bandwidth and less than -50 dB with two cavities and -100 dB with eight cavities at the maximum attenuations.