Design and Characterization of a Hairpin Filter at GHz Frequencies Using a THz Microscope for Near-Field Analysis

Microwave filters are widely used for telecommunication applications and measurement systems. Hairpin filters are very compact edge couple bandpass filters, which typically exhibit low insertion loss and a narrow bandwidth. In this work, a hairpin filter with a designed center frequency of 34.38 GHz is investigated. The scattering parameters are measured with a network analyzer and the near-field distribution is measured spatially at multiple frequencies with a THz microscope, using a superconducting Josephson cantilever. The measured results of the hairpin filter agree well with the 3D electromagnetic simulations. The THz microscopy setup is capable of measuring three-dimensionally the intensity and frequency of field distributions from 1 GHz to 5 THz. This setup is a versatile tool for characterizing and optimizing microwave circuits and can be used to validate simulation results.