A Novel Compact Low Loss Waveguide Image Rejection Filter Based on a Backward Coupler with Band Pass Filters for 100 GHz Band

Shin’Ichiro Asayama,Akira Mizuno,Hideo Ogawa,Yutaka Hasegawa,Toshikazu Onishi

doi:10.1007/s10762-015-0148-6

Abstract

This paper describes the design and test observation result of a novel waveguide image rejection filter (IRF). The IRF is based on a quadrature hybrid coupler as a backward coupling structure, followed by Band Pass Filters (BPFs), and matched loads for the image-frequency termination. A prototype IRF shows return loss of better than 18 dB, and an image rejection ratio of more than 25 dB over 4 GHz band for stratospheric ozone spectra at 110 GHz when the LO frequency and IF frequency are 104 GHz and 6 GHz, respectively. We installed the prototype IRF into an ozone-measuring system and successfully observed an ozone spectrum at 110 GHz in single sideband (SSB) mode. This IRF has the advantages of low transmission loss, compact size, and easy scalability for sub-millimeter frequencies.

Highlights

There is a strong demand to use single sideband (SSB) or sideband separating (2SB) receivers for spectral line observations in astronomical and remote sensing applications at millimeter/sub-millimeter wavelengths
The double sideband (DSB) SIS mixer adopted here was developed at Osaka Prefecture University [20]
The Image Rejection Filter (IRF) was inserted between the feed horn and the SIS mixer, image band signals were terminated to 4 K termination loads

Summary

Introduction

There is a strong demand to use single sideband (SSB) or sideband separating (2SB) receivers for spectral line observations in astronomical and remote sensing applications at millimeter/sub-millimeter wavelengths. In this decade, waveguide-type 2SB superconductor-insulator-superconductor (SIS) mixers have been developed [1,2,3,4,5,6,7], and adopted for the receiver bands of the Atacama large millimeter/sub-millimeter array (ALMA) [8]. The IRF is based on a quadrature hybrid coupler as a backward coupling structure, followed by Band Pass Filters (BPFs), and matched loads for the image-frequency termination The advantage of this approach is that the IRF can be fabricated with a small waveguide structure and low transmission loss, and, in addition, a stable and reliable image rejection ratio can be achieved. To demonstrate the feasibility of the IRF, we fabricated a prototype IRF and installed it into an ozone-measuring system with an SIS mixer and successfully observed an ozone spectrum at 110 GHz in SSB mode

IRF Based on a Backward Coupler with Band Pass Filters

Filter Design

Fabrication and Measurements

Receiver Performance

Ozone Observation

Conclusion