275–500 GHz Waveguide Diplexer to Combine Local Oscillators for Different Frequency Bands

Development of local oscillator (LO) sources based on amplifier multiplier chains (AMC) have steadily progressed in recent years, with more power and more bandwidth available at mm and sub-mm wavelengths. Recent radio astronomy receiver projects are, however, aiming at duplicating the RF bandwidth, with fractional bandwidth around 60%, even at frequencies as high as 500 GHz. In particular, National Astronomical Observatory of Japan (NAOJ) is developing wideband SIS mixers for a combined Atacama Large Millimeter/Submillimeter Array (ALMA) band 7 and 8 receiver, extending from 275 to 500 GHz. Currently, LO sources are not available for such a wide frequency band. To solve this problem, we have developed a waveguide diplexer to combine the output power of two different LO sources for ALMA band 7 (275-373 GHz) and band 8 (385-500 GHz). The proposed waveguide diplexer is based on hybrid couplers and waveguide filters. Two units of the designed component have been fabricated and S-parameters have been measured with good agreement with simulations. Initial tests to combine the LO power of band 7 and 8, LO sources at cryogenic temperature to pump a wideband 275-500 GHz SIS mixer have been successful.