Design of 345 GHz Sideband Separation SIS Mixer C. Risacher (1) , V. Vassilev (2) , V. Belitsky (3) , A. Pavolotsky (4) Group for Advanced Receiver Development, Onsala Space Observatory, Chalmers University of Technology, S 412 96, Gothenburg, Sweden Email: (1) risacher@oso.chalmers.se, (2) vevas@oso.chalmers.se, (3) belitsky@oso.chalmers.se, (4) pavolotsky@oso.chalmers.se Abstract We present a design of a 275-370 GHz sideband separation mixer (2SB) where all the superconductor-insulator-superconductor (SIS) junctions, tuning circuitry lines, local oscillator (LO) directional coupler and bias-T are integrated on a single substrate (mixer chip). The RF signal is divided equally with 90º-phase shift by a quadrature waveguide hybrid. The outputs of the hybrid are coupled to the mixer chip through a novel waveguide-to-microstrip transition with integrated bias-T. This structure allows coupling the input waveguide signal to the SIS junction and tuning circuitry lines via a radial type probe while having an isolated port at the opposite side of the substrate where the IF signal can be extracted and DC current injected to bias SIS junctions or suppress Josephson effect independently for each junction. The LO and RF are combined via a 17dB directional coupler, made with superconducting microstrip lines coupled trough lumped links achieved by two perforation slot-holes in the ground plane. Keywords: Sideband separation, SIS mixer, waveguide probe, bias-T. 1. INTRODUCTION This mixer is designed for APEX 12m telescope, which is currently under construction in the Chajnantor region of Northern Chile at an altitude of 5000m, in the Atacama Desert. This site, probably the driest on Earth, is one of the best for sub-millimetre wave observations, together with the South Pole and Mauna Kea in Hawaii [1]. The antenna is a prototype of the ALMA antennas, but with improved surface accuracy (18 µm rms) to allow higher frequency operation and with a modified sub-reflector to accommodate multi-channel arrays. This telescope will have both heterodyne and continuum instruments, covering the frequency range (230 GHz–1.5 THz). The work presented here is done for APEX band 3, covering the frequency range 275-370 GHz (which coincides with ALMA band 7), with centre frequency at approximately 345 GHz. This receiver was chosen to be of sideband separation type, a technology pioneered in millimetre waves by NRAO [2]. Indeed, the noise performance of double sideband (DSB) super heterodyne receiver can be limited by the atmospheric noise fed into the system via the image band. At 345 GHz, receiver sensitivity is improved with a 2SB mixer compared to a DSB mixer [3]. In the proposed design, sideband separation is achieved using a quadrature scheme as in Fig. 1. The RF signal is divided equally with 90° phase difference, and the local oscillator (LO) is also divided equally and applied to 2 identical DSB mixers. LO LO 180 o MIX1 MIX2 G1 G2 T 0 90º hybrid Upper side band LNA 1 LNA 2 Isolator 1 Isolator 2 Lower side band T 0 90º hybrid RF SIGNAL Pin T 0 1/2 P in 1/2 P in Fig. 1: Block diagram of the sideband separation mixer. The mixers outputs at intermediate frequency (IF) are connected to cryogenic IF low noise amplifiers (LNA) having isolators at their input to avoid oscillations. The outputs of the LNAs are then connected to a quadrature 3 dB hybrid. Since one of the sidebands is combined in phase and the other out of phase, sideband cancellation occurs and both sidebands appear separated. The degree of sideband suppression is dependent on the symmetry in amplitude and phase