17th International Symposium on Space Terahertz Technology P1-18 Development of Balanced SIS Mixers for ALMA Band-10 Sergey V. Shitov, Oleg V. Koryukin, Yoshinory Uzawa, Takashi Noguchi, Andrey V. Uvarov, Ilya A. Cohn down to only 2-3 p,W for a two polarized SIS receiver, if submicron SIS junctions are used in the balanced mixer configuration. There are a few options for waveguide balanced and side- band separating mixers employing the straightforward connection of the DSB mixers using hybrid couplers [2]. However, the low-loss operation of the waveguide-based connections and the required balancing of the mixers both are of a great concern in the THz frequency range. This is a good reason to look for the more compact design of the RF circuit for waveguide or/and consider an integrated quasioptical SIS mixer design. It is known that the conversion gain of a SIS junction is reducing essentially above the gap frequency, and it turns negligibly low above twice the gap frequency. The ALMA Band-10 DSB receiver specification for T R x is 230 K within 80% of the band 787-950 GHz and 345 K at any frequency of the band. The IF range of 4-12 GHz is required for the DSB receiver with ripples below 4 dB within each 2 GHz segment. All these make the accurate design of the mixer's RF and IF circuits very important. Since the Q-factor of a SIS junction at submillimeter wavelength is high (Q>>1), the twin-SIS junction [3]-[5] is a good solution providing about twice RF bandwidth of a single junction. However, it is known that two junctions in the twin-SIS are unequally coupled at RF across the band, thus providing less gain than one can expect from the equivalent junction of the same R. This effect is difficult to estimate experimentally, since two junctions are connected in parallel at DC. The growing importance of such accurate analysis in THz range can be motivated with concern of the essential drop of conversion gain of a Nb-based SIS mixer closer to 1 THz. To analyze the effect of unequal pump and dynamic resistance for a two-junction SIS mixer, the Tucker's 3-port model can be used yet being expanded to multi-junction mixing arrays. Abstract-A few concepts of a wide-band balanced SIS mixer employing submicron-sized SIS junctions are under development for 787-950 GHz frequency range. A quasioptical DSB balanced mixer with integrated cross-slot antenna is considered as the less laborious and cheaper option. The silicon lens-antenna beam efficiency is expected above 80 % across the whole band with first-order sidelobe below -16 dB. To use the conservative horn antenna solution, a single- chamber waveguide DSB balanced mixer is developed. Two equal probe-type SIS chips are inserted into a full-height waveguide through its opposite broad walls; these two mixers are driven by the signal waveguide in series. The LO current is transferred to the mixers in parallel via a capacitive probe inserted through the narrow wall of the signal waveguide from the neighboring LO waveguide. The HFSS model demonstrated the LO power coupling efficiency above -3 dB, almost perfect signal transfer and the LO cross talk below - 30 dB that take into account misalignment (misbalance) of the chips. It is demonstrated numerically using Tucker's 3-port model that unequal pump of junctions of a twin-SIS mixer can lead, in spite of the perfect signal coupling, to degradation of the gain performance up to -3 dB, especially at the top of the ALMA Band-10. Index Terms-SIS mixer, balanced mixer, lens antenna. I. INTRODUCTION T he LO power required for a THz-band SIS mixer can be below 1 gW. However, the simple LO coupling circuit, which uses a thin-film beam splitter, wastes usually more than 90% of the available LO power into a termination load. The commercially available wide-band tunerless LO sources [I] can presently provide only few microwatts in the THz range. To reduce the LO power requirements, a balanced mixer is that possible solution. One may estimate the figure Manuscript received May 11, 2006. This work was supported in part by the ALMA-J Office, National Astronomical Observatory of Japan and by the Russian State Pro g rams "Support of Leadin g Scientific Schools in Russia" (grant 1548.2003.2) and Program for Basic Research "Problems of Radio Physics (Department of Physical Sciences, Russian Academy of Science) in direction "Applications in Terahertz Range". Sergey V. Shitov is with the National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan and with the Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow 125009, Russia (phone: +81-422-34-3879; fax: +81-422-34-3864; e-mail: s.shitov@nao.acjp ). Yoshinori Uzawa and Takashi Noguchi are with the National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan (e- mails: y.uzawa@nao.ac.jp , t.noguchi@nao.ac.jp ). Oleg V. Koryukin, Andrey V. Uvarov and Ilya A. Cohn are with the Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow 125009, Russia (e-mails: lco giçjntech.glire.jg uvaroyghitech.cplire.ru , cohn@hitech.c.plire.ru ). II. BALANCED MIXER DESIGN A . Quasioptical SIS mixer A few concepts of a wide-band balanced SIS mixer employing submicron-sized SIS junctions (A 0.5 sq.pm) are under development at NAOJ for ALMA Band-10. Considering a general THz-range SIS mixer, the quasioptical (QO) lens-antenna approach seems less laborious, that is confirmed experimentally [6]. Quasioptical chips are easier to process, and they can be handled with much less caution. It is worth to add here that high-quality epitaxial films of NbN [7], which is a promising option for a 90