ALMA Memo 498 14 May 2004 The ALMA Band 6 ( 211 - 275 GHz ) Sideband - Separating SIS Mixer - Preamplifier

The ALMA Band 6 (211-275 GHz) receivers use sideband-separating SIS mixer-preamplifiers with dual 4-12 GHz IF outputs. The sideband-separating mixers are of the phasing type, with the LO driving two component mixers in-phase and the RF signal connected to the mixers through a quadrature hybrid. The IF outputs of the mixers are amplified, then combined in a quadrature hybrid which separates the upper and lower sideband signals. The RF circuit components are all in a single split waveguide block — quadrature hybrid, LO power divider, LO couplers, cold image termination, and the two mixer chips. To achieve the wide IF bandwidth, a low-parasitic mixer is used and the preamps are bolted directly to the mixer block. INTRODUCTION The Atacama Large Millimeter Array* will have 64 antennas and will cover 35-960 GHz in ten bands using dual-polarized heterodyne receivers. In Band 6 (211-275 GHz), sideband-separating SIS mixers with dual 4-12 GHz IF outputs are used. Sideband separation (or rejection) in the front end is desirable for spectral line observations to reduce the contribution of atmospheric noise in the image sideband to the overall system noise. There are three ways to suppress the image response of a broadband mixer receiver: (i) A filter can be inserted in front of the mixer to terminate the mixer reactively at the image frequency. This is difficult in widely tunable receivers. (ii) A tunable four-port diplexer with a cold image termination can be used. This can be done quasioptically, e.g., using a Martin-Puplett interferometer, but has limited IF fractional bandwith, requires mechanical tuning, and is cumbersome at millimeter wavelengths. (iii) A sideband-separating mixer can be used, and this is the approach used in the present work. Different approaches to sideband separation are described in [1]. At the 1998 ISSTT, we described a single-chip Band 6 sideband-separating mixer [2], and in 2000 proposed a waveguide version of a similar circuit but with balanced mixers [3]. Other waveguide based sideband-separating SIS mixers have been described by Claude et al. [4], Belitsky et al. [5], and Chin et al. [6]. The configuration used in the present work is shown schematically in Fig. 1. Of particular importance in Fig. 1 is the resistor RIM on the fourth port of the RF quadrature hybrid. From the symmetry of the circuit, it is clear that this resistor is the image source for the sideband-separating mixer; USB thermal noise from this resistor is downconverted to the LSB IF output port, while LSB thermal noise appears at the USB IF output. MIXER CIRCUIT DESIGN Although a single-chip design with all the RF components on the same substrate may seem attractive, the large size of the chip compared with that of a simple elemental mixer results in a relatively small number of mixers per wafer. As ALMA requires well over 100 mixers for each band, we explored the feasibility of machining the RF components as waveguide circuits in a single E-plane split metal block which also contains two elemental mixer chips. The most difficult component to fabricate with acceptable gain and phase imbalance is the waveguide quadrature hybrid, but this