As the Atacama Large Millimeter/submillimeter Array (ALMA) nears completion, 73 dual-polarization receivers have been delivered for each of Bands 3 (84-116 GHz) and 6 (211-275 GHz). The receivers use sideband-separating superconducting Nb/Al-AlOx/Nb tunnel-junction (SIS) mixers, developed for ALMA to suppress atmospheric noise in the image band. The mixers were designed taking into account dynamic range, input return loss, and signal-to-image conversion (which can be significant in SIS mixers). Typical SSB receiver noise temperatures in Bands 3 and 6 are 30 and 60 K, respectively, and the image rejection is typically 15 dB.
We have developed a 230 and 345 GHz very-longbaseline interferometry (VLBI) receiver for the South Pole Telescope (SPT). With the receiver installed, the SPT has joined the global Event Horizon Telescope (EHT) array. The receiver optics select the 230 or 345 GHz mixers by rotating the tertiary mirror around the optical axis, directing the chief ray from the secondary mirror to the feed horn of the selected frequency band. The tertiary is installed on top of the receiver cryostat, which contains both mixer assemblies. The feed horns are placed symmetrically across the centerline of the telescope optics and tilted inward by 5.7 degrees from the vertical plane so that their beams intersect at the chief ray intersection on the tertiary mirror. We have performed vector beam measurements of the SPT VLBI receiver in both frequency bands. The measurements preserved the relative location of the beams, to establish the relative locations of the phase centers of the two horns. Measurements in two parallel reference planes above the cryostat were used to suppress reflected light. To model the beam, we derive a general expression of the electric field vector on the measurement plane for a tilted beam and infer the feed horn position parameters for both frequency bands by fitting models to data with a Markov chain Monte Carlo (MCMC) method. The inferred parameters such as the tilt angle of the feed horn are in good agreement with the design. We present the measurement setup, amplitude and phase pattern of the beam, and the fitting result here.We now have the normalized electric field distribution for the tilted beam on the scanning plane
Initial results are presented for a 200-300 GHz SIS mixer/preamplifier with an IF bandwidth of 8 GHz. The mixer uses Nb/Al-oxide/Nb tunnel junctions in a circuit with low IF capacitance and inductance. The mixer block mounts directly on the body of a three-stage 4-12 GHz preamplifier which uses discrete InP HFET devices. Mixer bias is provided through the input circuit of the preamplifier. At a LO frequency of 230 GHz, the measured mixer-preamp gain is 30-35 dB, and the DSB receiver noise temperature is 45-57 K across the whole IF band. The preamp alone has 40 dB of gain, and dissipates 7.7 mW. With four amplifiers required in each ALMA cartridge, there is concern that the cooling capacity currently planned for ALMA receivers will be marginal. This mixer/preamplifier has demonstrated that an IF bandwidth of 8 GHz is indeed achievable for ALMA Band 6.
Fig. 1. Phasing type of sideband-separating mixer as used in the ALMA Band-3 and -6 SIS receivers. Fig. 2. For a typical Band-6 SIS receiver, contours of (a) mixer gain dB (solid orange lines) and (b) receiver noise temperature (solid blue lines), plotted on a Smith chart of the RF source impedance normalized to the optimum source impedance R opt . The gray contours are the IF output resistance of the mixer normalized to R opt : solid gray lines indicate positive output resistance and dashed gray lines indicate negative output resistance. The squiggly gray line indicates the transition from positive to negative output resistance. The dashed gray circle at |ȡ| = 0.4 indicates the desirable operating region.Abstract -As ALMA nears completion, 73 dual-polarization receivers have been delivered for each of Bands 3 (84-116 GHz) and 6 (211-275 GHz). The receivers use sideband-separating superconducting tunnel-junction (SIS) mixers, developed for ALMA to suppress atmospheric noise in the image band. The mixers were designed taking into account input return loss, dynamic range and signal-to-image conversion (which can degrade the image rejection). Results are given for typical production receivers.
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