An open structure SIS mixer for 350 GHz

Abstract

Today superconducting tunnel junctions are the best mixer elements in the 0.7 to 3 mm wavelength range. This open structure mixer, built on the basis of such a junction approaches the best waveguide systems (1). Its particular merrits are low cost, easy assembly, and convenient tuning by one single element. The mixer element is a series array of 3 PbBi junctions deposited on a fused-quartz substrate. The junctions are positioned in the middle of a planar dipole structure. In order to isolate the bias circuit and the IF amplifier band stop filters were deposited on both ends of the dipole. The usual perodic high-low impedance structure was used for these filters. A substrate holder presses the substrate with moderate force towards an elliptical immersion lens made from crystal quartz. This concept was pioneered by (2). Special features of our design are (i) the junction composition, providing a gap voltage of 10mV for the array, (ii) the use of a resonant dipole feed with choked bias connections, (iii) the imaging by one single aspheric lens of crystal quartz. The lens-junction assembly is mounted in a copper block which also carries the IF connector. A 3 mm screw in the substrate holder provides a movable backplane behind the dipole feed. On both sides of the mixer block superconducting coils with 6000 turns each provide the necessary magnetic field. The results below were measured at 100 mA corresponding to 110 Gauβ at the junction. At this flux density the Josephson effect is at its first minimum. Mixer, coils, and a low noise preamplifier are mounted in a ℓHe bath dewar which has a quiescent hold time of 50 hours. An uncoated 1.05 mm crystal quartz window on the 77 K radiation shield rejects 10 μm radiation and has its resonant transmission at 340 GHz. A 0.25 mm Mylar window, resonant at 350 GHz, seals the vacuum. The local oscillator, (4) is combined with the signal by a 35 μm Mylar foil transmitting nominally 96% of the signal and 4% of the ℒO power. Apart from the immersion lens in the mixer block there is no further lens in the signal path. The ℒO source is focused onto the mixer by a plastic lens between the beam combiner and the ℒO output. The receiver noise was measured for a number of frequencies (fig 1) using the Y-factor method where an absorber at 290 K and a wet absorber at 77 K stored in ℓN2 were brought into the signal path. The best value of 214 K at 342 GHz, measured with a bath temperature of 1.5K, coincides with the resonance of the IR-filter. The pumped and unpumped i/v curve together with the calibration curves for hot and cold load are given in fig.2. Receiver noise as a function of IF frequency (fig 3) was measured in 50 MHz bandwidth and coincides with the noise curve of the preamplifier. The angular diagram (fig 4) was measured using a chopped cold load with a 2 degree aperture moved in 1 degree steps. It meets the design goal of a clean, symmetrical f/10 beam and would match the Cassegrain beam of e.g. the 30 m IRAM telescope on Pico Veleta without additional optics.