Context. We describe the new SEPIA (Swedish-ESO PI Instrument for APEX) receiver, which was designed and built by GARD OSO in collaboration with ESO. It was installed and commissioned at the APEX telescope during 2015 with an ALMA Band 5 receiver channel and updated with a new frequency channel (ALMA Band 9) in February 2016. Aims. This manuscript provides a reference for observers who use the SEPIA receiver in terms of the hardware description, optics and performance as well as the commissioning results. Methods. Out of three available receiver cartridge positions in SEPIA, the two current frequency channels, corresponding to ALMA Band 5, the RF band 158-211 GHz, and Band 9, the RF band 600-722 GHz, provide state-of-the-art dual polarization receivers. The Band 5 frequency channel uses 2SB SIS mixers with an average SSB noise temperature around 45 K with IF (intermediate frequency) band 4-8 GHz for each sideband providing total 4 × 4 GHz IF band. The Band 9 frequency channel uses DSB SIS mixers with a noise temperature of 75-125 K with IF band 4-12 GHz for each polarization.Results. Both current SEPIA receiver channels are available to all APEX observers.

We describe the design, performance, and commissioning results for the new ALMA Band 5 receiver channel, 163–211 GHz, which is in the final stage of full deployment and expected to be available for observations in 2018. This manuscript provides the description of the new ALMA Band 5 receiver cartridge and serves as a reference for observers using the ALMA Band 5 receiver for observations. At the time of writing this paper, the ALMA Band 5 Production Consortium consisting of NOVA Instrumentation group, based in Groningen, NL, and GARD in Sweden have produced and delivered to ALMA Observatory over 60 receiver cartridges. All 60 cartridges fulfil the new more stringent specifications for Band 5 and demonstrate excellent noise temperatures, typically below 45 K single sideband (SSB) at 4 K detector physical temperature and below 35 K SSB at 3.5 K (typical for operation at the ALMA Frontend), providing the average sideband rejection better than 15 dB, and the integrated cross-polarization level better than –25 dB. The 70 warm cartridge assemblies, hosting Band 5 local oscillator and DC bias electronics, have been produced and delivered to ALMA by NRAO. The commissioning results confirm the excellent performance of the receivers.

We present a new design approach for the 90° directional coupler with very low amplitude imbalance. The primary feature of this quadrature coupler is the introduction of a controllable ripple in the operational band for achieving a better overall amplitude balance. This design concept is demonstrated through a 90° branch-line hybrid for the 4 -7.9 GHz band (65% fractional bandwidth) using microstrip transmission lines. Our simulations indicate that the amplitude imbalance of the designed hybrid is better than 0.3 dB over the most of the 4 -7.9 GHz band with a phase imbalance better than ±8.5°. Experimental verification of the hybrid shows excellent agreement with simulations.

Superconductor-Insulator-Superconductor (SIS) is the key component for millimeter and submillimeter mixers for radio astronomy and environmental science. The capacitance of the SIS mixer determines both the RF and IF performances. Previously, the measurements of this capacitance have been followed with high uncertainty levels. Herein, we present the characterization of the SIS junction capacitance at cryogenic temperature (~4 K) by direct measurement of the SIS junction impedance at microwave frequencies allowing accurate characterization of the SIS junction capacitance. The proposed calibration method uses only one short-circuit reference. The SIS junction capacitance measurement is realized by biasing the junction at the different parts of its current-voltage characteristic, thus eliminating a separate measurement of shortcircuit standard. In order to verify the acquired measurement results, thin-film capacitors with known capacitance were also characterized. The capacitances of four SIS junctions with various areas were measured. The absolute uncertainty of the proposed measurement method was found to vary from 5 to 6.8 % amongst different junction areas.

The junction specific capacitance (Cs) is an essential parameter in designing tuning circuitry for Superconductor-Insulator-Superconductor (SIS) mixers. However, our knowledge of the junction capacitance only relies on the few available empirically obtained Cs vs. specific normal resistance (RnA) relations, which are inconsistent especially at low RnA values, RnA < 40 Ω.µm 2. In this paper, we report the Nb/Al-AlOx/Nb SIS junction capacitance data from our recently presented direct microwave (4 GHz) measurements at 4K for junctions with various RnA values ranging from 8.8 to 68 Ω.µm 2. New insight is provided into the extraction of the true geometrical specific capacitance of SIS junctions. We show that, even at such low microwave frequencies, the so-far-neglected nonlinear susceptance is significant, especially for junctions with low RnA values. This susceptance originates from the real part of the response function, IKK, which can be calculated through the Kramers-Kronig transform of the DC tunnel current. The new specific capacitance, which accounts for this contribution is presented as a function of RnA. We provide an improved and more accurate Cs (RnA) relation, which can be a reliable and useful tool for circuit designers. The obtained Cs (RnA) relation is compared with those available in the literature, and the possible reasons giving rise to the disparity among these relations are discussed. By comparing the modelled and the measured noise temperature of the APEX SHeFI band 3 (385-500 GHz) DSB mixer, we show that the new Cs (RnA) relation offers a great potential for improving the performance of SIS mixers.