Ghassan Yassin

Abstract:

We present the design of an integrated dual-polarisation superconductor-insulator-superconductor (SIS) mixer operating at ALMA Band 10 frequency range. All the RF components, including the orthomode transducer (OMT), the quadrature hybrid and the SIS mixer circuits, are designed as superconducting planar circuits to form a single planar mixer chip. The mixer circuits will be fabricated using standard thin film deposition technology on an ultra-thin silicon-on-insulator (SoI) substrate. In this paper, we discuss in detail the design of the various RF components that make up the complete dual-polarisation SIS mixer chip, and provide the predicted performance using rigorous electromagnetic modelling. The optimised designs of these components are then integrated in a superconducting quantum mixer circuit modelling (SuperMix) environment for assessing the performance of the heterodyne parameters of the mixer such as mixer gain and noise temperature.

Abstract:

We present the design of a four-port planar circuit beam splitter comprising a microstrip and a coplanar waveguide (CPW) crossing each other. The CPW is fabricated in the ground plane (bottom layer) and the microstrip is deposited on top of the dielectric layer. A small section of the microstrip line is bent and aligned parallel to the central conductor of the bottom CPW, allowing the level of power coupling to be easily controlled by changing the length of the aligned section. The simple layout of the planar beam splitter makes it easy to fabricate in a wide frequency range from microwave to submillimetre (sub-mm) wavelengths. In this paper, we describe in details the electromagnetic design of the planar beam splitter and its predicted performances in the frequency range of 600– 700 GHz. We discuss the potential usage of the planar beam splitter as a replacement to the free-space beam splitter in receivers, in particular those using superconductor-insulatorsuperconductor (SIS) mixer arrays. To investigate the integrity of our design in a controlled way we scaled the design to operate in the Ku-band and measured the performance of several prototypes experimentally. Our tests showed good agreement between the measured performance and simulations.

Abstract:

In this paper, we compare simulations to experimental results for a new SIS frequency multiplier. To simulate these devices, we developed software based on spectral-domain analysis, which is ideal for simulating higherorder harmonics such as those present in a multiplier. In addition, we included the embedding circuit and interpolated the experimental I-V curve to allow the simulation to capture the experimental system as closely as possible. For the experimental data, results were taken from a new SIS frequency multiplier that has recently been developed at the Chalmers University of Technology. Previously, these experimental results were compared to simulations based on Tucker theory. Here, we compare these results to spectraldomain simulations. Qualitatively, the inclusion of embedding impedances and the use of spectral-domain analysis improves the agreement between simulation and experiment. The software can now be used to design multipliers with high output power and high conversion efficiency.

Abstract:

We have developed anSIS receiver with a wide intermediate-frequency (IF) bandwidth.This is important for reducing image integration time and simultaneously measuring multiple spectral lines. The receiver is a finline mixer-based design, which allows for ultra-wide radio-frequency (RF) bandwidth and has lower mechanical requirements compared to radial stub designs. Simulations of this receiver showed quantum limited noise in the RF frequency range of 140 to 260 GHzand from DC to 10GHz in the IF spectrum.We measured the noise temperature by comparing the receiver's response to hot and cold loads. The best noise temperature was 37.9 K at 231.0 GHz, and all of the results were below 100 K from 213 to 257 GHz (the bandwidth of our local-oscillator). We measured the IF bandwidth using a spectrum analyser, and found good results from around 3-10 GHz. The lower frequency was restricted by our IF amplifier's bandwidth but the higher frequency limit was lower than we expected from simulations. We believe that this discrepancywas due to the inductance of the bondwires that we used to connect the mixer chip to the IF board. We are currently investigating techniques to reduce and compensate for this inductance.

Abstract:

We have designed and fabricated a feed horn array to operate between 1300 - 1500 GHz. It consists of four smooth walled, two section feeds with a single flare angle discontinuity near the throat of each horn. The design was carried out using a modelling package that employs modal matching together with genetic algorithm and downhill simplex optimisation techniques. This determines the optimum horn dimensions given a set of stringent requirements on beam circularity and cross polarisation levels. The simulated far-field radiation patterns exhibit very good beam circularity, low sidelobes and low cross polarisation levels across the frequency range of operation. The array has been fabricated out of brass by the workshop of Oxford University using a direct machining technique that employs drill tools shaped to the profile of the designed horn. Far-field beam pattern measurements presented in this paper were carried out at the Harvard-Smithsonian Center for Astrophysics and suggest good agreement with the theoretical predictions.