Paul K. Grimes

Compact broadband planar orthomode transducer

The design and test results of a compact C-band orthomode transducer are presented. The transducer comprises four rectangular probes orthogonally arranged in a circular waveguide, designed to work in the WG13 band. Measurements of the system in the frequency range 4.64–7.05 GHz agree very well with simulation results and show a cross-polarisation level below −58 dB, a return loss of about −20 dB, and an insertion loss difference of less than 0.18 dB between the orthogonal polarisation modes across the full waveguide band.

Greenland telescope project: Direct confirmation of black hole with sub‐millimeter VLBI

A 12 m diameter radio telescope will be deployed to the Summit Station in Greenland to provide direct confirmation of a Super Massive Black Hole (SMBH) by observing its shadow image in the active galaxy M87. The telescope (Greenland Telescope: GLT) is to become one of the Very Long Baseline Interferometry (VLBI) stations at sub-millimeter (submm) regime, providing the longest baseline >9000 km to achieve an exceptional angular resolution of 20 µas at 350 GHz, which will enable us to resolve the shadow size of ~40 µas. The triangle with the longest baselines formed by the GLT, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the Submillimeter Array (SMA) in Hawaii will play a key role for the M87 observations. We have been working on the image simulations based on realistic conditions for a better understanding of the possible observed images. In parallel, retrofitting of the telescope and the site developments are in progress. Based on 3 years of opacity monitoring at 225 GHz, our measurements indicate that the site is excellent for submm observations, comparable to the ALMA site. The GLT is also expected to make single-dish observations up to 1.5 THz.

A 700-GHz SIS antipodal finline mixer fed by a Pickett-Potter horn-reflector antenna

We report the successful operation of a superconductor-insulator-superconductor (SIS) finline mixer operating near the superconducting energy gap of Nb. The mixer employs a new type of Pickett-Potter horn-reflector (PPHR) antenna, which exhibits low sidelobes and low cross-polarization levels, and yet is easy to fabricate. The SIS tunnel junction and all of the integrated superconducting tuning circuits are fabricated from Nb using planar-circuit technology. The mixer employs an antipodal finline section, deposited on one side of a quartz substrate, which transforms the high impedance of the waveguide (/spl ap/300 /spl Omega/) to the low impedance of the microstrip line (/spl ap/20 /spl Omega/). The Nb/Al-oxide/Nb tunnel junction is fabricated at the same time as the finline circuit. In this paper, we describe the design and testing of the mixer, and pay particular attention to the electromagnetic design of the PPHR antenna. We investigate the noise temperature and gain of the mixer over 642-714 GHz, and analyze the experimental results using rigorous theories that were developed specifically for the purpose. Our investigation demonstrates that finline mixers have good performance both below and above the superconducting energy gap.

Preliminary Measurement Results of a 650 GHz Planar Circuit Balanced SIS Mixer

We present the design and preliminary experimental results of a broadband 650 GHz balanced SIS mixer utilizing two back-to-back unilateral finline tapers. The new design employs fully integrated circuit components onto a 15 μm silicon-on-insulator (SOI) substrate, hence greatly simplifying the balanced mixer block requirements. Independent testing of the on-chip RF quadrature hybrid and the DC/IF blocks showed performances consistence with the computational predictions. The fabricated mixer chip tests have also demonstrated successful operation in balanced mixing mode as the LO noise was separated from the IF output. Preliminary tests of the devices sensitivity gave a best double sideband (DSB) noise temperature of 714 K measured at 635 GHz, and remained at a similar level throughout the operating frequency band. The higher than expected noise temperature was mainly caused by a significant shift in the junction position during fabrication, in both the longitudinal and lateral directions, resulting in substantial shift in the tuning frequency and degradation in the coupling of power to the junction.

Design and Performance of a 3-Junction Series Distributed SIS Mixer for Wide IF Applications

Series distributed superconductor-insulator-superconductor mixers have been used to provide very wide instantaneous bandwidth of operation. In this paper, we explore a design based on a 3-junction array in which the junctions themselves form part of the RF tuning network of the mixer. We have developed mixers based on these designs for use in the 200 and 300 GHz bands. Receivers incorporating these wideband mixers demonstrate very low noise temperature. At a local oscillator frequency of 300 GHz, the lowest measured intermediate frequency noise temperature is about 42 K and the usable intermediate frequency bandwidth extends from 6 to 16.5 GHz. Both the design theory and experimental results are presented.

A 650 GHz Unilateral Finline SIS Mixer Fed by a Multiple Flare-Angle Smooth-Walled Horn

We report the design and successful operation of an superconductor-insulator-superconductor (SIS) mixer operating near the superconducting gap of niobium. A key feature of this design is the employment of a unilateral finline taper to transform the waveguide modes to microstrip signals. This transition is easy to design since it can be rigorously modeled, and also easy to fabricate being a single-layer structure. We will show that unilateral finline mixers have important advantages at THz frequencies since they exhibit wideband operation at both radio frequency (RF) and intermediate frequency (IF), allow elegant on-chip integration of the mixer circuits and result in an extremely simple mixer block that does not require a backshort or any mechanical tuners. The mixer we describe below is fed by a multiple flare-angle smooth-walled horn which exhibits beam pattern characteristic comparable to the conventional corrugated horn and yet is much easier to fabricate. In this paper, we shall present a brief discussion of the testing of the multiple flare-angle horn and detailed description of the design and testing of the mixer, covering ~100 GHz bandwidth centered at 650 GHz. In particular, we will present full electromagnetic de sign description of the mixer chip including the superconducting effects, and the heterodyne properties of the mixer using quantum mixing theory. Mixer performance tests that we carried out from 595 to 702 GHz gave a best receiver noise temperature of 145 K at 600 GHz, corrected for a 75 μm beam splitter. Finally, we performed a thorough analysis of the mixer performance, comparing the experimental results with theoretical models. Our investigation demonstrated that unilateral finline mixers fed by a multiple flare-angle horn can yield performance comparable to conventional designs, hence are suitable for large format mixer array at THz frequencies.

Wideband SIS Receivers Using Series Distributed SIS Junction Array

We have developed an SIS mixer based on a 3-junction series distributed array. In this mixer layout, the three junctions are connected together by passive network elements, such that each junction is subjected to different local oscillator (LO) drive, and slightly different DC bias voltages. This design helps to reduce the output capacitance of the mixer at the intermediate frequency (IF) so as to achieve a wider IF bandwidth, while maintaining adequate RF bandwidth. The receiver performance of this type of mixer was evaluated in the 220 GHz band. The lowest noise temperature measured was ~30 K, and the noise temperature remains below 50 K over the IF range from 3 to 11 GHz. Receivers incorporating this new mixer design are currently in routine operation at the Submillimeter Array.

Wideband submillimeter receivers based on series distributed SIS junctions

The IF bandwidth of an SIS mixer is limited by its capacitance. A series distributed design reduces the effective junction capacitance and the junctions can be reused as low impedance matching elements, limiting the capacitance of the RF matching network. In this paper, the analysis of this novel design is explored. Two receivers incorporating two types of series distributed SIS mixers were tested at 220 and 350 GHz. IF bandwidths of up to 16 GHz and low noise performance have been demonstrated.

Performance of microstrip-coupled TES bolometers with finline transitions

We have fabricated TES bolometers with finline transitions for the CℓOVER project. We have measured the optical response of CℓOVERs first prototype 97-GHz detectors and find that they have a detection efficiency close to 100%. We have also investigated the effects of misalignment of the finline in the waveguide and of thinning the substrate. The prototype detectors have dark NEPs as low as 1.5 x 10-17W/√Hz and satisfy the requirement of photon-noise limited operation on CℓOVER. We describe the optical tests of CℓOVERs prototype 97-GHz detectors and discuss their implications for the design of the science-grade detectors.

Waveguide-to-planar circuit transition for millimetre-wave detectors

A novel design of a waveguide to microstrip or coplanar waveguide transition using a unilateral finline taper is presented. The transition from the unilateral finline mode to the TEM microstrip mode is done directly, avoiding the antipodal finline tapers that have commonly been employed. This results in significant simplification of the design and fabrication, and shortening of the chip length, thereby reducing insertion loss. Also presented are designs at 90 GHz that can be employed in superconducting tunnel junction mixers or transition edge sensor bolometers, and scale-model measurements at 15 GHz.