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Abstract:

We present a new software package, called QMix, for simulating the behavior of superconductor/insulator/ superconductor (SIS) mixers. The software uses a harmonic balance procedure to calculate the ac voltage across the SIS junction and multitone spectral domain analysis to calculate the quasiparticle tunneling currents. This approach has two major advantages over other simulation techniques: 1) it can include an arbitrary number of higher order harmonics, and 2) it can simulate the effect of multiple strong nonharmonic frequencies. This allows the QMix software to simulate a wide range of SIS mixer operation, including the effects of harmonics in the local-oscillator (LO) signal and gain saturation with respect to the RF signal power. In this article, we compare the simulated results from QMix to the measured performance of a 230-GHz SIS device, both to validate the software and to investigate the experimental data. To begin, we simulated the conversion gain of the mixer and we found excellent agreement with the experimental results. We were also able to recreate the broken photon step effect by adding higher and lower order harmonics to the LO signal. We then simulated a range of incident RF signal powers in order to calculate the gain saturation point. This is an important metric for SIS mixers because we require linear gain for reliable measurements and calibration. In the simulated results, we found both gain compression and gain expansion, which is consistent with other studies. Overall, these examples demonstrate that QMix is a powerful software package that will allow researchers to simulate the performance of SIS mixers, investigate experimental results, and optimize mixer operation. We have made all of the QMix software open-source and we invite others to contribute to the project.

Abstract:

DC blocks are used frequently in planar circuits to enable separate DC voltage/current biasing of active components inserted along the transmission lines. In this Letter, we present a slotline DC block design where the conductors of the transmission line can be physically broken, while allowing the propagation of the RF signal across the discontinuity with negligible insertion loss. The DC block comprises two break-lines with narrow gaps, patterned on the two ground planes of a slotline with each breakline connected to an RF choke. The RF chokes present open circuit nodes that prevent the RF power from leaking into the break-lines gaps. We have fabricated and tested the DC block, and demonstrated that the measured performance agrees very well with simulated results. The insertion loss was close to – 0.5 dB in the designated range of 12–16 GHz, demonstrating that the RF leakage through the DC block is indeed negligible.

Abstract:

We present a planar Yagi antenna that has an end-fire beam characteristic and broad operational bandwidth. Two dipoles with different lengths are cascaded together to broaden the bandwidth. Printed directors and truncated ground plane are employed to achieve high front-to-back gain ratio. The double-dipole antenna is fed directly with a microstrip, and hence is readily adaptable to most of the millimetre and sub-millimetre detector circuits. In this paper, we present the antenna design in the context of designing a 1.05 THz Superconductor-Insulator-Superconductor (SIS) mixer that is suitable for Atacama Large Millimetre/Sub-millimetre Array (ALMA) Band 11 receiver. Detailed analysis of the antenna design is discussed, and simulated far field beam patterns and bandwidth performance is presented.

Abstract:

We present a new software package for simulating the performance of Superconductor / Insulator / Superconductor (SIS) mixers. The package is called QMix (“Quasiparticle Mixing”) and it uses multi-tone spectral domain analysis (MTSDA) to calculate the quasiparticle tunneling current through the SIS junction. This technique is very powerful and it allows QMix to simulate multiple strong tones and multiple higher-order harmonics. We have compared this software to the experimental data from a 230 GHz SIS mixer, both to validate the software and to explore the measured results. Overall, we found very good agreement, demonstrating that QMix can accurately simulate the performance of SIS mixers. We believe that QMix will be a useful tool for analyzing experimental data, designing new SIS mixers, and simulating new applications for SIS junctions, such as frequency multiplication.