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.
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.
We present the design of a broadband superconductor-insulator-superconductor (SIS) mixer operating near 700 GHz. A key feature of our design is the utilisation of a new type of waveguide to planar circuit transition comprising a unilateral finline taper. This transition is markedly easier to design, simulate and fabricate than the antipodal finline we employed previously. The finline taper and the superconducting circuitry are deposited on a 15 μm thick silicon substrate. The employment of the very thin substrate, achieved using Silicon-On-Insulator (SOI) technology, makes it easy to match the incoming signal to the loaded waveguide. The lightweight mixer chip is held in the E-plane of the waveguide using gold beam leads, avoiding the need for deep grooves in the waveguide wall. This new design yields a significantly shorter chip, free of serrations and a wider RF bandwidth. Since tuning and all other circuits are integrated on the mixer chip, the mixer block is extremely simple, comprising a feed horn and a waveguide section without any complicated mechanical features. We employ a new type of smooth-walled horn which exhibits excellent beam circularity and low cross polarisation, comparable to the conventional corrugated horn, and yet is easier to fabricate. The horn is machined by standard milling with a drill tool shaped into the horn profile. In this paper, we describe the detailed design of the mixer chip including electromagnetic simulations, and the mixer performance obtained with SuperMix simulations. We also present the preliminary measurements of the smooth-walled horn radiation patterns near the mixer operating frequencies.
Superconductor-Insulator-Superconductor (SIS) tunnel junction mixers are now commonly used in as- tronomical receivers at (sub)millimeter wavelengths because of their superb sensitivity, high dynamic range and stability of operation. Niobium SIS mixers operating at frequencies well below the super- conducting gap (∼680 GHz) have already achieved quantum limited sensitivity. Therefore to further enhance the receiver sensitivity, increasing the Intermediate Frequency (IF) bandwidth of SIS mixers has became crucial. This thesis focuses on the theoretical modeling, design and experimental verifi- cation of Nb SIS mixers operating around 230 GHz with a wide IF bandwidth of 1–15 GHz. These mixers were designed for a single baseline heterodyne interferometer (GUBBINS), which is being built to observe the Sunyaev-Zel’dovich effect in the Cosmic Microwave Background. The combination of wide IF bandwidth SIS mixers and complex analogue correlators will allow GUBBINS to feature high surface brightness sensitivity, that helps to distinguish the weak SZ effect from the background noise. The SIS mixer detector system was assembled inside the GUBBINS cryostat together with the IF electronics and RF/LO optical systems. Low noise temperatures of around 71 K were then measured in the GUBBINS system. The Nb SIS mixer we have developed uses a unilateral finline and fully integrated planar circuits deposited on a silicon substrate, to couple the electromagnetic radiation from the waveguide into the SIS junction. The finline mixer allows a broad-band RF coupling, an easy integration of the on-chip planar circuits and an easy-to-fabricate mixer block. To achieve a wide IF bandwidth, the output impedance of the SIS mixer was well matched to the input impedance of the amplifier by a multi-stage microstrip circuit. Additionally, the planar circuit of the SIS mixer was also designed to have a small lumped inductance and capacitance. The SIS mixer chip was extensively simulated by rigorous electromagnetic software (HFSS) and the S-parameter was exported to a quantum mixing package SuperMix to produce a full-wave model of the mixer. Experimental testing yielded a best noise temperature of 50 K with an average noise temperature of 75 K over an RF bandwidth of 160 GHz– 260 GHz. We have performed thorough experimental and computational investigation of the IF system in particular the constraints on the bandwidth caused by the lumped element capacitance of the mixer chip and the matching of the output impedance of the mixer to the IF amplifier. Our conclusion is that a bandwidth of 1–15 GHz could be achieved using our mixer design, subject to the performance of the amplifier. Finally, a variable temperature load system was successfully developed and tested inside the cryostat, to avoid the losses from the room-temperature optics. We have showed that the noise temperature of the SIS detector could be reduced by as much as 15 K by testing the mixer using a variable temperature load inside the cryostat.
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.
In this paper, we describe the design and operation of a 350-GHz finline superconductor-insulator-superconductor mixer. The mixer is fed by a horn-reflector antenna, and the superconducting circuit is fabricated using planar-circuit technology and fully integrated tuning. An important feature of the mixer is that it 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 (/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 procedure in some detail. We pay particular attention to the synthesis of the finline taper and the electromagnetic design of the horn-reflector antenna. We have tested a finline mixer over the frequency range of 330-370 GHz and measured a receiver noise temperature of 90 K, which remained unchanged over the whole frequency range. Our investigation has demonstrated that it is possible to make superconducting finline mixers for frequencies as high as 350 GHz.
The successful operation of a 650 GHz unilateral finline superconductor–insulator–superconductor mixer with a wide IF bandwidth is reported. The mixer was fabricated using silicon‐on‐insulator (SOI) technology, with planar circuit on‐chip integration. The mixer was tested over the frequency range of 630–702 GHz, and the best recorded DSB noise temperature was 226 K. The measured RF and IF behaviour of the mixer agreed very well with the simulation predictions. This demonstrates that superconducting finline mixers fabricated on an SOI substrate can achieve state‐of‐the‐art performance, and yet are much more compact and highly reproducible, and hence suitable for large‐format arrays in astronomy instruments, at THz frequencies.
Fig. 1. (Top) A 700 GHz finline mixer chip. The finline taper runs from the left to center of the chip, and the mixer tuning circuit, RF block and bond pads are on the right (Bottom) Cross-sections through the finline taper, showing the metallization and the E-field lines. The finline taper gradually transforms the waveguide mode at A through unilateral finline (B) to antipodal finline (C), before transitioning from antipodal finline to microstrip (D-F). Abstract—SIS mixers using finline waveguide to microstrip transitions show a number of advantageous features, particularly in providing a broadband, fixed impedance feed to the SIS mixer tuning circuit. The large chip area and insensitivity to mixer block machining tolerances simplifies the assembly of the mixer, as well as allowing more advanced mixer designs to be fabricated on a single chip. Finline SIS mixers have shown good performance in the 230 and 350 GHz bands[1][2], and we have also previously reported results from two finline SIS mixers in the 600-720 GHz band[3][4], obtaining best receiver noise temperatures of ~250 K. In this paper we present results from several finline mixers in the 600-720 GHz band, using both variants of our original 3-stage Chebyshev filter tuned single junction mixers and new Belitsky tuned mixers. These dual junction tuned mixers show significantly better performance than our previously reported results, with receiver noise temperatures below 200 K. We present detailed SuperMix simulations of the receiver performance and compare these with measured results. We find good agreement between simulations and measured performance can be achieved by introducing a small amount of attenuation in the RF circuit of the mixer. We also present simulations of the IF bandwidth of finline mixers, and suggest a method by which this can be greatly improved.