Superconductor-insulator-superconductor Mixer Research Articles

SIS junction reactance complete compensation

Superconductor-insulator-superconductor (SIS) junction geometrical capacitance with out of phase current I/sub kk/ impedance component forms sufficient junction reactance X/sub SIS/=( omega C+B/sub Q/)/sup -1/. It is suggested that the way to resonate out both omega C and B/sub Q/ is by using an additional identical SIS junction connected to the first through a long line impedance inverter and RF+DC biased symmetrically to the first. Pumped I-V curves without quantum reactance and frequency impedance patterns of the system are calculated. Calculations demonstrated the presence of high and even negative induced dynamic resistance regions at high-order quasiparticle steps for the case of SIS junction reactance complete compensation. The suggested method may be used in SIS mixers and detectors for a better RF matching.

Characterization of NbN films and tunnel junctions

Properties of NbN films and NbN/MgO/NbN tunnel junctions are discussed. NbN junctions are being developed for use in high-frequency, superconductor-insulator-superconductor (SIS) quasiparticle mixers. To properly design mixer circuits, junction and film properties need to be characterized. The specific capacitance of NbN/MgO/NbN junctions has been measured as a function of the product of the normal-state resistance and the junction area (R/sub n/A), and it is found to vary by more than a factor of two (35-85 fF/ mu m/sup 2/) over the range of R/sub n/A measured (1000-50 Omega mu m/sup 2/). This variation is important because the specific capacitance determines the RC speed of the tunnel junction at a given R/sub n/A value. The magnetic penetration depth of NbN films deposited under different conditions is also measured. The magnetic penetration depth affects the design of microstrip line used in RF tuning circuits. Control of the magnetic penetration depth is necessary to fabricate reproducible tuning circuits. Additionally, the critical current uniformity for arrays of 100 junctions has been measured. Junction uniformity will affect the design of focal-plane arrays of SIS mixers. Finally, the relevance of these measurements to the design of Josephson electronics is discussed.

A full-band waveguide SIS receiver with integrated tuning for 75-110 GHz

The performance of a broadband SIS (superconductor-insulator-superconductor) receiver with no mechanical tuning elements was modeled and tested. The mixer mount had a broadband-waveguide-to-microstripline transition consisting of a four-step Chebyshev single-ridge transformer. The last step of the ridge connected the waveguide to a microstripline circuit. The on-chip circuit consisted of a microstripline that transmitted the RF and the local oscillator signals to the SIS mixer. Both the SIS element and the inductor had RF grounds provided by 90 degrees radial stubs. The inductor tuned out the junction capacitance to allow operation over the full frequency band. The SIS element can be a single junction or series array using Nb-AlO/sub x/-Nb trilayer tunnel junctions with areas as small as 0.5 mu m/sup 2/ and V/sub m/(2 mV)=39 mV. Preliminary results indicated a DSB receiver noise temperature of 65-80 K across the band measured at 4.4 K with an internal cryogenic RF hot/cold source and with T/sub if/=21 K. With another device, a mixer noise temperature of 35 K at 100 GHz was achieved, increasing to 45 K at 79.5 and 110 GHz. Coupled mixer gain of up to +3 dB and negative dynamic resistance on the first photon step were observed. The lowest noise temperature was obtained for an untuned single-junction mixer at 80 GHz: T/sub R/=41 K and T/sub M/=20 K were measured.

100 GHz SIS mixer with improved RF matching

A novel superconductor-insulator-superconductor (SIS) mixer coupling scheme with an integrated millimeter-wave transformer has been designed and tested. The impedance transformation allows larger junction sizes and easier fabrication than is possible with single junction or array mixers. Experiments with the front-end transformer mixers demonstrated mixer gains up to 10 dB and instantaneous bandwidths of 700 MHz at W-band. In addition to heterodyne mixing, direct detection measurements were made to measure instantaneous signal bandwidth. Current responsivities of 1000 A/W were measured with a 3-dB bandwidth of 8 GHz. The direct detection performance falls to within experimental uncertainty at the quantum limit of one-electron-per-photon.

Nb edge junction process for submillimeter wave SIS mixers

The authors describe a junction fabrication process that produces high-quality Nb edge junctions of areas less than 0.2 mu m/sup 2/ on thin quartz (0.10 nm) substrates. This process utilizes a 10:1 projection wafer stepper for high-resolution and high-accuracy layer-to-layer registration. Large numbers of junctions were fabricated reliably with high-quality I-V (current-voltage) characteristics and with impedances suitable for use in SIS (superconductor-insulator-superconductor) mixers for submillimeter astronomy. Junctions produced by this process can be stored on the shelf for more than two years with no special precautions taken, and with an impedance change of less than 10%. No failures have been observed after thermal cycling. Currently these junctions are being used in an astronomical receiver that demonstrates state-of-the-art performance in the atmospheric window centered at 345 GHz. With some improvements to the process, it is possible to make junctions with areas sufficiently small for omega RC approximately=3 at 800 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Beating the quantum limit in SIS mixers

The 2LO (double local oscillator) mixer is analyzed in the low-LO-power limit for a perfect SIS (superconductor-insulator-superconductor) diode. It is shown that a 2LO SIS mixer can beat the noise performance of a one-LO mixer by a factor of two. It is suggested that even lower noise is possible with an SIS when operated in some other way.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

MM wave quasioptical SIS mixers

The authors have tested the performance of planar SIS (superconductor-insulator-superconductor) mixers with log-periodic antennas at near millimeter and submillimeter wave frequencies from 90 to 360 GHz. The large omega R/sub N/C product ( approximately 10 at 90 GHz) of the Nb/NbO/sub x//Pb-In-Au junctions requires an integrated inductive tuning element to resonate the junction capacitance at the operating frequencies. Two types of integrated tuning element were used which were designed with the aid of measurements using a Fourier transform spectrometer. Preliminary results indicate that the tuning elements can give very good mixer performance up to at least 200 GHz. An inductive wire in parallel with a 5-junction array gives a minimum mixer noise temperature of 115 K (DSB) at 90 GHz with a FWHM (full width at half maximum) bandwidth of 8 GHz. An open-ended microstrip stub in parallel with a single junction gives minimum mixer noise temperatures at 150 and 200 K (DSB) near 90 and 180 GHz with FWHM bandwidths of 4 and 3 GHz, respectively. The relatively high mixer noise temperatures compared to those of waveguide SIS mixers in a similar frequency range are attributed mainly to the losses in the optical system. >

RF-sputter-deposited magnesium oxide films as high-quality adjustable tunnel barriers

High-quality RF-sputtered MgO films as tunnel barriers to fabricate small-area nitride tunnel junctions. A magnetism oxide barrier deposited as a single layer or as a multilayer film results in devices with similar characteristics. Annealing trilayers at temperatures in excess of 250 degrees C for several hours decreases junction current density and improves device quality, presumably by increasing barrier heights through reducing resonant tunneling states. A self-aligned process utilizing only two mask levels is used to produce junctions as small as 0.5 mu m/sup 2/ with excellent critical current uniformity. These junctions exhibit energy gaps of 5.1 mV and low subgap currents at current densities in excess of 1000 A/cm/sup 2/, which make them suitable for a variety of applications such as SIS (superconductor-insulator-superconductor) mixers and logic circuits.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Numerical computation of two-LO SIS mixer gain and noise

The authors present numerical calculations of the gain and noise of a two-LO (local oscillator) mixer based on an ideal SIS (superconductor-insulator-superconductor) IV curve. They demonstrate the phase-sensitive response of the mixer with two-LO pumping and present calculations of the noise for various DC bias conditions and embedding impedances. Preliminary results indicate that for certain DC bias conditions and signal terminations, the noise added to a signal by the two-LO mixer operated in the single-sideband mode can be a factor of 25 less than that added by a conventional single-sideband single-LO mixer. >

Scheme for phase-locking a local oscillator in submillimetre-wave receiver

A technique for phase-locking a low-power fundamental local oscillator in a superconductor-insulator-superconductor (SIS) junction submillimetre-wave receiver is described. The arrangement uses the low-noise SIS mixer itself to downconvert the local oscillator signal from a Josephson junction or quantum well oscillator. A high-frequency phase-locked reference tone is required but for a typical SIS receiver the tone power is less than 10 nW. Even at terahertz frequencies this could easily be generated using a simple multiplier. >

Noise parameters of SIS mixers

It has been shown that low-noise receivers can be constructed at millimeter wavelengths by using mixers containing superconducting tunnel junctions as the nonlinear elements. This is possible because of both the low intrinsic noise of these devices and their potential for high conversion gain. Here, the quantum theory of mixing is used to derive the full-noise parameters and small-signal parameters of sinusoidally pumped SIS (superconductor-insulator-superconductor) junctions. These are then put into a form that allows the theory of two-port linear networks to be brought to bear, allowing calculation of such useful parameters as minimum noise temperature, optimum source impedance, available (or exchangeable) gain at minimum noise, and stability factor. These quantities are properties of the pumped junction that do not depend on the source or load impedance, but do depend on the terminations at the image and harmonic sideband frequencies. The harmonic sidebands are taken to be shorted, and the image termination dependence is studied. Numerical results are presented for both ideal (BSC theory) and practical (measured current-voltage characteristic) junctions. The noise parameters of the cascade connection of an SIS mixer and a (noisy) IF amplifier are considered, leading to a specification of the optimum coupling network between the two. It is noted that the SIS mixer is usually not unconditionally stable, but that oscillation can be avoided by careful design of the IF coupling network. >

Saturation of the SIS mixer by out-of-band signals

The tendency of superconductor-insulator-superconductor (SIS) mixers to saturate at low input signal levels is shown to depend on the total signal voltage across the junction, including frequency components outside the band of interest. If large dynamic range is to be achieved, mixers should be designed with embedding networks that present low impedances to the junction at out-of-band frequencies. >

High-frequency limits of superconducting tunnel junction mixers

We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.

SIS Mixer to HEMT Amplifier Optimum Coupling Network (Short Paper)

The coupling network between a superconductor-insulator-superconductor (SIS) mixer and a high-electron-mobility-transistor (HEMT) amplifier is investigated from the point of view of minimizing the overall noise temperature and also increasing the saturation level of the mixer. The effect of a negative output impedance of the mixer upon the amplifier noise is considered and an optimum negative source resistance is found. The amplifier noise at this optimum negative source resistance is shown to be related to the noise wave coming out of the amplifier input terminals. Key words: SIS, HEMT, low-noise, negative resistance.

Saturation of the superconductor quasiparticle direct radiation detector

The saturation properties of the superconductor-insulator-superconductor (SIS) direct detector are calculated. It is shown that this device begins to saturate only at powers far larger than one would expect from a simple analogy with the SIS mixer. Since the SIS direct detector is known to have very low noise, this result should lead to its immediate practical exploitation for direct detection of millimeter wavelength radiation.

Effects of Magnetic Field on the Josephson Noise from an SIS Mixer

The effects of a magnetic field on the noise due to the Josephson current in a superconductor-insulator-superconductor (SIS) mixer were investigated. The noise was found to be suppressed by an applied magnetic field small enough not to affect the DC characteristics of the mixer adversely. The quantum mixing effect between 70 GHz and 10 GHz radiations was clearly observed at bias voltages of 1∼3 mV, as a result of the noise suppression.

A general numerical analysis of the superconducting quasiparticle mixer

Virtually all analyses of the superconductor-insulator-superconductor (SIS) quasiparticle mixer have employed the quantum theory of mixing in its three-frequency approximate form. This approximation is valid only in the limit of very large junction capacitance. For finite capacitance, these analyses may be seriously in error. To remedy this, a computer program has been developed which uses the quantum theory of mixing in its most general form, treating the large-signal properties of the mixer in the time domain. The terminating impedances at the harmonics of the local oscillator frequency and their sidebands can be arbitrary. Using this analysis, the effect of finite junction capacitance on one SIS mixer's performance is described. This gives an insight into the range of validity of the three-frequency model.

Impedance conversion using quantum limit nonreciprocity for superconductor-insulator-superconductor mixer compensation

It is shown how a superconductor-insulator-superconductor (SIS) mixer when inductively terminated at the if port has inductive reactance at the signal port. This reactance may be used to compensate for the geometric capacitance of a conventionally operated SIS mixer over twice the bandwidth available through signal port resonance techniques.

Superconductor-insulator-superconductor mixing with arrays at millimeter-wave frequencies

We report high-frequency mixing at 75 GHz using superconductor-insulator-superconductor (SIS) tunnel junction arrays of various length and composition. Short (6-junction) arrays, both of Pb (In) and of Pb, and long (36-junction) arrays of Pb are used. The array geometry and antenna coupling can be scaled to 300 GHz with existing technology. Comparing the experiments to each other and to theory provides criteria for SIS mixer design. The measured single side band (SSB) conversion loss is as good as 2.0±0.9 dB. This is marginally the best published conversion result for any resistive mixer without image rejection, and quantitatively agrees with the quantum theory of mixing. We examine Josephson effect noise in this device and give a rough upper frequency limit for single-junction SIS mixers. Conversion maxima are also noted close to the half-gap singularity present in Pb (In) junctions.