S. Rudner

Arrays of superconducting tunnel junctions as low‐noise 10‐GHz mixers

Quasiparticle mixing (biasing at the gap voltage) was studied in arrays of small Pb‐O‐Pb tunnel junctions. Coherent mixing at 9 GHz gave a conversion loss as low as −5.8 dB and a single sideband mixer noise temperature of 10–40 K. The optimal conversion efficiency was obtained where the bias current microwave response was maximal. The arrays of junctions were rugged, and gave a good impedance match to external circuitry.

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.

Arrays of Josephson tunnel junctions as parametric amplifiers

Arrays of 10 or 40 series‐connected small densely packed Josephson tunnel junctions were successfully operated as parametric amplifiers at 10 GHz. Between 50 and 90% of the junctions in an array were observed to work coherently in the amplification process. Signal gain exceeding 24 dB was achieved with a pump power as low as −50 to −70 dBm. A power width (i.e., the range of the pump power for a gain within 3 dB of the peak gain) of 0.25 dB and a bandwidth of 15 MHz were measured for 20‐dB gain. The power width was in accordance with theory but the bandwidth was reduced, probably due to noise saturation effects. The latter effects can be reduced by lowering the input noise temperature or by including an external band limitation. The noise temperature was also increased by saturation; however, with an external bandwidth of 60 MHz, a noise temperature of 30±20 K was measured. The application of a magnetic field has proved extremely useful as a means of tuning the amplifiers to stable high‐gain operating poin...

The antenna-coupled SIS quasiparticle array mixer

We report SIS mixing with Pb(In) junction arrays at 73.5 GHz. The array geometry and antenna coupling can be scaled to 300 GHz with existing technology. The measured SSB conversion loss is as good as 2.0±0.9 dB. This is marginally the best conversion result for a resistive mixer without image rejection yet published and agrees well with the quantum theory of mixing. We examine Josephson effect noise and thereby give a rough upper frequency limit for single-junction SIS mixers.

Parametric amplification in arrays of Josephson tunnel junctions

Arrays of 30 series‐connected unbiased small Josephson tunnel junctions give parametric amplification at 10 GHz. A pump power of 10−8 W was required and gains of up to 24 dB were achieved. The gain was constant for a large range of signal power, up to a signal output power of 10 dB below the pump power. A maximum brightness temperature of 1.7×106 K, in a bandwidth of 12 MHz, was deduced from variations of the gain with input noise level.

A subharmonic Josephson relaxation oscillator—amplification and locking

The inductance of the shunt loop in a resistively shunted Josephson tunnel junction may cause relaxation oscillations at subharmonics of the Josephson frequency. The relaxation period, the injection locking to a subharmonic of a strong external signal, and the amplification of weak signals were studied in detail. Gains up to 15 dB were achieved, but the noise temperature, which depended upon the harmonic of the relaxation frequency at which the amplification occurred, was high.

Shunted Josephson tunnel junctions: High‐frequency, self‐pumped low noise amplifiers

The high‐frequency amplification properties of transformer coupled, resistively shunted Josephson tunnel junctions have been investigated. The importance of the shunt loop inductance is stressed. It allows a high cutoff frequency, of significance for good high‐frequency performance. The self‐pumped parametric amplifier showed none of the excessive noise rise, which has hitherto plagued the development of externally pumped Josephson junction amplifiers. Around 10 GHz, we estimated a noise temperature less than 30 K for an amplifier pumped by a Josephson oscillation with a frequency well above twice the signal frequency. The corresponding gain of 5 dB may be increased in a better impedance matched circuit. The gain was very stable against variations in the bias conditions. A gain‐bandwidth product as high as 0.3 was registered. The experimental results agreed well with the established theory for self‐pumped parametric Josephson amplifiers. It should be possible to extend the low noise amplification by this ...

Basic Josephson tunnel parameters at microwave frequency—Strong temperature variations

Unexpected partly dramatic temperature dependences were registered for the basic tunnel parameters used to characterize a Josephson junction. Measurements of the magnetic field variation of the microwave‐power reflection, i.e., a mapping of the Josephson plasma resonance, enabled determinations of the rf small‐voltage resistance RJ, the cosφ amplitude ζ, and the effective junction capacitance C in both Pb‐O‐Pb and PbTl‐O‐Pb junctions. The uncertainties in the parameter values are large, particularly in the low temperature values of ζ, for which we confirm earlier estimates of ζ, but as the superconducting transition temperature Tc is approached, ζ increases rapidly and goes through a positive maximum close to +1 before it decreases towards zero at Tc. Also, RJ displays a pronounced maximum in the same temperature interval; it decreases rapidly within a narrow range below the temperature of the maximum before it again increases slowly well below Tc. The location in temperature for the two maxima depends up...

Low‐noise self‐pumped Josephson tunnel junction amplifier

We have studied the parametric amplification properties of an internally pumped Josephson tunnel junction, where the shunt contained resistive and inductive components. The amplification was, in this case, limited by circuit component values to less than 8 dB at 10 GHz, but it was very stable against variations in bias conditions. The amplifier gain‐bandwidth product was large and the noise level very low. We estimated the noise temperature to be less than 50 K, which should make the device attractive for mm‐wave amplification.

Basic Josephson tunnel parameters at microwave frequency—A reassessment

A previously published analysis of the magnetic field variation of the reflected microwave power from small unbiased Pb‐O‐Pb Josephson junctions found strong temperature dependences for the basic tunnel parameters. We reanalyze those measurements employing independent variables, compensating for spurious external reflections, and accounting for noise reduction of the dc critical current. The cosφ amplitude ζ is positive near Tc and becomes negative at lower temperatures. In contrast to the previous work ζ is less sharply peaked, the junction rf resistance varies smoothly, and the capacitance is temperature independent. Our results clearly disagree with recent theoretical work.