A. R. Kerr

Cryogenic cooling of mixers for millimeter and centimeter wavelengths

It is shown theoretically that cryogenically cooling a Schottky-barrier mixer only slightly increases the conversion loss while giving a considerable reduction in mixer noise. The d.c. bias and local oscillator drive must be appropriately scaled. Experimental results indicate that in conjunction with a cooled paramp IF amplifier, single-sideband (SSB) receiver noise temperatures of ~350 K at 85 GHz, and ~260 K at 33 GHz are presently obtainable-an improvement by a factor of 6 at 85 GHz and 4 at 33 GHz over current room-temperature mixer receivers. An unexplained source of noise within the diodes has been observed and if this can be eliminated a further factor of 2 improvement in noise temperature will be obtained.

Low-Noise Room-Temperature and Cryogenic Mixers for 80-120 GHz

A description is given of two new mixers designed to operate in the 80-120-GHz range on the 36-ft radio telescope at Kitt Peak, Ariz. It is shown that for a hard-driven diode the parasitic resistance and capacitance are the primary factors influencing the design of the diode mount. A room-temperature mixer is described which achieves a single-sideband (SSB) conversion loss (L) of 5.5 dB, and a SSB noise temperature (T/sub m/) of 500 K (excluding the IF contribution) with a 1.4-GHz IF. A cryogenically cooled version, using a quartz structure to support the diode chip and contact whisker, achieves values of L = 5.8 dB and T/sub m/ = 300 K with a 4.75-GHz IF. The mixers use high-quality Schottky-barrier diodes in a one-quarter-height waveguide mount.

Noise and Loss in Balanced and Subharmonically Pumped Mixers: Part I--Theory

In this paper, the theory of noise and frequency conversion is developed for two-diode balanced and subharmonically pumped mixers. Expressions for the conversion loss, noise temperature, and input and output impedances are derived in a form suitable for numerical analysis. Schottky diodes are assumed, having nonlinear capacitance, series resistance (which may be frequency dependent due to skin effect), and shot and thermal noise. In Part II, the theory is applied to several practical examples, and computed results are given which show the very different effects of the loop inductance (between the diodes) in balanced and subharmonically pumped mixers. It is also shown that the ideal two-diode mixer using exponential diodes has a multiport noise-equivalent network (attenuator) similar to that of the ideal single-diode mixer.

Suggestions for revised definitions of noise quantities, including quantum effects

Recent advances in millimeter- and submillimeter-wavelength receivers and the development of low-noise optical amplifiers focus attention on inconsistencies and ambiguities in the standard definitions of noise quantities and the procedures for measuring them. The difficulty is caused by the zero-point (quantum) noise hf/2 W/Hz, which is present even at absolute zero temperature, and also by the nonlinear dependence at low temperature of the thermal noise power of a resistor on its physical temperature, as given by the Planck law. Until recently, these effects were insignificant in all but the most exotic experiments, and the familiar Rayleigh-Jeans noise formula P=kT W/Hz could safely be used in most situations, Now, particularly in low-noise millimeter-wave and photonic devices, the quantum noise is prominent and the nonlinearity of the Planck law can no longer be neglected. The IEEE Standard Dictionary of Electrical and Electronics Terms gives several definitions of the noise temperature of a resistor or a port, which include: 1) the physical temperature of the resistor and 2) its available noise power density divided by Boltzmanns constant-definitions which are incompatible because of the nature of the Planck radiation law. In addition, there is no indication of whether the zero-point noise should be included as part of the noise temperature. Revised definitions of the common noise quantities are suggested, which resolve the shortcomings of the present definitions. The revised definitions have only a small effect on most RF and microwave measurements, but they provide a common consistent noise terminology from dc to light frequencies.

Some recent developments in the design of SIS mixers

SIS mixers in which superconducting tuning elements are integrated with the tunnel junctions have resulted in very low noise heterodyne receivers in the range 68–260 GHz. Above ∼120 GHz the need for extremely small reduced-height waveguides is avoided by mounting the SIS junctions in a suspended-stripline circuit coupled to a full-height waveguide by a broadband probe. The special characteristics of coplanar transmission line permit the design of SIS mixers with low parasitic reactances. Such a mixer operates over the full WR-10 band (75–110 GHz) without mechanical tuners.

Low‐noise 115‐GHz receiver using superconducting tunnel junctions

A 110–118‐GHz receiver based on a superconducting quasiparticle tunnel junction mixer is described. The single‐sideband noise temperature is as low as 68±3 K. This is nearly twice the sensitivity of any other receiver at this frequency. The receiver was designed using a low‐frequency scale model in conjunction with the quantum mixer theory. A scaled version of the receiver for operation at 46 GHz has a single‐sideband noise temperature of 55 K. The factors leading to the success of this design are discussed.

An 85-116 GHz SIS receiver using inductively shunted edge junctions

A superconductor-insulator-superconductor (SIS) mixer with a broadband integrated tuning structure is described. The mixer is tunable from 85 to 116 GHz and at 114 GHz has a noise temperature >

Integrated tuning elements for SIS mixers

The use of inductive elements to tune out the junction capacitance in SIS mixers is examined. Two new integrated tuning structures are introduced which overcome the limitations of earlier designs.

A Simple Quasi-Optical Mixer for 100-120 GHz

An open-structure quasi-optical mixer for 100-120 GHz is described. The mixer uses a GaAs diode coupled to a cavity-backed two-slot radiator. The design should be usable to sub-millimeter wavelengths with appropriate frequency scaling, and should be suitable for cryogenic operation.

Fabrication of Nb/Al-Al/sub 2/O/sub 3//Nb junctions with extremely low leakage currents

Nb/Al-Al/sub 2/O/sub 3//Nb trilayer films were deposited using DC magnetron sputtering guns in a UHV (ultrahigh vacuum) system which is capable of 5*10/sup -10/ Torr. SIS (superconductor-insulator-superconductor) junctions as small as 3.2*3.2 mu m/sup 2/ were isolated from the trilayer by standard photolithography. The junctions typically have V/sub m/=70-90 mV at 4.2 K, while at 2.0 K, V/sub m/ is as large as 1 V. This corresponds to a subgap current of 0.15% of the quasiparticle current rise. The subgap leakage current is compared to the predictions of the BCS (Bardeen-Cooper-Schrieffer) theory. The specific capacitance is preliminarily measured to be 45+or-5 fF/ mu m/sup 2/. >