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IEEE Transactions on Microwave Theory and Techniques

Conversion gain and noise of niobium superconducting hot-electron-mixers

H. Ekstrom, Boris S. Karasik, E. Kollberg, K. S. Yngvesson

Published Apr 1, 1995• 142 citations
Publisher

Abstract

A study has been done of microwave mixing at 20 GHz using the nonlinear (power dependent) resistance of thin niobium strips in the resistive state. Our experiments give evidence that electron-heating is the main cause of the nonlinear phenomenon. Also a detailed phenomenological theory for the determination of conversion properties is presented. This theory is capable of predicting the frequency-conversion loss rather accurately for arbitrary bias by examining the I-V characteristic, Knowing the electron temperature relaxation time, and using parameters derived from the I-V-characteristic also allows us to predict the -3-dB IF bandwidth. Experimental results are in excellent agreement with the theoretical predictions. The requirements on the mode of operation and on the film parameters for minimizing the conversion loss (and even achieving conversion gain) are discussed in some detail. Our measurements demonstrate an intrinsic conversion loss as low as 1 dB. The maximum IF frequency defined for 3-dB drop in conversion gain, is about 80 MHz. Noise measurements indicate a device output noise temperature of about 50 K and SSB mixer noise temperature below 250 K. This type of mixer is considered very promising for use in low-noise heterodyne receivers at THz frequencies.>

DOI
10.1109/22.375258

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Applied Physics Letters•Sep 21, 1998•16 citations
A study has been done of microwave mixing at 20 GHz using the nonlinear (power dependent) resistance of thin niobium strips in the resistive state. Our experiments give evidence that electron-heating is the main cause of the nonlinear phenomenon. Also a detailed phenomenological theory for the determination of conversion properties is presented. This theory is capable of predicting the frequency-conversion loss rather accurately for arbitrary bias by examining the I-V characteristic, Knowing the electron temperature relaxation time, and using parameters derived from the I-V-characteristic also allows us to predict the -3-dB IF bandwidth. Experimental results are in excellent agreement with the theoretical predictions. The requirements on the mode of operation and on the film parameters for minimizing the conversion loss (and even achieving conversion gain) are discussed in some detail. Our measurements demonstrate an intrinsic conversion loss as low as 1 dB. The maximum IF frequency defined for 3-dB drop in conversion gain, is about 80 MHz. Noise measurements indicate a device output noise temperature of about 50 K and SSB mixer noise temperature below 250 K. This type of mixer is considered very promising for use in low-noise heterodyne receivers at THz frequencies.>
Mixing at 20 GHz in niobium superconducting thin-film strips in the resistive state is studied. Experiments give evidence that electron-heating is the main cause of the nonlinear phenomena. The requirements on the mode of operation and on the film parameters for small conversion loss and the possibility of conversion gain are discussed. Measurements indicate a conversion loss between 1-8 dB and a DSB mixer noise temperature between 100 and 450 K at 20 GHz. The device output noise temperature at the mixer operating point can be as low as 30-40 K. A simple theory that is based on the assumption that the small signal resistance is linearly dependent on power is presented. This type of mixer is considered very promising for use in low-noise heterodyne receivers at THz frequencies.>
The mixing of 20 GHz radiation in a Nb superconducting film in the resistive state was studied. The experiment gave evidence of electron-heating to be the origin of the non-linear phenomenon. The requirements on the operation mode and on the film parameters in order to obtain small conversion losses or even gain are determined. Our measurements indicate a conversion loss of about 6-8 dB. The hot-electron bolometer is considered to be very promising for use in heterodyne receivers in a wide frequency range from microwaves to terahertz frequencies.
Bolometric mixers based on Nb and NbN superconducting thin films in the resistive state have been prepared for 20, 100 GHz and 350-500 GHz operation. The mixing mechanism is presumably of electron heating origin. Our measurements indicate that a conversion loss of about 6-8 dB can rather easily be achieved, and that the noise is reasonably low. The requirements on the operation mode and on the film parameters in order to obtain small conversion losses or even gain are discussed. For NbN films the availability of nearly 1 GHz IF bandwidth is experimentally demonstrated. NbN hot-electron mixers combined with slot-line tapered antenna on Si membrane or with double-dipole antenna on SiO2 substrate have been fabricated. The devices we study are considered to be very promising for use in heterodyne receivers from microwaves to terahertz frequencies.
A theoretical analysis for the noise temperature of hot—electron superconducting mixer has been presented. The contributions of both Johnson noise and electron temperature fluctuations have been evaluated. A set of criteria ensuring low noise performance of the mixer has been stated and a simple analytic expression for the noise temperature of the mixer device has been suggested. It has been shown that an improvement of the mixer sensitivity does not necessarily follow by a decrease of the bandwidth. An SSB noise temperature limit due to the intrinsic noise mechanisms has been estimated to be as low as 40-90 K for a mixer device made from Nb or NbN thin film. Furthermore, the conversion gain bandwidth can be as wide as is allowed by the intrinsic electron temperature relaxation time if an appropriate choice of the mixer resistance has been made. The intrinsic mixer noise bandwidth is of 3 GHz for Nb device and of 5 GHz for NbN device. An additional improvement of the theory has been made when a distinction between the impedance measured at high intermediate frequency (larger than the mixer bandwidth) and the mixer ohmic resistance has been taken into account. Recently obtained experimental data on Nb and NbN bolometer mixer devices are viewed in connection with the theoretical predictions. The noise temperature limit has also been specified for the mixer device where an outdiffusion cooling mechanism rather than the electron—phonon energy relaxation determines the mixer bandwidth. A consideration of the noise performance of a bolometer mixer made from YBaCuO film utilizing a hot—electron effect has been done.
Mixing experiments have been performed at frequencies from 4 to 20 GHz on Nb thin-film superconducting hot-electron bolometers varying in length from 0.08 to 3 μm. The intermediate frequency (IF) bandwidth is found to vary as L−2, with L the bridge length, for devices shorter than √12 Le−ph≊1 μm, with Le−ph the electron-phonon length. The shortest device has an IF bandwidth greater than 6 GHz, the largest reported for a low-Tc superconducting bolometric mixer. The conversion efficiencies range from −5 to −11 dB (single sideband, SSB). For short bridges, the mixer noise temperature is found to be as low as 100 K (double sideband, DSB), with little length dependence. The local oscillator power required is small, ≊10 nW. Such mixers are very promising for low-noise THz heterodyne receivers.
Conversion gain bandwidth and noise characteristics are reported for lattice-cooled superconducting YBa2Cu3O7 hot-electron bolometers acting as low-power, wide-bandwidth heterodyne mixers at 75 and 585 GHz. Near 77 K, these mixers show intrinsic conversion gain as high as −16 dB using ∼1 μW of local oscillator power. The measured gain bandwidth is well described by a single Lorentzian with −3 dB frequency of 5–9 GHz. Measurements of output noise combined with conversion gain indicate an optimal input noise temperature of ∼5000 K, significantly lower than calculations predict.