Detection of frequency-modulated signals by means of a Josephson junction

Valery V. Pavlovskiy, I. Gundareva, O. Yu. Volkov +1 more

Detection mechanisms in Josephson junctions with energies Ej comparable to thermal energies kT have been studied. The responses ΔV of YBa2Cu3O7−x bicrystal junctions to monochromatic radiation with frequencies f ranging from 94 GHz to 3.1 THz can be described in terms of classical rectification on a static nonlinear V–I curve at low frequencies and frequency modulation of the ac Josephson current at high frequencies, with an interplay between these mechanisms at intermediate frequencies. An electrical noise-equivalent power of (9 ± 3) × 10−15 W/Hz1/2, a responsivity of (3.4 ± 0.5) × 105 V/W, and a dynamic power range of 5 × 104 have been demonstrated for the square-law classical detection of 94 GHz radiation with the junctions at 50 K. The effect of background radiation on the V–I curves of YBa2Cu3O7−x bicrystal junctions was found to have an optical noise-equivalent temperature of ≤30 mK/Hz1/2. The main contribution to the effect comes from the interplay between the classical and Josephson detection mechanisms. The spectral and power dependencies of the responses ΔV of Josephson junctions have been numerically simulated within the resistively shunted junction model at various values of kT/Ej, and the results are in acceptable agreement with the experimental data.

Н. И. Иванов, I. I. Iurchenko

Vasyl M. Kychak, Mikola V. Vasylkivskyi, Maksym Huz +1 more

The structural scheme of a pulse-code modulator (PCM) based on high-temperature superconductors and Josephson junctions is proposed. For the implementation of comparators superconducting quantum interferometers are used based on Josephson junctions, which give a possibility to process weakly intense signals in the terahertz frequency range. Analytical dependences of blurring of the threshold characteristic from the critical current of Josephson junction and temperature are obtained. It is established that the amount of blurring increases directly proportional to square root of the product of critical current and the junction temperature.The analytical dependences of the PCM sensitivity from the critical current, temperature and characteristic voltage of Josephson junction were obtained. It is shown that the sensitivity increases directly proportional to the square root of the product of the critical current and the junction temperature and decreases in proportion to the square root of the characteristic voltage of Josephson junction.Research results show that devices based on low-temperature superconductors are characterized by a lower threshold of sensitivity in comparison with similar devices based on high-temperature superconductors.The threshold of sensitivity are ones of nA which is much lower than similar devices based on semiconductors and makes it possible to use such PCMs to process weakly intense signals in the terahertz frequency range.

Harm Tolner, Cornelis D. Andriesse, H. Schaeffer

C. C. Grimes, Sidney Shapiro

Experiments are reported in which two millimeter-wave signals incident on point-contact Josephson junctions produced changes in the junction dc voltage versus current characteristic and an intermediate frequency output whose amplitude depended sensitively on both junction bias and applied power. Equations are derived, based on Josephson's phenomenological equations, for the Josephson current in a junction exposed to two applied rf signals. When the applied signals differ appreciably in frequency, additional constant-voltage steps in the $V\ensuremath{-}I$ curve are predicted which are spaced at the difference frequency. These steps have been observed in experiments employing sources at 64 and 72 Gc/sec. Results of mixing experiments utilizing two sources nearly equal in frequency are reported at 23 and at 72 Gc/sec. In this case the two waves beat together and are equivalent in their effect to a single signal amplitude modulated at the difference frequency. Also explained on the same basis are experiments in which the third harmonic of a signal at 24 Gc/sec mixed with a signal at 72 Gc/sec. These results demonstrate the existence of the Josephson mixing mechanism as opposed to classical nonlinear mixing, and they show that it is operative at microwave and millimeter-wave frequencies over a wide range of power.

F. Ya. Nad