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Japanese Journal of Applied Physics

Experimental Study of Josephson Junction 2nd-Harmonic Mixers at 70 GHz

Sadamu Kita, Kazuo Fujisawa

Published Jan 1, 1981• 7 citations
Publisher

Abstract

Josephson junction (JJ) harmonic mixers with even-harmonic numbers can operate at zero bias voltage. Among them, low harmonic ones are expected to have a good conversion efficiency. Hence, we have done experiments on the 2nd-harmonic mixer at 70 GHz. It is found that mixing behaviors are classified into three types: zero-bias, finite-bias and intermediate operation. It is also found that the conversion efficiency of the 2nd-harmonic mixer is not so much less than that of the fundamental mixer. We obtained a conversion gain of +1.5 dB with the 2nd-harmonic mixer, and +2.0 dB with the fundamental mixer.

DOI
10.1143/jjap.20.l1

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Josephson junction (JJ) harmonic mixers with even-harmonic numbers can operate at zero bias voltage. Among them, low harmonic ones are expected to have a good conversion efficiency. Hence, we have done experiments on the 2nd-harmonic mixer at 70 GHz. It is found that mixing behaviors are classified into three types: zero-bias, finite-bias and intermediate operation. It is also found that the conversion efficiency of the 2nd-harmonic mixer is not so much less than that of the fundamental mixer. We obtained a conversion gain of +1.5 dB with the 2nd-harmonic mixer, and +2.0 dB with the fundamental mixer.
We have carried out a series of experiments at 70.03 GHz on Josephson junction harmonic mixers with harmonic numbers 1–8, using an identical Josephson junction. It was found that Josephson junction harmonic mixers operate differently depending on whether their harmonic numbers are odd or even. With odd-harmonic mixers, only finite-bias operation is possible, while with even-harmonic mixers, finite-bias and zero-bias operations are both possible. It is noted that a 2nd-harmonic mixer has a conversion efficiency equal to or higher than that of a fundamental mixer. The dependence of the conversion efficiency (η) on the harmonic number (N) was also studied, and the following approximate relations were determined: η∼N -1.6 for finite-bias operation and η∼N -1.4 for zero-bias operation.
We have studied the JJ harmonic mixing with an analog simulator devised by Magerlein. For even-harmonic mixing, there are two operation modes: zero-bias operation and finite-bias operation. The former is peculiar to even-harmonic mixing, while the latter is common to both even- and odd-harmonic mixing. Zero-bias operation is possible with a supply of LO current that suppresses the zero-voltage step completely, and it has the advantage of eliminating the dc bias supply and its accompanying heating effect and shot noise. For relatively low LO frequency (\varOmegaLO≃0.6), zero-bias operation has a higher conversion efficiency than does finite-bias operation. 2nd harmonic mixing under zero-bias operation has a conversion efficiency nearly equal to the fundamental mixing in the range \varOmegaS\lesssim0.5.
A new type of harmonic mixer using a series array of short superconducting weak-links is proposed for mixer applications at far-infrared and submillimeter-wave frequencies. The problem of limited mixer dynamic range can be eliminated by the use of a Josephson junction series array structure. Using a eleven-element series array, a preliminary experiment on harmonic mixing was carried out for two signals at 105.9 GHz and 5.4-12.6 GHz. IF (intermediate frequency) output was measured for each harmonic order from 9th to 19th by optimizing local power and bias voltage. In this experiment, 5-dB heightening of the signal-to-noise ratio was obtained at the IF output when the harmonic mixing order n was equal to the series element number, N=11.>
Two high-Tc superconducting (HTS) YBa2Cu3O7-δ(YBCO) grain boundary (GB) Josephson junctions in series as a harmonic mixer is investigated. The research shows that the mixer consisting of two series YBCO Josephson junctions has a larger harmonic number than that with only one junction in heterodyne mixing at zero-voltage bias. One of the advantages of the series-Josephson-junction mixer is that the total critical current is improved without sacrificing the sensitivity of the junctions in the array that makes the mixer easier to get more microwave induced-steps required by larger harmonic number. Furthermore, the underdamped YBCO GB series-Josephson-junction mixer operated at zero-bias voltage, where the zero-order Shapiro step exists, achieves mutual phase locking of the junctions in the array, resulting in an excellent mixing performance. Thus, the series-underdamp-Josephson-junction mixer with zero bias is predominant and compact in terahertz (THz) application due to its excellent performance and no dc bias supply.
Abstract : The term 'Josephson effects' refers to a number of unusual properties of current flow through a barrier or weak link between two superconductors. Such a structure--called a Josephson junction--gives rise to frequency conversion effects that suggest its use as an oscillator-mixer in the millimeter and submillimeter region. Such a device would offer high sensitivity, broad bandwidth, tunability, and, in certain modes of operation, the possibility of conversion gain. These frequency conversion effects were observed in the range of 100 GHz to 600 GHz and beyond. The junctions were driven by a 75 GHz or 25 GHz klystron while coupled either to a coaxial cavity resonant at 20 GHz or to a resonance at about 500 GHz associated with the junction structure. Lines in the junction voltage-current characteristic were observed which demonstrated that, at certain values of junction bias, power at the rf--or its harmonics--was converted to power at the resonance frequency. An explanation of the experiment is given together with the necessary background, which provides the basis for a discussion of such operational considerations as upper frequency limit, sensitivity, temperature of operation, and saturation behavior at high signal levels. (Author)
Abstract This paper presents a monolithic high-temperature superconducting K-band sub-harmonic Josephson mixer using a pair of reverse-biased Josephson junctions (JJs), which are biased using reversed DC current. The sub-harmonic mixer can provide higher conversion gain and stronger rejection of the even-order harmonics of the local oscillator signal. A matching circuit with several bandpass filters (BPFs) is designed to match the junctions and achieve a maximum conversion gain. Based on the circuit configuration and using the established Verilog-A JJ mixer model, transient simulation has been done in the circuit simulator. An electromagnetic (EM) model is built and simulated in a full-wave simulation environment, and the results reveal that the design realizes very low loss at 25 GHz, excellent port isolation and high conversion efficiency. A prototype has been fabricated and tested in a cryocooler at operating temperatures of 40 K and 20 K. Measurements have experimentally verified the novel radio frequency design and simulation results. Maximum conversion gains of −10.5 dB and −6.5 dB were obtained at 40 K and 20 K, respectively, with a bandwidth greater than 400 MHz and outstanding gain linearity. It is also predicted that the mixer performance can be further improved with higher critical current I c values and identical or similar I c values between two parallel junctions.