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Japanese Journal of Applied Physics

Verification of the Model of Inductive Coupling between a Josephson Oscillator and a Stripline

Keisuke Kudo, Keiji Yoshida, Keiji Enpuku, Kaoru Yamafuji

Published Jan 1, 1993• 0 citations
Publisher

Abstract

In order to realize an efficient coupling between a flux-flow-type Josephson oscillator (FFO) and a stripline, we have carried out experiments to verify the mathematical model of the inductive coupling scheme between FFO and a stripline resonator in the frequency range between 50 GHz and 350 GHz. It is shown that the simulation using the proposed equivalent circuit for the inductive coupling scheme well explains the experimental results. The experimentally obtained center frequency and the bandwidth of the matching circuit were as large as 120 GHz and 40 GHz, respectively, which are also in reasonable agreement with those obtained in the simulation.

DOI
10.1143/jjap.32.60

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In order to realize an efficient coupling between a flux-flow-type Josephson oscillator (FFO) and a stripline, we have carried out experiments to verify the mathematical model of the inductive coupling scheme between FFO and a stripline resonator in the frequency range between 50 GHz and 350 GHz. It is shown that the simulation using the proposed equivalent circuit for the inductive coupling scheme well explains the experimental results. The experimentally obtained center frequency and the bandwidth of the matching circuit were as large as 120 GHz and 40 GHz, respectively, which are also in reasonable agreement with those obtained in the simulation.
A theoretical and experimental study of the coupling of the flux-flow-type Josephson oscillator (FFO) to an external stripline has been carried out to overcome the large impedance mismatch between them. For efficient coupling and impedance matching at millimeter and submillimeter wavelengths, the authors have proposed a scheme where the Josephson oscillator is inductively coupled to the stripline by a thin superconducting electrode. Theoretical modeling of the coupling section and the design of the stripline impedance-transformer have been carried out. In a preliminary experiment, the authors obtained a power of about 10/sup -7/ W emitted from the FFO into the stripline in the frequency range from 184 GHz to 193 GHz.>
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and a stripline impedance transformer, the authors have made a theoretical study of the inductive coupling between FFO and a stripline. It is shown that by choosing the output impedance of the FFO Z o smaller than the characteristic impedance of the coupling section Z 1 , the frequency dependence of the output power at the end of the coupling section can be extremely flat over a large frequency band. A model simulation employing a NbN/Pbin Josephson junction and a Nb/NbN stripline demonstrates that in the case of optimum design Z 0 =0.6 Z 1 , the bandwidth of the coupling section, which is defined by the frequency range where the deviation is within +or-5%, is shown to be as large as 80% of the centre frequency.
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and an external stripline, the authors carried out experiments to investigate the inductive coupling scheme in a strong coupling regime. By adopting NbN films with large penetration depths as electrodes for strong coupling, they were able to realize coupling sections with a coupling coefficient alpha =M/ square root L/sub 1/L/sub 2/ as large as 0.7, with M, L/sub 1/, and L/sub 2/ being mutual inductance, self-inductances of, the FFO, and the stripline, respectively. Using this design for the coupling, the authors succeeded in detecting AC fields at frequencies around 150 GHz, which were emitted from FFO to the stripline via the coupling section.
By using a superconducting integrated sub-mm wave receiver, the linewidth of a flux-flow type Josephson oscillator (FFO) was measured to be less than 2.1 MHz in the band 280-330 GHz. The output power coupled to a 10 i2 microstripline was about 0.5 jiW, and could be adjusted by changing the current bias of the oscillator junction. The integrated receiver consists of two identical Josephson flux-flow oscillators and an SIS (superconductor-insulator-superconductor) mixer. We can determine the composite linewidth of the oscillators by mixing them down to an intermediate frequency in the 515 mixer. The FPOs are connected to the SIS mixer via centerfed interdigital capacitors. One FFO is well coupled to the SIS mixer by using a 3-step Chebychev rnicrostripline transformer and a fairly large interdigital capacitor, while the second H0 has about 15 20 dB weaker coupling. A strip inductor, rf-terminated by a radial stub resonates out the SIS junction capacitance at about 340 GHz. Radial stubs were also used as RF chokes for the if-line (4 GHz). The FFOs and the SIS element are made of Nb/Nb-oxide/PbBi tunnel junctions. The circuit was optimized by careful CAD modeling (HP Microwave Design System). Normal conductors were used with the resistivity set to zero. Microstripline widths and lengths were recalculated for the superconducting case with the London penetration depths of the electrodes (Nb and PbBi) taken into account. After adjusting the area of the SIS element and the length of the tuning strip inductor, a good response was observed from 280 to 330 GHz, with the LC resonance at 340 GHz. The power was high enough to pump the SIS mixer.
An experiment on the coupling of the flux-flow type Josephson oscillator to SIS detectors by inductive coupling scheme has been carried out. Radiated powers from the Josephson oscillator were detected by an array of Josephson tunnel junctions fabricated on top of the upper electrode of the oscillator. The thickness of the common electrode was made thin enough to allow inductive coupling through the electrode. The maximum value of the obtained voltage amplitude of the emitted wave from the oscillator was 1.7mV, which is in reasonable agreement with the theoretical one. The present method can be an efficient coupling scheme between the Josephson oscillator and external circuits.
A theoretical study is made of a travelling-wave-type oscillator, which utilizes a flux flow in a long Josephson junction for use as a local oscillator in the integrated superconducting receiver system. An internal electromagnetic field of the oscillator junction in the flux-flow state is investigated both numerically and analytically. It is shown that the voltage amplitude of the internal oscillation increases gradually in the direction of the flux flow and reaches a maximum value at the junction end. An equivalent circuit of the oscillator is also obtained, which gives dependences of the emitted radiation on frequency, magnetic field, and load. It is shown that the output power attains the value of the order of 10−6 W in the frequency range between 100 and 500 GHz, and that the output power and the radiation frequency can be controlled by both the bias voltage and the applied magnetic field. These theoretical results explain quantitatively the experimental ones with a Pb-alloy long junction of length 24 λJ.