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Superconductor Science and Technology

Design of the flux-flow type Josephson oscillator inductively coupled to a stripline impedance transformer

Keisuke Kudo, Keiji Yoshida, Keiji Enpuku, K. Yamafuji

Published Nov 1, 1991• 2 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/4/11/035

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By using an integrated superconducting sub-mm wave receiver, the composite linewidth of two Josephson flux- flow oscillators (FFO) was measured to be less than 2.1 MHz in the band 280-330 GHz. The output power coupled to a 10 R microstripline was about 0.5 pW, and could be adjusted by changing the current bias of the oscillator junction. The receiver consists of two identical FFO's and an SIS (superconductor- insulator-superconductor) mixer. The composite linewidth of the oscillators was determined by mixing the two signals in the SIS mixer to a lower intermediate frequency (if). The FFO's 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 microstriplie transformer and a fairly large inter- digital capacitor, while the second FFO 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 FFO's and the SIS element are made of NbNb-oxidelPbBi tunnel junctions. The receiver circuit was optimized by careful CAD modeling (HP Microwave Design System). and gives rise to the emission of a pulse of electromagnetic energy when it hits the ends of the junction. If the arrival of solitons at one end of the junction is periodic, rf radiation is emitted. The propagation mode of the fluxon in a LJJ depends on the junction quality factor QJ, which is related to the junction losses as (3) QJ = ((wo/uJ)2P -t a)-'.

•Jan 1, 1994•0 citations
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.
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 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.
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.
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.
To develop an efficient local oscillator (LO) integrated with a SIS mixer operating above the gap frequency of Nb, we have investigated the radiation power P of NbN-based flux-flow-type Josephson oscillators (FFO's). The designed and fabricated chip incorporates FFO's, SIS power detectors (DET's), and their coupling circuits. Both FFO's and DET's consist of epitaxial NbN/AlN/NbN junctions with high critical current density J/sub C/ (15200 nW, enough for the optimum pumping of a SIS mixer with rf-resistance of 50 /spl Omega/, is coupled to DET's for 0.5-0.9 THz. The coupling bandwidth is larger than 20% of its central frequency. In the band, the radiation frequency is tuned by the control current through the FFO of 10-100 mA. The maximum coupling frequency of the present experiment is quantitatively agreed with the theoretical one. The peak power of 1.3 /spl mu/W is detected at 0.76 THz. The dissipated power in a FFO is smaller than 500 /spl mu/W, which is 10/sup -4/ times less than that of semiconductor sources. These results indicate the feasibility of NbN-based FFO's for a practical on-chip LO.
By using an integrated superconducting sub-mm wave receiver, the composite linewidth of two Josephson flux- flow oscillators (FFO) was measured to be less than 2.1 MHz in the band 280-330 GHz. The output power coupled to a 10 R microstripline was about 0.5 pW, and could be adjusted by changing the current bias of the oscillator junction. The receiver consists of two identical FFO's and an SIS (superconductor- insulator-superconductor) mixer. The composite linewidth of the oscillators was determined by mixing the two signals in the SIS mixer to a lower intermediate frequency (if). The FFO's 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 microstriplie transformer and a fairly large inter- digital capacitor, while the second FFO 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 FFO's and the SIS element are made of NbNb-oxidelPbBi tunnel junctions. The receiver circuit was optimized by careful CAD modeling (HP Microwave Design System). and gives rise to the emission of a pulse of electromagnetic energy when it hits the ends of the junction. If the arrival of solitons at one end of the junction is periodic, rf radiation is emitted. The propagation mode of the fluxon in a LJJ depends on the junction quality factor QJ, which is related to the junction losses as (3) QJ = ((wo/uJ)2P -t a)-'.