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IEEE Transactions on Magnetics

SQUID voltage-controlled-oscillator

Arnold H. Silver, Robert D. Sandell, J. Wilcox

Published May 1, 1983• 6 citations
Publisher

Abstract

We have investigated the SQUID as a voltage-controlled source of microwaves. The low impedance "resistive" SQUID can be a relatively high power (\sim nW), tunable, and monochromatic source for both on-chip and off-chip applications. Studies of the time-dependent junction phase and the available power spectra as they vary with such device parameters as loaded Q and the SQUID-β= 2πLic/φoestablish design rules for a well-behaved oscillator. For a VCO βQ\beta Q\geq2degenerate parametric subharmonic oscillations and chaotic instabilities are observed. Power increase is suggested by the use of voltage-clamped dc SQUIDs and arrays.

DOI
10.1109/tmag.1983.1062474

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We have investigated the SQUID as a voltage-controlled source of microwaves. The low impedance "resistive" SQUID can be a relatively high power (\sim nW), tunable, and monochromatic source for both on-chip and off-chip applications. Studies of the time-dependent junction phase and the available power spectra as they vary with such device parameters as loaded Q and the SQUID-β= 2πLic/φoestablish design rules for a well-behaved oscillator. For a VCO βQ\beta Q\geq2degenerate parametric subharmonic oscillations and chaotic instabilities are observed. Power increase is suggested by the use of voltage-clamped dc SQUIDs and arrays.
This is a new contract for a projected multi-year investigation of Superconducting Quantum Interference Device (SQUID) arrays and their application to microwave technology. The focus of this contract is the investigation and demonstration of a SQUID voltage-controlled oscillator (VCO). The array is then projected to increase the available power and operating impedance. The approach adopted will utilize monolithic superconducting integrated circuits which will be designed, fabricated, and tested at TRW. The project has three tasks: (1) Analysis of dc SQUID Arrays; (2) Design of dc SQUID Generator; and (3) SQUID VCO Fabrication and Measurement which are being carried out.
The output voltage of a dc superconducting quantum interference device (SQUID) containing a resistive section Rs oscillates at a frequency νq =Vs/ φ0 (φ0=flux quantum) when a current controlled voltage Vs is maintained across Rs. These oscillations have been observed from ∼10−1 to 1011 Hz, using Rs values ranging from ∼10−10 to ∼10−3 Ω in low inductance (∼10−10 H) point contact SQUID loops. At high frequencies, they manifest themselves in the dc current-voltage characteristics by inducing extra current steps.
The dynamics and noise of a dc SQUID (the Superconducting Quantum Interference Device), with the McCumber parameter /spl beta//sub c/=2/spl pi/R/sup 2/l/sub c/C//spl Phi//sub 0/ close to the unity (where I/sub c/, R, C is the critical current, the shunt resistance and the capacitance of the Josephson junctions comprising the SQUID, respectively, /spl Phi//sub 0/=2.07 10/sup -15/ Wb is the magnetic flux quantum) integrated with a planar spiral input coil, have been experimentally studied. The length of the spiral input coil was chosen so as to match its /spl lambda//4 microwave resonance frequency to the plasma resonance frequency of the SQUID. The input coil resonance is found to enhance the overall quality factor Q of the Josephson oscillations in the SQUID and, as a result, to increase the dynamic resistance R/sub d/ and the gradient of the flux-to-voltage characteristics, /spl delta/V//spl delta//spl Phi/, without hysteresis. A dc SQUID with a loop inductance L=30.5 pH, /spl beta//sub c/=0.72, and a 6 turn input coil demonstrated a non-distorted quasi-sinusoidal flux-to-voltage transfer function with an exceptionally large modulation depth of 140 /spl mu/V peak-to-peak. The spectral density of the magnetic flux noise was as low as 3.5/spl times/10/sup -7/ /spl Phi//sub 0//Hz/sup 1/2/ in the double stage configuration, measured at a temperature 4.2 K using direct read out electronics. In combination with an intermediary transformer, the current resolution of the SQUID is as low as 1.25 pA/Hz/sup 1/2/ with an input coil inductance of 100 nH.
We have fabricated and studied a promising kind of direct current superconducting quantum interference device (dc-SQUID) in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and π in the temperature range of 0.1–4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.
An all thin-film resistive SQUID with two Josephson tunnel junctions has been designed, fabricated and studied. As well as the two junctions, this device consists of a low ohmic (1.9*10 -6 Omega ) resistive part connected by superconducting leads to both junctions in a loop with very low ( approximately 10 -11 H) inductance. A direct current flowing through the resistive part fixes the voltage difference between the two Josephson junctions that are additionally fed by a bias current. The voltage difference is converted into a signal frequency that can be picked up from the SQUID with a conversion factor of ( phi o ) -1 =483.6 MHz mu V -1 , defined only by the electron charge and Planck's constant. An output signal level (peak-to-peak) of about 50 mu V has been obtained. The low frequency range (below 100 kHz) of the output signal frequency has been investigated. The possibilities of considerably extending this frequency range are also discussed.