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IEEE Transactions on Applied Superconductivity

Inductive Isolation in Stacked SQUID Amplifiers

Quentin Herr, Donald L. Miller, Aaron A. Pesetski, John X. Przybysz

Published Jun 1, 2007• 23 citations
Publisher

Abstract

Amplification of SFQ pulses to higher signal levels has been achieved using the general technique of inductive isolation. A multiple-flux-quantum generator, which produces quantum-accurate waveforms suitable for ADC applications, and an output data link were demonstrated. Both circuits were implemented using critically damped, dc-powered SQUIDs stacked in series. In the past, the voltage output achievable with this approach has been severely limited by parasitic capacitance to ground and to the control lines. Parasitic capacitance inhibits SQUID operation both at turn-on and turn-off. Using inductive isolation between SQUIDs, parasitic capacitance was completely tuned out. A 100-flux-quantum waveform generator, based on a series stack of 100 SQUIDs, operated at 6 Gb/s, fabricated in a 4.5 kA-per-square-cm Josephson junction process. The output amplifier produced a 1 mV swing at 12 Gb/s non-return-to-zero. This circuit is based on a stack of only two SQUIDs, and uses both inductive isolation and asymmetric resistive shunting to enhance the out-of-phase oscillation mode.

DOI
10.1109/tasc.2007.898542

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•1 citation
Amplification of SFQ pulses to higher signal levels has been achieved using the general technique of inductive isolation. A multiple-flux-quantum generator, which produces quantum-accurate waveforms suitable for ADC applications, and an output data link were demonstrated. Both circuits were implemented using critically damped, dc-powered SQUIDs stacked in series. In the past, the voltage output achievable with this approach has been severely limited by parasitic capacitance to ground and to the control lines. Parasitic capacitance inhibits SQUID operation both at turn-on and turn-off. Using inductive isolation between SQUIDs, parasitic capacitance was completely tuned out. A 100-flux-quantum waveform generator, based on a series stack of 100 SQUIDs, operated at 6 Gb/s, fabricated in a 4.5 kA-per-square-cm Josephson junction process. The output amplifier produced a 1 mV swing at 12 Gb/s non-return-to-zero. This circuit is based on a stack of only two SQUIDs, and uses both inductive isolation and asymmetric resistive shunting to enhance the out-of-phase oscillation mode.
We have fabricated and characterized the noise performance of well-coupled Nb-Pb alloy edge junction dc superconducting quantum interference devices (SQUIDs) with a resistively shunted inductance. The extra damping resistor removes the resonances in the current-voltage characteristics resulting in a very low value of the extrinsic energy sensitivity in flux-locked-loop operation. Numerical simulations, with the junction capacitance included, indicate that the damping resistance does not degrade noise performance until it is significantly less than the junction-shunting resistance. We also tested different SQUID designs and demonstrated that a large SQUID inductance, proposed earlier to be desirable when resistively shunted, can seriously affect the low-frequency flux noise.
We report the enhanced voltage swing of a rapid-single-flux-quantum (RSFQ) distributed amplifier by replacing a single superconducting quantum interference device (SQUID) with a double-stack-SQUID (DSS). A DSS is composed of two stacked 2-junction SQUIDs sharing one sensing inductor. Thanks to its stack structure, a DSS is expected to generate twofold output voltage. We have designed a 12-stage RSFQ distributed amplifier equipped with DSSs. The maximum output voltage swing reached 10.2 mV in simulation. Test chips were fabricated using a 25-µA/µm2 Nb integration process. In measurements, a test chip was cooled in a liquid helium bath. The experimental output voltage swing was up to 8.34 mV.
Arrays of dc-superconducting quantum interference devices (dc-SQUIDs) were investigated, in which each dc-SQUID element has a multiturn input coil. The dependence of flux noise on the number of turns of the input coil was studied for current amplifiers. SQUID arrays were fabricated by a Nb/AlO x /Nb Josephson tunnel junction fabrication process. Results show that the flux noise of the SQUID array amplifier does not depend on the turn number of the input coil. A SQUID array amplifier consisting of 128 dc-SQUID elements was fabricated, in which each element had a 7-turn input coil and a mutual inductance between the input coil and the SQUID loop of 460 pH. This amplifier showed an ultrahigh current resolution of 1 pA/Hz0.5 and a maximum input current of 11 µA for signals of 50 kHz, which is sufficient bandwidth for X-ray detection applications. These values correspond to a large dynamic range of 87 dB. The flux-voltage characteristics of this SQUID array, however, showed asymmetric behavior, which originated from electronic resonance in stray capacitance and the mutual inductance between the SQUID loop and the multiturn input coil.
A suitable data link from RSFQ to semiconductor electronics remains challenging. We report an output amplifier that uses a new pulse multiplier circuit. The pulse multiplier consists of several stages arranged in series; a double-flux-quantum gate at each stage promotes the SFQ pulse to the next stage. Characteristics of the circuit design are: 1) DC power. As with the DFQ gate, every unshunted Josephson junction in the amplifier is loaded by critically-damped junctions that prevent voltage-state modes. 2) Equal rise and fall times that scale with output amplitude. In our case, 50 ps rise and fall time and 1-2 mV output amplitude can be realized, which is ideal for a data rate of 10 Gb/s. 3) Quantum accurate voltage multiplication independent of bias current. 4) Non-return-to-zero (NRZ) operation. The circuit was designed and successfully tested in our 8 kA/cm/sup 2/ foundry process. A 60 GHz pulse train, generated on-chip, was gated with RSFQ logic to produce a data pattern that was then fed into a pulse multiplier of either ten or twenty stages. Voltage multiplication was observed with operating margins of /spl plusmn/21% on bias current. Fast rise time was directly observed; unfortunately, fall time was spoiled by ringing on a 1 ns time scale. It is plausible that small changes to the physical layout of the circuit would produce desired operation.
In a superconductor digital-RF transmitter, the power amplifier chain can be implemented in hybrid temperature, hybrid technology environment, where the superconductor switching amplifier forms the first stage of the amplification chain. We have designed several flavors of SQUID based switching amplifiers, each targeted for different power output and speed. One of the amplifiers is a differential amplifier featuring dynamic equalization. To minimize the intersymbol interference, the amplifier employs a pre-emphasis technique, thereby compensating for the bandwidth limitations of the channel. The differential output voltage of this amplifier is 16 mV at low speed (200 Mbps), which rolls off to 2 mV at 16 Gbps data rate, for an “1100” pattern (no consecutive transitions on the input data sequence). For high speed operation of the SQUID amplifiers, simultaneous switching of SQUIDs is desired to reduce the output rise and fall times. Hence, in another amplifier design, called the Differential H-Tree Amplifier, the SQUIDs are arranged in an “H-tree” structure, to equalize the propagation delay of the control signal to each SQUID. The amplifier's differential output voltage is 8 mV at low speed (200 Mbps), and rolls off to 2 mV at 10 Gbps data rate. A third type of amplifier is a differential SFQ-to-DC amplifier; it consists of a pair of synchronously driven SFQ-to-DC converters that produce complementary positive and negative voltage waveforms respectively. The differential output voltage of this amplifier is 0.8 mV, and it can be operated at very high speeds. Moreover, the complexity of this amplifier enables it to be yielded in higher critical current density process. Both, the speed and the output voltage scale as the square root of the process critical current density.
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.