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Physics Procedia

Novel Stable and Reliable Readout Electronics for HTS rf SQUID

Hans‐Joachim Krause, N. Wolters, Yi Zhang

Published Jan 1, 2012• 5 citations
Publisher

Abstract

Conventional readout electronics for radio-frequency SQUIDs exhibit long-term instabilities due to a temperature-dependent frequency drift of their voltage controlled oscillator. A novel electronics with a quartz-controlled synthesizer was developed to solve this problem. The frequency can be adjusted from 450 to 900 MHz with 2 ppm frequency stability, the amplitude range covers more than 60 dB. An adjustable phase shifter for demodulation is implemented to compensate for variable cable length. The total amplification is about 100 dB. The electronics with touchpanel and remote USB control is commercially available with up to three preamplifier units. Because thermal drift is eliminated, it is well suited for outdoor applications, e.g. in geophysics.

DOI
10.1016/j.phpro.2012.06.164

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Conventional readout electronics for radio-frequency SQUIDs exhibit long-term instabilities due to a temperature-dependent frequency drift of their voltage controlled oscillator. A novel electronics with a quartz-controlled synthesizer was developed to solve this problem. The frequency can be adjusted from 450 to 900 MHz with 2 ppm frequency stability, the amplitude range covers more than 60 dB. An adjustable phase shifter for demodulation is implemented to compensate for variable cable length. The total amplification is about 100 dB. The electronics with touchpanel and remote USB control is commercially available with up to three preamplifier units. Because thermal drift is eliminated, it is well suited for outdoor applications, e.g. in geophysics.
In order to improve magnetic field sensitivity and stability of the high-temperature RF SQUID(Rradio-Frequency Superconducting Quantum Interference Devices) magnetometer,a radio-frequency oscillator used to provide bias signal to resonant circuit of the superconducting quantum interference devices was designed.The voltage-controlled oscillator with adjustable continuously frequency was used as signal source,and radio-frequency signal attenuation circuit used to attenuate oscillation signal to meet the power requirements of the oscillation signal of the SQUID was also designed.The experiments showed that the oscillator can continuously produce adjustable 549.75～947 MHz radio-frequency signal,and the minimum signal power is-98.97 dBm,which meets the work requirements of the SQUID devices.
We have designed and tested a superconducting quantum interference device (SQUID) readout circuit, which can be operated at liquid helium temperatures. Although room-temperature SQUID electronics perform well, it is sometimes desirable to keep the wires between SQUID, readout electronics, and feedback coil as short as possible to minimize phase shifts and time delays. Cooling the readout circuit to low temperatures can also decrease its thermal noise. Our readout circuit uses conventional ac-flux modulation, which significantly reduces low frequency excess noise and drift in the preamplifier. In this case, simple complementary metal–oxide–semiconductor circuits with low power dissipation can be used as amplifier, phase-sensitive detector and integrator. The power dissipation of the complete readout is less than 15mW at 5V supply voltage.
We measured the amplitude–frequency characteristics of radio frequency superconducting quantum interference devices (rf SQUIDs) over a temperature range between 65 and 79 K. Using the expressions derived from the recently developed rf SQUID theory, valid also at large thermal fluctuations, we determined from these data the basic parameters of high-transition-temperature superconductor (HTS) rf SQUIDs. These parameters were: (a) the high-frequency coupling coefficient between the rf SQUID and the tank circuit resonator, k, (b) the SQUIDs hysteretic parameter, β, (c) the critical current of the Josephson junction, Ic, (d) its normal resistance, Rn, and (e) its noise parameter, Γ. We found a good agreement with the values of β(Ic) and Rn determined directly after destructively opening the SQUID loop. In accordance with the theoretical predictions, our experimental results show that at large thermal fluctuation levels (T≅77 K), rf SQUIDs with large loop inductance operate in nonhysteretic mode up to β values exceeding 3. Furthermore, we have shown that the optimal energy sensitivity is attained in the nonhysteretic mode at a value of β distinctly higher than 1. A quantitative comparison of white noise predicted by the theory with that obtained from the experiment showed a reasonable agreement. We also discussed the contribution of the phase information to the SQUID’s signal and noise at optimum operation conditions, when a mixer was used as a signal detector.
Ultrahigh frequency rf-SQUID magnetometer employing a cryogenic preamplifier and a commercially available thin film rf-SQUID sensor have been designed, manufactured, and tested. The system has been operated at 316 MHz, which is the maximum pump frequency accepted by the thin film sensor. The cryogenic preamplifier has been built around a microwave high-electron-mobility transistor which was made unconditionally stable at ultrahigh frequency band by means of an appropriate circuit design, namely, a cascode configuration. The total flux noise of the magnetometer has been found to be 8.4 × 10−6Φ0/√Hz at 4.2 K and 5.8× 10−6Φ0/√Hz at 1.4 K. The temperature dependence of the fractional step rise α of the current-voltage rf characteristic has been measured to be proportional to the 2/3 power of the SQUID sensor temperature. The noise performance of the system at 4.2 K has been limited by the intrinsic SQUID noise only, in good agreement with theoretical predictions.
This article presents a low-temperature superconductor (LTS)-based microwave phase shifter. The device utilizes a superconducting hybrid coupler monolithically integrated with two tunable reflective loads. An array of radio frequency superconducting quantum interference devices (rf-SQUIDs) is used to achieve inductive tuning in reflective loads, resulting in a broadband true time delay phase shift. The rf-SQUIDs are current controlled using an on-chip dc bias circuitry. The proof-of-concept fabricated device provides excellent RF performance in theX{X}X- and Ku -band with a phase shift of 49° at 10 GHz and 67° at 15 GHz. The insertion loss is < 1.5 dB for the entire range from 8 to 18 GHz. The methods to further improve the obtained phase shift are discussed. The overall device has a chip footprint of 0.6 mm2 and is fabricated using MIT-Lincoln Lab (MIT-LL) SFQ5ee eight-layer Niobium (Nb)-based superconducting fabrication process.
An improved read-out electronics without flux modulation for dc superconducting quantum interference devices (dc SQUIDs) is presented where the preamplifier is directly coupled to the SQUID. Compared to our previous designs, the 1/f voltage and current noise levels of the preamplifier were reduced by about a factor of 4 (rms) by using bipolar matched transistors instead of operational amplifiers at the input. Three types of flux-locked-loop electronics were developed and successfully tested: a low-power (120 mW) version for large multichannel arrays, a high-speed version with a 15 MHz bandwidth and a very fast (300 ns) reset mode, and a low-noise (0.44 nV/Hz) version with a fast and precise bias reversal circuit. The flux-locked-loop electronics are compatible with each other, so that only one type of low-power (180 mW) control unit is required to operate them. All essential specifications of practical SQUID systems (noise, dynamic behavior, and temperature stability) were determined by using liquid-helium and liquid-nitrogen cooled SQUIDs, respectively.