Recently, a SQUID direct readout scheme called voltage-biased SQUID Bootstrap Circuit (SBC) is introduced to reduce preamplifier noise contribution. In this paper, we describe a concept of SBC with bias reversal technique which can suppress SQUID intrinsic 1/f noise. When applying a symmetrically rectangular voltage across SBC, two I-Φ characteristics appear at the amplifier output. In order to return to one I - Φ curve, a demodulation technique is required. Because of the asymmetry of typical SBC I-Φ curve, the demodulation method is realized by using a flux compensation of one half Φ0 flux shift. The output signal is then filtered and returned to one I-Φ curve for ordinary FLL readout. It was found, the reversal frequency fR can be dramatically enhanced when using a preamplifier consisting of two operational amplifiers. A planar Nb SQUID magnetometer with a loop-inductance of 350 pH, fR =50 kHz and a second order low pass filter with 10 kHz cut off frequency was employed in our experiment. Results prove the feasibility of SBC bias reversal method. Comparative experiment on noise performance will be carried out in further studies.
Recently, a SQUID direct readout scheme called voltage-biased SQUID Bootstrap Circuit (SBC) is introduced to reduce preamplifier noise contribution. In this paper, we describe a concept of SBC with bias reversal technique which can suppress SQUID intrinsic 1/f noise. When applying a symmetrically rectangular voltage across SBC, two I-Φ characteristics appear at the amplifier output. In order to return to one I - Φ curve, a demodulation technique is required. Because of the asymmetry of typical SBC I-Φ curve, the demodulation method is realized by using a flux compensation of one half Φ0 flux shift. The output signal is then filtered and returned to one I-Φ curve for ordinary FLL readout. It was found, the reversal frequency fR can be dramatically enhanced when using a preamplifier consisting of two operational amplifiers. A planar Nb SQUID magnetometer with a loop-inductance of 350 pH, fR =50 kHz and a second order low pass filter with 10 kHz cut off frequency was employed in our experiment. Results prove the feasibility of SBC bias reversal method. Comparative experiment on noise performance will be carried out in further studies.
Recently, we demonstrated and analysed the superconducting quantum interference device (SQUID) bootstrap circuit (SBC). It is a direct readout scheme for dc SQUID in the voltage bias mode, permitting one to suppress the preamplifier noise. The SBC enables us to control the two key parameters of a voltage-biased SQUID: the flux-to-current transfer coefficient and the dynamic resistance. The flux-to-current, I??, characteristics of SBC are made asymmetric by introducing the additional current feedback. Depending upon the choice of the working point, this feedback can be positive (working point W2 on the steeper I?? slope) or negative (W1 on the less steep slope). The dynamic resistance is controlled by the additional voltage feedback. In our publications to date we presented only the SBC operation at W2, while in this paper we demonstrate operation at W1 and show that also in this regime the preamplifier noise suppression is possible. We used a liquid-helium-cooled Nb SQUID with a loop inductance of 350 pH and attained white flux noise of 2.5? ??0?Hz ? 1/2 both at W2 and at W1. In the latter case, the linear flux range exceeded one half-flux quantum ?0. This large linear range should lead to a significantly improved stability and slew rate of the system and also make the tolerable spread in circuit parameters much wider than in all SQUID direct readout schemes known to date. Consequently, operation in this regime opens a new path to possible SBC optimization.
The voltage biased (SQUID) bootstrap circuit (SBC) was recently introduced as an effective means to reduce the preamplifier noise contribution. We analyze the tolerances of the SBC noise suppression performance to spreads in SQUID and SBC circuit parameters. It is found that the tolerance to spread mainly caused by the integrated circuit fabrication process could be extended by a one-time adjustable current feedback. A helium-cooled niobium SQUID with a loop inductance of 350 pH is employed to experimentally verify the analysis. From this work, design criteria for fully integrated SBC devices with a high yield can be derived.
The flux modulation scheme (FMS) is the standard readout technique of dc SQUIDs, where a step-up transformer links the SQUID to the preamplifier. The transformer's primary winding shunts the SQUID via a large capacitor while the secondary winding connects it to the preamplifier. A modulation flux having a frequency of typically 100 kHz generates an ac voltage across the SQUID, stepped up by the transformer. The SQUID with FMS is customarily operated in the current bias mode, because a constant dc bias current flows only through the SQUID due to the capacitor isolation. With FMS, however, the transformer ac shunts the SQUID so that in reality the operating mode is neither purely current-biased nor voltage-biased but rather nominal current-biased or “mixed biased.” Our objective is to experimentally investigate the consequences of ac shunting of the dc SQUID in FMS and the transformer's transfer characteristics. For different shunt values we measure the change in the SQUID bias current due to the ac shunt using another SQUID in the two-stage readout scheme, and simultaneously monitor the SQUID output voltage signal. We then explain our measurements by a simplified graphic analysis of SQUID intrinsic current-voltage (I–V) characteristics. Since the total current flowing through the SQUID is not constant due to the shunting effect of the transformer, the amplitude of SQUID flux-to-voltage characteristics V(Φ) is less as compared to the direct readout scheme (DRS). Furthermore, we analyze and compare V(Φ) obtained by DRS and FMS. We show that in FMS, the transfer characteristics of the SQUID circuit also depend on the isolation capacitance and the dynamic resistance of the SQUID.
Noise matching between superconducting quantum interference device (SQUID) sensor and readout electronics has always been an important problem. Conventionally, in order to avoid the hysteresis, SQUIDs should be operated in the range of βccis the Stewart-McCumber parameter. Recently, Liu et al. extended the SQUID operating area to βc > 1 due to a large noise parameter Γ*. When βc> 1, SQUID exhibits a large ∂V/∂Φ, which is beneficial in reducing the noise contribution of the readout electronics, δΦe. In this work, we compare the SQUID system noise δΦsmeasured with four different readout electronics. Here, the four readout electronics employed are as follows: 1) the flux modulation scheme (FMS); 2) the direct readout scheme (DRS), which uses six parallel connected bipolar transistor as its preamplifier (DRS-PCBT); 3) the DRS, which uses a single preamplifier AD797 (DRS-AD797); and 4) the two-stage scheme. The preamplifier voltage noise of DRS-PCBT and DRS-AD797 are about 0.4 nV/√Hz and 0.9 nV/√Hz, respectively. For a two-stage scheme, the equivalent flux noise reaches about 0.25 μΦ0/√Hz, the measured SQUID intrinsic noise, δΦi, will dominate δΦs. In other readout schemes, however, δΦsdepends on the value of βc. Finally, we found that FMS is suitable to match SQUIDs with βccc> 3.
A cold preamplifier based on superconducting quantum interference devices (SQUIDs) is currently the preferred readout technology for the low-noise transition edge sensor (TES). In this work, we have designed and fabricated a series SQUID array (SSA) amplifier for the TES detector readout circuit. In this SSA amplifier, each SQUID cell is composed of a first-order gradiometer formed using two equally large square washers, and an on-chip low pass filter (LPF) as a radio-frequency (RF) choke has been developed to reduce the Josephson oscillation interference between individual SQUID cells. In addition, a highly symmetric layout has been designed carefully to provide a fully consistent embedded electromagnetic environment and achieve coherent flux operation. The measured results show smooth V–Φ characteristics and a swing voltage that increases linearly with increasing SQUID cell number N. A white flux noise level as low as 0.28 μ Φ 0/Hz1/2 is achieved at 0.1 K, corresponding to a low current noise level of 7 pA/Hz1/2. We analyze the measured noise contribution at mK-scale temperatures and find that the dominant noise derives from a combination of the SSA intrinsic noise and the equivalent current noise of the room temperature electronics.
Applications of Superconducting Quantum Interference Devices (SQUIDs) usually require high sensitivity at relatively low frequencies, often down to 1 Hz or lower. Excess noise, typically with a spectral density scaling inversely as the frequency, can substantially reduce the sensitivity of SQUIDs at low frequencies. We have studied 1/f noise in niobium rf-SQUIDs in some detail. To reduce any noise contribution of the readout electronics, a cryogenic preamplifier was used. When one measures the signal voltage of the SQUID directly, a pronounced 1/f noise was observed in all samples, and was nearly independent of the bias frequency used. The crossover between 1/f and white noise moved towards higher frequencies as the bias frequency was increased, because of a lower white noise contribution. The 1/f noise scaled approximately as the inductance of the SQUID. When operated in a flux locked loop, however, no 1/f noise could be observed above 0.5 Hz. Operating a rf SQUID in a flux locked loop can thus substantially reduce 1/f noise in rf SQUIDs.