3682 EEE "SACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 7, NO. 2, JUNE 1997 '

Magnetometers

Masoud Radparvar and Sergey V. Rylov HYPRES, Inc., Elmsford, NY 10523

Abstract -- A single-chip digital SQUID magnetometer integrates a sensitive analog SQUID sensor with a comparator gate and feedback circuitry on the same chip. The comparator gate i s an asymmetric SQUID gate driving two DC-to-SFQ converters in series with the feedback coil. In an optimized digital magnetometer chip, sensitivity and noise level are determined by the input analog SQUID. The dynamic range of such digital chips is extremely wide enabling them to be operated in a relatively high magnetic field environment without extensive magnetic shielding. Their slew rate is determined by the frequency of an external two-phase clock. This chip simplifies room temperature electronics and, due to its digital output, can be easily multiplexed on-chip. In this paper, we describe this digital SQUID magnetometer chip and summarize our experimental results demonstrating a digital SQUID chip with sensitivity of 20 pQo/dHz and a system slew rate of 5 x 106 Q0/s at the pick up coil.

I. INTRODUCTION

An analog SQUID magnetometer chip has a SQUID gate coupled to a transformer that is matched to a pick up coil. The field imposed on the SQUID is transformed to a voltage by the SQUID and is sensed by an electronic circuit outside the dewar. In order to utilize a SQUID as an amplifier, its periodic transfer characteristic should be linearized. This linearization also substantially increases the dynamic range of the SQUID circuit. Figure 1 exhibits a DC SQUID system with its peripheral electronics. The function of the feedback coil is to produce a field which is equal but opposite to the applied field. The output is proportional to the feedback current, and hence, to the amount of flux required to cancel the measured field, and is independent of the transfer characteristic.

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The linearization of the transfer characteristics can also be accomplished by a circuitry integrated on-chip. Several versions of this single-chip digital magnetometer have been demonstrated.[ 1-31 However, none of them exhibited sensitivity and/or dynamic range suitable for practical applications. Previously, we introduced a new version of the digital magnetometer[4-51 which overcomes many of these limitations. In this paper, we present the experimental result on this novel digital single-chip magnetometer which exhibits improved sensitivity and dynamic range that is suitable for many practical applications.

11. CIRCUIT DESIGN AND OPERATION

Fig. 2 shows the block diagram for this digital SQUID. The two distinctive advantages of this digital SQUID amplifier over more traditional conventional digital SQUIDS are better sensitivity and ultra-wide dynamic range, obtained by using i) the analog SQUID preamplifier and ii) the series connection of the single-flux-quantum feedback device with the SQUID pickup loop. These two crucial features have been introduced previously based on RSFQ elements.[6] We have modified I this design by replacing its all-RSFQ approach with a more efficient combination of latching and RSFQ devices which allows to reduce the number of Josephson junctions and to have considerably higher output voltage. The circuit diagram of our latest design using this hybrid approach is shown in Fig. 3. The circuit consists of a comparator SQUID (C) coupled to the output of an analog SQUID preamplifier that is integrated to a pick up coil with an effective turn ratio of 140. The output of the comparator controls inputs of two separate SQUID write gates (one of them through a buffer circuit). I

I

................................ FEEDBACK CURRENT

Fig. 1 A conventional DC SQUID circuit with peripheral electronics. A feedback circuitry is needed to linearize SQUID characteristics. The dotted line shows the components normally at cryogenic temperature.

Manuscript received August 27, 1996. Research supported in part by NIH and NASA.

FEEDBACK LOOP I

CLOCK 1 OUTPUT-1 t

DC

Fig 2 Block diagram of a single-chip SQUID magnetometer Two DC sources and a two-phase clock with the timing shown are needed for proper operation of the magnetometer

Fig. 2 also illustrates the biasing circuits for the digital A two-phase clock and two DC bias lines are SQUID.

1051-8223/97$10.00 0 1997 IEEE

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required for the operation of a single-chip digital SQUID magnetometer. The clocks and the DC bias lines are provided from standard signal generators. The outputs of the digital SQUID circuit are two complementary binary signals at several MHz with a level of about 2 mV. One of these two digital outputs can be integrated to produce an analog reconstruction of the input signal. Alternatively, one can process the outputs using a digital signal processor to obtain a digital representation of the input signal.

FEEDBACK LOOP

DC BIAS

ANALOG SQUID CLOCK BIAS

COMPARATOR SQUID (C)

WRITE GATE

Fig.3 Circuit diagram for a high sensitivity digital SQUID magnetometer with analog DC SQUID fiont-end, sensitive comparator and feedback circuitry. The right (lee) write gate cancels positive (negative) magnetic fields by launching fluxons (antifluxons) into the feedback transformer. The comparator consists of 8 washers, each 12 pH, to improve its sensitivity. There is no intrinsic limit to the dynamic range for this type of SQUID magnetometer since the input flux is canceled by the feedback flux. However, for testing purposes, dynamic range is limited by the current-carrying capacity of the input line used to coupled to the pick up coil. All resistors, inductors and currents are in Q, pH and mA, respectively, unless otherwise specified.

The operation of the single-chip magnetometer is similar to the digital SQUID chips previously described in separate papers[4-51 and briefly, is as follows. The comparator and buffer SQUIDs have asymmetric threshold characteristics and are biased slightly over their critical currents using the two phase clocks shown in Fig. 2. In the absence of an external

field, the comparator switches to a voltage state causing its corresponding write gate to induce a flux quantum into the feedback loop and, simultaneously, prohibiting the buffer gate from switching to a voltage state. This flux creates a circulating current in the loop that is amplified by the analog SQUID and is applied back to the comparator SQUID. In the following clock cycle, this magnetic field keeps the comparator &om switching and causes the buffer circuit to switch and, hence, its corresponding write gate to induce an antifluxon in the loop annihilating the original fluxon. As long as there is no applied magnetic field, this process of fluxodantifluxon creatiodannihilation continues which represents the steady state operation of the digital SQUID circuit. This operation is shown in Fig. 4a and clearly demonstrates that the comparator and the buffer gates, alternately, switch to a voltage state causing their corresponding write gates to emit a fluxon or an antifluxon into the loop.

In presence of an applied magnetic field, the comparator causes its corresponding write gate to generate pulses into the feedback loop to cancel the applied magnetic field. The injection of fluxons continues in each clock period as long as the gate current of the comparator is below its threshold current. With proper polarity, the SFQ-induced current in the superconducting feedback loop can eventually cancel the applied current and restore the comparator SQUID close to its original state. When the current in the feedback loop is close to zero, both write gates, alternately, emit fluxons and antifluxons into the loop in each clock period, keeping the feedback current at its steady state.

In order for single-chip digital SQUID magnetometers to be of commercial value, they must, at least, have an energy sensitivity approaching their counter-part analog SQUIDS. This requirement puts the stringent criteria on the components of the single-chip magnetometer. The pre-amplifier determines the energy sensitivity. However, the comparator’s current (or magnetic field) sensitivity should also be adequate to exploit the pre-amplifier’s sensitivity. This should be accomplished without sacrificing the margins on the bias current of the comparator. To improve the sensitivity of the comparator, it is designed to have eight parallel washers, each 12 pH, in the SQUID loop to allow coupling of multi-turn coils to it.[5]

In an optimized digital SQUID circuit, the Least Significant Bit (LSE) of the output must be equal to the flux noise of the front-end analog SQUID (S,‘”). If the LSB is much less than SB”*, then the digital SQUID circuit unnecessarily tracks its own noise compromising system slew rate. If the LSB is much larger than S,’”, then the sensitivity of the complete digital SQUID is compromised. In the latter case, however, such a low sensitivity digital SQUID can be produced and will operate properly, albeit with non-optimal sensitivity. Such an exercise is useh1 to undertake in the process of circuit optimization, as it enables the progressive and independent optimization of the analog front-end SQUID, the one-bit comparator, the feedback coil and all of their interface designs. As a matter of fact, the results

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exhibited in Fig. 4 represent data from two separate digital SQUIDs: a) a low sensitivity SQUID exploited to optimize circuit components and b) a high sensitivity digital SQUID used to demonstrate sensitivity and full circuit operation. Low sensitivity digital SQUIDs are also useful demonstrations to establish the feasibility of the digital SQUID circuits. Fig. 4b shows the operation of a high sensitivity digital SQUID which clearly demonstrates an external 60 Hz signal.

the circuit generate an opposing current. As a result, the flux in the loop during normal operation will at no time be larger than one flux quantum. Thus, the maximum current in the feedback loop, which also corresponds to the LSB associated with the digital SQUID, is also about 1 nA. The minimum detectable current is determined by this current divided by the square root of the oversampling ratio (clock frequency/ sampling rate). Thus, a few pA sensitivity is easily achievable by a 10 MHz clock at 500 Hz sampling rate for a typical signal (with a bandwidth of 100-200 Hz).

Fig.4 a- Steady state outputs of a low sensitivity digital SQUID circuit (with 25 nH pick up and transformer coils) in absence of an extemal field. The traces from top to bottom are clock 1, clock 2, input and comparator and buffer outputs. As expected the outputs, altemately, induce fluxons and antifluxons into the loop. b- Steady state outputs of a high sensitivity digital SQUID (0.4 pH pick up and transformer coils). In this figure only outputs are shown. The chip is unshielded, consequently, it “measures” extemal60 Hz noise.

The digital SQUID chip slew rate with respect to its feedback loop is given by S, = aP., x F,, where F, is the clock frequency. The required slew rate is determined by S , = N, x A, where N, and A are the external noise slew rate and the pick up coil area, respectively. The system must track the signal as well as the noise (with slew rate of a few pT/s) to be able to operate in an unshielded environment. This requirement sets the minimum clock frequency required to accommodate the slew rate associated with the extemal noise and is typically a few MHz.

Magnetic signals of interest have bandwidths around several 100 Hz. This signal bandwidth determines the sampling frequency requirement. The signal has to be sampled at a minimum of twice its frequency to satisfy the Nyquist criterion in signal reconstruction. This requirement sets the output sampling frequency to about 1 kHz for typical magnetic signals. Intemally, the digital SQUID samples the signal at a much higher rate (oversampling), and, as a result, will generate a higher accuracy measurement, with the accuracy improvement factor equal to the square root of the oversampling ratio. The internal sampling frequency is the same as the circuit clock frequency.

The pickup coil inductance of a useful digital SQUID is about L, = 0.4-0.6 pH and is matched to the feedback transformer. The current due to each flux quantum (Q0) in the feedback loop is (I, = Qd(2 x L,)) which is about 1 nA. The (superconducting) feedback loop is designed to continuously null the flux that threads through it by having

111. EWERIMTNETAL RESULTS

Fig. 5 shows a photograph of a successful high sensitivity digital SQUID chip. This chip was fabricated using HYPRES‘ standard niobium technology using 1 kA/cm2 Josephson tunnel junctions and a 10-layer process with all niobium electrodes and wiring, aluminum oxide tunnel barrier, MO resistors, Au metallization and SiO, insulating layers. [7]

Fig 5 Photograph of a high sensitivity digital SQUID chip magnetometer. The loop inductance of the SQUID comparator is made of eight washers in parallel to facilitate its coupling to the large coupling transformer (1 2 pH) The input and the feedback coils are each 0.4 pH and each is fabricated from two coils in series The analog SQUID is coupled to the feedback coils through the two washers under these coils.

I In order to measure the flux sensitivity of a digital SQUID

circuit, a signal with bandwidth of about 100 Hz is applied between points A and B in Fig. 3. The comparator is biased with a 9 MHz clock and its output is measured. This periodic output is shown in Fig. 6a. The periodicity is associated with the input high sensitivity analog SQUID. The period corresponds to a single flux quantum a0 in the input S and is measured, in this case, to be generated by a 1 input current. The comparator output (the same point as clock bias) is integrated and its transition width is measured. Note that this is a digital output. The transition width of the

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digital output is a measure of the noise floor of the complete circuit. Such a transition is detailed in Fig. 6b, and is measured to correspond to an input current of 0.03 mA. Consequently, the open loop flux noise in the input SQUID/comparator SQUID combination is S,”* = (0.03/1.5) Q,,/(d9 MHz) = 6.6 x l o 6 OddHz at 100 Hz.

SQUID while the top waveform is the integrated output. The 60 Hz noise is apparent on the output waveform.

Fig. 6a - Voltage across a comparator SQUID coupled to an analog SQUID driven by a 9 MHz clock vs. current in an open feedback coil. Horizontal scale is 0.5 mA/div. b- Expanded transition width associated with the combined analog and comparator SQUIDS. Horizontal scale is 50 pA/div.

Figure 7 shows the noise spectrum for the single-chip magnetometer. In this case, the output of the comparator is low-pass filtered and then measured by a spectrum analyzer. A current of known magnitude is also coupled to the pick up coil via the input coil (Fig. 3) for calibration. Based on the applied 1 kHz current, the white noise level is estimated to be around 20 p@JdHz when feedback loop is closed and the clock is running at 5 MHz. The improvement in sensitivity is easily measurable as a function of clock frequency and no improvements is observed beyond 1 MHz due to the cut off frequency associated with the feedback loop and the coupling transformer. The 60 Hz noise together with its harmonics are evident in the spectrum. The sensitivity of the single-chip magnetometer is believed to be limited by the sensitivity of its analog SQUID front-end as similar SQUID chips fabricated using the same niobium technology have exhibited comparable noise characteristics.

Fig. 8 The integrated output (top trace) of a high sensitivity digital SQUID circuit in response to a 4 Hz input signal (bottom trace). The output is reconstructed by low-pass filtering and integrating the comparator output.

In summary, we have demonstrated a single-chip digital SQUID magnetometer with flux sensitivity of 20 p@,/dHz in flux-locked loop operation. With a 5 MHz clock, the slew rate was 5 x lo6 Ods at the pick up coil or 3.5 x lo4 Oo/s at the SQUID. The chip’s sensitivity and slew rate make it attractive for utilization in magnetometer/gradiometer systems for many practical applications. To the best of knowledge, this is the first demonstration of a digital SQUID with practical pick up coil (0.4 pH) and white noise level approaching its analog SQUID front-end.

IV. ACKNWLEDGMENT

The authors would like to thank Prof. John Wikswo at Vanderbilt University and Dr. Mark DiIorio at Biomagnetic Technologies for many useful discussions.

Fig. 7 Noise spectrum of the single-chip magnetometer. 60 Hz noise and its harmonics are evident in this spectrum. The 1 kHz signal is applied for calibration purposes.

Fig. 8 shows the signal measured by the single-chip digital magnetometer. The bottom trace is the input signal to the

REFERENCES

D. Drung, E. Crocoll, R. Herwig, M. Neuhas, and W. Jutzi, “Measured performance parameters o f gradiometers with digital output,” IEEE Trans. on Mag. MAG-25, pp. 1034-1037, March 1989. N. Fujimaki, H. Tamura, T. Imamura and S. Hasuo, “A single-chip SQUID magnetometer,” IEEE Trans. Elect. Dev. 35, 2414-2418 Dec. 1988. P.F. Yuh and S.V. Rylov, “An experimental digital SQUID with large dynamic range,” IEEE Trans. on Appl. Supercond., 5, 2129-2132, June 1995. M. Radparvar, “A wide dynamic range single-chip SQUID magnetometer,” IEEE Trans. on Applied Superconductivity 4, 87-91, June 1994. M. Radparvar and S. Rylov, “An integrated digital SQUID Magnetometer with high sensitivity input,’’ IEEE Trans. on Applied Supercond., 5,2142-2145, June 1995. S V. Rylov. “Analysis of high performance counter-type A/D converters using RSFQ logic/memory elements,” IEEE Trans. on Mag. MAG-27,2431-2434, March 1991. M. Radparvar, “Superconducting niobium and niobium nitride processes for medium scale applications,“ Cryogenics, 35, 535-540, August 1995.