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Comparative study of bias-reversing schemes for low frequency noise reduction in dc SQUIDS
PHYSICA
Physica B 194-196 (1994) 89-90 North-Holland
COMPARATIVE STUDY OF BIAS-REVERSING SCHEMES FOR LOW FREQUENCY NOISE REDUCTION IN DC SQUIDS J.M. Lockharta,b, D.N. Hipkins a, G. M. Guttb, B. Muhlfelderb, and N. Jeunerjohn a aphysics & Astronomy Dept., San Francisco State Univ., San Francisco, CA 94132, USA bHansen Physics Laboratory, Stanford Univ., Stanford, CA 94305, USA
We have investigated the effectiveness for low frequency noise reduction of three different schemes of periodic bias current reversal in dc SQUIDs operated in the flux-locked mode. The three techniques incorporate different frequency and phase relationships between the bias current waveform and the flux modulation waveform. We find that some SQUIDs show significant low frequency noise reduction under bias reversing, while other SQUID devices of the same design inherently achieve the lower level of low frequency noise and hence do not benefit from bias reversal.
1. INTRODUCTION Over the past few years, several schemes [1,2,3,4] have been developed to reduce the low frequency noise in dc SQUIDs caused by junction critical current fluctuations. These techniques all couple the usual flux modulation with a periodic reversal of the dc bias current, with the different methods utilizing different bias current rate and phase relationships with respect to the flux modulation waveform. Perhaps the most straightforward of these are the "Dynabias" or "YAMS" techniques [1,2] which simply switch the bias direction at an even submultiple of the flux modulation frequency. This process greatly reduces the effect of asymmetric critical current fluctuations in the SQUID on the detected signal produced by flux modulation. The results presented here are based on the use of the YAMS scheme with a 500 kHz square wave flux modulation and a 1.9 kHz bias reverse; further work using much higher bias reverse frequencies is in progress. Other authors have demonstrated the effectiveness of the "SHAD" technique [3] in which a four-step sequence of flux and current bias states is utilized, and a related technique in which the flux and bias current are reversed at the same frequency but with a 90 degree phase offset [4]. These methods yield potential low frequency noise reductions comparable
to those which we demonstrate, but they are somewhat more complex to implement. 2. M E T H O D
The dc SQUIDs used in these measurements were model 50 thin film dc devices from Quantum Design with their inputs open-circuited. They were mounted inside a metal Dewar surrounded by a double mumetal shield with a residual field of about 5 mG. The dewar was pumped through a mechanical vacuum regulator to obtain temperatures as low as 1.8 K for some of the measurements. The SQUIDs were operated in the flux-locked mode using commercial control electronics from Quantum Design and Applied Physics Systems as well as units fabricated in our laboratory. The output signals were lowpass filtered, digitized at 16-20 bits, and sampled by computer at typical rates of 2 Hz. The SQUIDs were thermally cycled to above 10K and back after the initial cooldown in order to minimize trapped flux. In order to test the effectiveness of the bias reversal scheme in reducing noise from asymmetric critical current fluctuations, we were able to find one SQUID device which appeared to have minimal fluctuations of this type. It's low frequency noise was compared to that of several t32aical devices with and without bias reversing.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSD1 0921-4526(93)E0618-Q
90
3. RESULTS Fig. 1 shows the noise spectra for the two different SQUID devices obtained using the Quantum Design control electronics with and without bias reversing. SQUID device S1 appears to be almost completely free of asymmetric critical current fluctuations and hence does not benefit noticeably from the use of the bias reversing technique. SQUID $2, however, shows an order of magnitude reduction in low frequency noise when bias reversing is employed. This behavior remained consistent over many repeated runs. The low frequency noise of SQUID $2 with bias reversing is quite similar to the "matched" SQUID S 1.
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Fig. 1. Noise energy spectral density at 4.2 K temperature for SQUIDs S1 and S2 with and without bias current reversing (BR). The curves shown are fits to the experimental data; the low frequency slopes for the cases S1, S1BR, and S2BR are 1.0 + 0.1 while the slope for the case $2 is 1.1 + 0.1. Fig. 2 shows the nature of the fluctuations in the flux-locked output voltage for the two different SQUID devices with and without bias reversing. Fig. 2a is typical of the SQUID $2 which exhibits asymmetric critical current fluctuations when no bias reversing is employed. The fluctuations have a Gaussian distribution, as evidenced by the good agreement with the Gaussian fit shown, but the spread in the distribution is considerably greater than that of Fig. 2b, which is typical of the three other cases (SQUID S1 with or without reversing and SQUID $2 with bias reversing). No significant departures from the Gaussian distribution were observed in any of the cases.
Fig. 2. (a) Distribution of flux-locked output levels for SQUID $2 without bias reversing. The curve shown is a Gaussian fit to the data with a standard deviation of 1.7. (b) Similar plot characteristic of cases S1, S1BR, and S2BR. Here the Gaussian fit has a standard deviation of 1.3. Experiments with other bias reversing schemes have so far resulted in higher noise levels at low frequency, but the other electronics units have not been fully optimized for use with the Quantum Design SQUIDs. Studies with these alternate schemes are continuing. REFERENCES
I. M.B. Simmonds and R.P. Giffard, U.S. Patent No. 4 389 612 (1983). 2. Quantum Design DC SQUID Controller Manual (Quantum Design, San Diego, CA, 1991), p. 8. Also see R.H. Koch, et al., J. Low Temp. Phys. 51,207 (1983). 3. V. Foglietti, et al., Appl. Phys. Lett. 49, 1393 (1986). 4. O. D6ssel, et al., IEEE Trans. Magn. MAG-27, 2797 (1990).
Physica B 194-196 (1994) 89-90 North-Holland
COMPARATIVE STUDY OF BIAS-REVERSING SCHEMES FOR LOW FREQUENCY NOISE REDUCTION IN DC SQUIDS J.M. Lockharta,b, D.N. Hipkins a, G. M. Guttb, B. Muhlfelderb, and N. Jeunerjohn a aphysics & Astronomy Dept., San Francisco State Univ., San Francisco, CA 94132, USA bHansen Physics Laboratory, Stanford Univ., Stanford, CA 94305, USA
We have investigated the effectiveness for low frequency noise reduction of three different schemes of periodic bias current reversal in dc SQUIDs operated in the flux-locked mode. The three techniques incorporate different frequency and phase relationships between the bias current waveform and the flux modulation waveform. We find that some SQUIDs show significant low frequency noise reduction under bias reversing, while other SQUID devices of the same design inherently achieve the lower level of low frequency noise and hence do not benefit from bias reversal.
1. INTRODUCTION Over the past few years, several schemes [1,2,3,4] have been developed to reduce the low frequency noise in dc SQUIDs caused by junction critical current fluctuations. These techniques all couple the usual flux modulation with a periodic reversal of the dc bias current, with the different methods utilizing different bias current rate and phase relationships with respect to the flux modulation waveform. Perhaps the most straightforward of these are the "Dynabias" or "YAMS" techniques [1,2] which simply switch the bias direction at an even submultiple of the flux modulation frequency. This process greatly reduces the effect of asymmetric critical current fluctuations in the SQUID on the detected signal produced by flux modulation. The results presented here are based on the use of the YAMS scheme with a 500 kHz square wave flux modulation and a 1.9 kHz bias reverse; further work using much higher bias reverse frequencies is in progress. Other authors have demonstrated the effectiveness of the "SHAD" technique [3] in which a four-step sequence of flux and current bias states is utilized, and a related technique in which the flux and bias current are reversed at the same frequency but with a 90 degree phase offset [4]. These methods yield potential low frequency noise reductions comparable
to those which we demonstrate, but they are somewhat more complex to implement. 2. M E T H O D
The dc SQUIDs used in these measurements were model 50 thin film dc devices from Quantum Design with their inputs open-circuited. They were mounted inside a metal Dewar surrounded by a double mumetal shield with a residual field of about 5 mG. The dewar was pumped through a mechanical vacuum regulator to obtain temperatures as low as 1.8 K for some of the measurements. The SQUIDs were operated in the flux-locked mode using commercial control electronics from Quantum Design and Applied Physics Systems as well as units fabricated in our laboratory. The output signals were lowpass filtered, digitized at 16-20 bits, and sampled by computer at typical rates of 2 Hz. The SQUIDs were thermally cycled to above 10K and back after the initial cooldown in order to minimize trapped flux. In order to test the effectiveness of the bias reversal scheme in reducing noise from asymmetric critical current fluctuations, we were able to find one SQUID device which appeared to have minimal fluctuations of this type. It's low frequency noise was compared to that of several t32aical devices with and without bias reversing.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSD1 0921-4526(93)E0618-Q
90
3. RESULTS Fig. 1 shows the noise spectra for the two different SQUID devices obtained using the Quantum Design control electronics with and without bias reversing. SQUID device S1 appears to be almost completely free of asymmetric critical current fluctuations and hence does not benefit noticeably from the use of the bias reversing technique. SQUID $2, however, shows an order of magnitude reduction in low frequency noise when bias reversing is employed. This behavior remained consistent over many repeated runs. The low frequency noise of SQUID $2 with bias reversing is quite similar to the "matched" SQUID S 1.
~'~1"
A.
8.38
B. (I.) 0 . 2 0
--7 "r-
''""
~..
.
-. . . . . .
"'.
/ .........
a.18
$2
S1,BR
e
c~
-9
la
9
8 7
6
S 4 3 2
18
1Z
3
,t S 6 7 8 9
10
Fluctuation (0.5 p.V) -10
--11
~3
--L2 Log Frequency
--~1
0'
(Hz)
Fig. 1. Noise energy spectral density at 4.2 K temperature for SQUIDs S1 and S2 with and without bias current reversing (BR). The curves shown are fits to the experimental data; the low frequency slopes for the cases S1, S1BR, and S2BR are 1.0 + 0.1 while the slope for the case $2 is 1.1 + 0.1. Fig. 2 shows the nature of the fluctuations in the flux-locked output voltage for the two different SQUID devices with and without bias reversing. Fig. 2a is typical of the SQUID $2 which exhibits asymmetric critical current fluctuations when no bias reversing is employed. The fluctuations have a Gaussian distribution, as evidenced by the good agreement with the Gaussian fit shown, but the spread in the distribution is considerably greater than that of Fig. 2b, which is typical of the three other cases (SQUID S1 with or without reversing and SQUID $2 with bias reversing). No significant departures from the Gaussian distribution were observed in any of the cases.
Fig. 2. (a) Distribution of flux-locked output levels for SQUID $2 without bias reversing. The curve shown is a Gaussian fit to the data with a standard deviation of 1.7. (b) Similar plot characteristic of cases S1, S1BR, and S2BR. Here the Gaussian fit has a standard deviation of 1.3. Experiments with other bias reversing schemes have so far resulted in higher noise levels at low frequency, but the other electronics units have not been fully optimized for use with the Quantum Design SQUIDs. Studies with these alternate schemes are continuing. REFERENCES
I. M.B. Simmonds and R.P. Giffard, U.S. Patent No. 4 389 612 (1983). 2. Quantum Design DC SQUID Controller Manual (Quantum Design, San Diego, CA, 1991), p. 8. Also see R.H. Koch, et al., J. Low Temp. Phys. 51,207 (1983). 3. V. Foglietti, et al., Appl. Phys. Lett. 49, 1393 (1986). 4. O. D6ssel, et al., IEEE Trans. Magn. MAG-27, 2797 (1990).