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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003

1039

Properties of a High-Tc dc SQUID Radiofrequency Amplifier
Alexey S. Kalaboukhov, Michael A. Tarasov, Ants Lohmus, Zdravko G. Ivanov, and Oleg V. Snigirev
Abstract--We present an experimental investigation of a radiofrequency amplifier based on a bicrystal high- dc SQUID designed for the frequency range 500­3000 MHz. The SQUID input coil comprises only one layer of a normal metal forming open-ended microstrip line. Both dc and microwave properties were investigated and analyzed. Maximum power gain was found to be 16 dB at 520 MHz for a SQUID with 8 turn input coil. Index Terms--High-temperature superconductors, microwave measurements, SQUIDS.

I. INTRODUCTION HE first practical radiofrequency amplifier based on a low- Nb dc SQUID was demonstrated more than fifteen years ago [1]. Since then, several approaches have been developed in order to achieve high power gain and low noise temperature [2], [3]. In spite of the ultimate noise performance, such amplifiers have various disadvantages such as low dynamic range, high intermodulation products mainly due to the limited SQUID voltage-to-flux transfer function, and absence of feedback. Recently, a new design for a low- Nb SQUID amplifier with a microstrip input coil was suggested [4]. This design reduced the SQUID loop inductance and the capacitance between the input circuit and the loop. Such a design with a normal metal coil replacing superconducting coil is very attractive for high- superconductor technology where fabrication of multilayer superconducting structures is complicated. In our previous work [5], [6] we developed a technological approach for high- dc SQUID radiofrequency amplifier based on bicrystal dc SQUID with normal metal input coil separated by an amorphous dielectric layer from the SQUID washer. The purpose of the present work is to analyze the margins of the
Manuscript received August 5, 2002. This work was supported in part by Award no. RE2-2223 of the U.S. Civilian Research & Development Foundation for the Independent States of the Former Soviet Union (CRDF). The work of M. A. Tarasov was supported by INTAS foundation Grants no. 00-686, 00-384. The work of A. Lohmus was supported by Estonian Scientific Foundation, Grant 5015. A. S. Kalaboukhov is with the Department of Microelectronics and Nanoscience, Chalmers University of Technology, S412 96 Goteborg, Sweden, on leave from Department of Physics, Moscow State University, 199899 Moscow, Russia (e-mail: ascry@ fy.chalmers.se). M. A. Tarasov is with Institute of Radio Engineering and Electronics RAS, Mokhovaja 11-7, 101999 Moscow, Russia. A. Lohmus is with Institute of Physics, University of Tartu, Riia 142 Tartu 51014, Estonia. Z. G. Ivanov is with Department of Microelectronics and Nanoscience, Chalmers University of Technology, S412 96 Goteborg, Sweden. O. V. Snigirev is with Department of Physics, Moscow State University, 199899 Moscow, Russia. Digital Object Identifier 10.1109/TASC.2003.814145

T

Fig. 1. Microphotograph of the high-T dc SQUID amplifier on YSZ bicrystal substrate with 30 misorientation angle.

high- dc SQUID with a normal metal input coil used as a radiofrequency amplifier. II. D
EVICE

DESIGN

AND

FABRICATION

A detailed description of our technological process can be found elsewhere [5]. A 180 nm YBCO film was deposited by conventional laser nm), ablation with an excimer KrF mixture laser ( buffer oxygen pressure 0.3 mbar and substrate temperature C. A thin gold film was deposited both by e-beam evaporation and magnetron sputtering. SQUID structure was patterned with optical photolithography and ion-beam etching in Ar ions with energy 250 eV and current density 0.2 mA/cm . The input coil was patterned by a lift-off. A microphotograph of the SQUID amplifier on a bicrystal ZrO substrate with 30 misorientation angle is shown in Fig. 1. III. BACKGROUND A. Input Coil Design The choice of the input coil parameters is contradictory. In order to achieve the highest power gain it is necessary to use coils with a large turn number to increase the mutual inductance between the coil and SQUID washer. In our previous work we analyzed the input coil in terms of a band pass filter. One important outcome was that the coil length significantly limits the operational frequency range of the amplifier. On the other hand, the input coil size is correlated with SQUID inductance,

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003

Fig. 2.

Configuration of the experimental setup for power gain measurements.

which is constrained by SQUID performance considerations. The best solution is to adjust the coil line width in order to vary its length keeping the total SQUID inductance fixed near the optimal value. B. Optimal SQUID Parameters The SQUID inductance is not the only the factor affecting on the SQUID amplifier performance. In order to obtain better microwave matching the dynamic resistance at the optimal bias point should be close to 50 Ohm for both the amplifier input and output. The SQUID inductance should coincide with the optimal value for a given . Thus, the overall performance is value, where is the normal determined by of the resistance of the Josephson junction. is about 1­2 For high- SQUIDs at 4.2 K the typical mV depending on the Josephson junction quality and misorientation angle. According to this, the critical current should not ­ pH. exceed 50­70 A, SQUID total inductance IV. E
XPERIMENTAL Fig. 3. Amplitude-frequency response of the comprehensive measurement circuit for blank YSZ substrate. The signal power is 50 dBm referred to the generator output. The total microwave losses through the circuit are 13 dB.

0

0

RESULTS

The experimental setup is shown in Fig. 2. For power gain measurements a calibrated generator was used. The small signal dBm referred to SQUID input) from generator (typically was directly coupled to the input coil through dc block and dB cold attenuator. There filters were used to decrease the noise level from room temperature electronics to avoid SQUID satdB gain was uration. First stage low noise amplifier with installed on the cold stage. Two room-temperature amplifiers dB were used to bring the signal up to the with total gain network or spectrum analyzer. Thus, the overall amplifiers gain dB. The SQUID was installed inside superconducting was and -metal shields. Before SQUIDs were tested, the full circuit amplitude-to-frequency response was investigated in the range 200­3000 MHz (Fig. 3) corresponding to the cold LNA bandwidth. A blank YSZ substrate was used instead of the SQUID chip in this measurement. The response is relatively flat with a small gap at 500 MHz, presumably from the cold LNA. A SQUID made on YSZ bicrystal substrate with 30 misorientation angle was investigated in detail. The input coil has 8 turns and 5 m linewidth. The insulator thickness is 170 nm. A and normal resisThe SQUID has critical current , giving the mV. tance To measure the power gain we used a spectrum analyzer to observe the SQUID response directly. The bias point was ad-

Fig. 4. Power gain of high-

T

dc SQUID amplifier with

N

= 8.

justed for a maximum voltage-to-flux response (150 V/ ); V corresponding to a Josephson frequency of 100 GHz. When the SQUID was introduced in the microwave circuit the total losses increased up to 25 dB. The origin of the additional losses is apparently inside the SQUID structure. In the present investigation we have not made any special efforts to reveal the loss mechanism. The result of the power gain measurements in the particular 200­600 MHz range is shown in Fig. 4. This dependence was obtained by subtraction of the output signal level observed at and different values of the external magnetic flux . The maximum power gain dB was observed at 520 MHz. Above 550 MHz there was no significant gain. V. D
ISCUSSION

In this paper we presented an investigation of a high- dc SQUID amplifier with a microstrip normal metal input coil. In spite that the SQUID did not have ultimate performance,


KALABUKHOV et al.: PROPERTIES OF A HIGH-

dc SQUID RADIOFREQUENCY AMPLIFIER

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V, we observed significant power gain of 16 dB at 520 MHz. With respect to our previous results with smaller coils (4­5 turns), the resonant frequency became lower but the power gain increased. The mentioned in [7], this is mainly connected with the stray capacitance and inductance of the input coil. It is more important to understand the reason for high insertion losses in the SQUID. This can be due to the normal metal input coil. Additionally, we did not specially examine the high frequency behavior of the amorphous dielectric used between the input coil and SQUID washer.

REFERENCES
[1] C. Hibert and J. Clarke, "DC SQUIDs as radiofrequency amplifiers," Journal of Low Temp. Phys., vol. 61, no. 3/4, pp. 263­280, 1985. [2] M. A. Tarasov, V. Y. Belitsky, and G. V. Prokopenko, "DC SQUID RF amplifiers," IEEE Trans. Appl. Supercond., vol. 2, no. 2, pp. 79­83, June 1992. [3] G. V. Prokopenko, D. V. Balashov, S. V. Shitov, V. P. Koshelets, and J. Mygind, "Two-stage S-band DC SQUID amplifier," IEEE Trans. Appl. Supercond., vol. 9, no. 2, pp. 2902­2905, June 1999. [4] M. Mueck, M.-O. Andre, J. Clarke, J. Gail, and C. Heiden, "Radiofrequency amplifier based on a niobium dc SQUID with microstrip input coupling," Appl. Phys. Lett., vol. 72, pp. 2885­2887, June 1998. [5] M. A. Tarasov, O. V. Snigirev, A. S. Kalabukhov, S. I. Krasnosvobodtsev, and E. A. Stepantsov, "Design and fabrication of the high-T radiofrequency amplifier with the microstrip input coil based on bicrystal dc SQUID," in Inst. Phys. Conf. Ser., Spain, Sept. 14­17, 1999, pp. 533­536. Paper presented at European Conference on Applied Superconductivity. [6] M. A. Tarasov, O. V. Snigirev, A. S. Kalabukhov, S. I. Krasnosvobodtsev, and E. A. Stepantsov, "A radiofrequency dc SQUID amplifier with a microstrip input coil: Simulations and experiment," in Proc. 7th Int. Superc. Electr. Conf., Berkeley, CA, USA, 1999, pp. 540­542. [7] M. Muck and J. Clarke, "The superconducting quantum interference device microstrip amplifier: Computer models," Journal of Applied Physics, vol. 88, no. 11, pp. 6910­6918, Dec. 2000.

A

CKNOWLEDGMENT

A. S. Kalaboukhov would like to thank technical engineer S. Pehrson for invaluable assistance in preparing the experimental setup.