Wideband SIS Receivers Using Series Distributed SIS ... - IEEE Xplore

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IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 3, NO. 4, JULY 2013

Wideband SIS Receivers Using Series Distributed SIS Junction Array Cheuk-Yu Edward Tong, Member, IEEE, Paul Grimes, Raymond Blundell, Ming-Jye Wang, and Takashi Noguchi

Abstract—We have developed an SIS mixer based on a 3-junction series distributed array. In this mixer layout, the three junctions are connected together by passive network elements, such that each junction is subjected to different local oscillator (LO) drive, and slightly different DC bias voltages. This design helps to reduce the output capacitance of the mixer at the intermediate frequency (IF) so as to achieve a wider IF bandwidth, while maintaining adequate RF bandwidth. The receiver performance of this type of mixer was evaluated in the 220 GHz band. The lowest noise temperature measured was 30 K, and the noise temperature remains below 50 K over the IF range from 3 to 11 GHz. Receivers incorporating this new mixer design are currently in routine operation at the Submillimeter Array. Index Terms—Distributed mixers, submillimeter receivers, superconductor–insulator–superconductor (SIS) mixers, ultra wideband receivers.

I. INTRODUCTION

T

HE first distributed mixer based on the superconductor–insulator–superconductor (SIS) junction was introduced in 1990s. The advantage of such a mixer is that it shows very wide input RF bandwidth and that it does not require SIS junctions with very high critical current density. Two variations of this design were introduced: one based on SIS nonlinear transmission lines [1] and one exploiting parallel-connected SIS junction arrays [2]. About 10 years ago, the series-connected distributed SIS mixer was introduced [3]. This new class of mixer is designed to operate with very wide IF bandwidth. The original design was a 4-junction array requiring junctions with high critical current density, which were hard to produce. In this paper, we exploit an alternate 3-junction design, which uses junctions with lower current densities. While SIS mixers based on series junction array are quite common [4], [5], such designs usually assume that the junction array is equivalent to a single junction. As frequency increases, the inductance introduced by the interconnecting lines between the individual junctions will affect the operation of the mixer, with each junction seeing slightly different embedding

Manuscript received December 27, 2012; revised April 01, 2013; accepted April 02, 2013. Date of publication May 15, 2013; date of current version June 27, 2013. C.-Y. E. Tong, P. Grimes, and R. Blundell are with Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 USA (e-mail: [email protected]; [email protected]; [email protected]). M.-J. Wang is with Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), Taipei, 10617, Taiwan (e-mail: [email protected]). T. Noguchi is with National Astronomical Observatory of Japan (NAOJ), Mitaka, Tokyo, 181-8588, Japan (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TTHZ.2013.2259624

Fig. 1. Schematic representation of the parallel connected (left) and series-connected (right) distributed SIS mixer. The SIS junction is represented by the tunnel barrier in parallel with its geometrical capacitance . Only two junctions are shown here in each of the two mixers but multiple sections may be cascaded to form a longer transmission line mixer.

impedance. By contrast, our design explicitly allows for different embedding impedance as well as different RF and DC voltages for each of the junctions. We report the performance of 220 GHz receivers incorporating this design, including data from their field deployment. II. PARALLEL VERSUS SERIES-CONNECTED DISTRIBUTED MIXERS The main idea of distributed SIS mixing is to incorporate SIS junctions into a transmission line. Since an SIS junction has a large geometrical capacitance, a low impedance transmission line mixer is readily formed by connecting a number of SIS junctions in parallel with inductive microstrip sections. An extremely long and narrow SIS junction, forming a superconducting nonlinear transmission line, is the limiting case of a parallel-connected distributed mixer with infinitely close packed junctions. A series-connected distributed mixer is formed when SIS junctions are incorporated as series elements in a transmission line. In this case, the junction reduces the series reactance of the resultant transmission line and increases the characteristic impedance of the hybrid transmission line. A schematic representation of a short section of both types of mixer is given in Fig. 1. The obvious advantage of a series-connected distributed SIS mixer is its higher dynamic range. Its power handling capability , where is the number of junctions in the array scales with [4], [6]. A parallel-connected mixer has the property of low output impedance because of the large geometrical capacitance of the SIS junction. This also places a severe limit on its IF bandwidth. In contrast, the output impedance of a series-connected mixer is high. By careful choice of the characteristic impedance of the transmission line mixer, the IF output capacitance can be controlled to provide wide IF bandwidth. Parallel-connected distributed mixers work well with SIS junctions with low critical current densities as a larger shunting junction RF resistance increases the efficiency of the mixer. For

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TABLE I COMPARISON BETWEEN PARALLEL- AND SERIES-CONNECTED SIS DISTRIBUTED MIXER

the series-connected mixer, higher current density devices are more desirable, as they offer a lower product. Our target is to keep this product to be below 3, so as to maintain a high series junction RF resistance along the transmission line mixer. A mixer with too many junctions connected in series is generally difficult to use because of the higher local oscillator (LO) power requirement and the high output impedance. Therefore, series-connected distributed mixers use shorter transmission lines made up of a few junctions. As a result, the achievable input RF bandwidth is narrower. On the other hand, a long parallel-connected transmission line mixer, measuring a few guide wavelengths long, can present extremely wide input RF bandwidth, potentially over an octave of frequency. The differences between the 2 types of distributed mixer are summarized in Table I. III. SERIES-DISTRIBUTED MIXER WITH THREE JUNCTIONS We have explored the possibility of designing a series distributed SIS mixer with as few as 3 junctions and with relatively low critical current density devices. Our main aim with these new designs of mixer is to achieve wide IF bandwidth operation, while maintaining adequate RF bandwidth and state of the art noise performance. Fig. 2 gives the equivalent circuit diagram of this series-distributed mixer design (top) and a photo of the mixer layout (bottom). Note that the circuit implementation differs from the circuit shown in Fig. 1. The change is driven by the need to place two series-connected junctions (J1 and J2) on an island. Each junction is represented by a parallel circuit made up of a geometrical capacitance and a resistance , where is the RF junction resistance proportional to , the normal state resistance of the junction. The ratio depends on the LO power and frequency. The resultant small signal impedance of the SIS junction is (1)

Fig. 2. Equivalent circuit of our series distributed mixer (top) and photo of the RF circuit of a completed device (bottom). The first two junctions J1 and J2 and are located between 2 low impedance sections shown as capacitors in the schematic. The coplanar line sections between and are shown as and . The long narrow line on the left is a quarter-wave inductors transformer section to match the distributed network to the waveguide input port.

and J2. In this case, heterodyne mixing, which appears as a loss of the transmission line, is enhanced. In our design, the target critical current density is 7 kA/cm , and the junctions are nominally 1.7 m in diameter. This gives a normal state resistance of about 12 per junction. These parameters are easily achievable with our Nb/Al/AlOx/Nb fabrication process using optical lithography [7]. For 220 GHz operation, we have found that and with a specific junction capacitance of 85 fF/ m , the product is 2. From (1), the real part of is 1.5 . As shown in Fig. 2 (bottom), the shunt capacitances and are constructed with two low impedance sections which have equivalent capacitance of 75 fF each. The coplanar line sections linking J1 and J2 form the series inductors , , and . The total inductance is 10 pH. This series reactance is somewhat reduced by the capacitance of the series SIS junction. The final characteristics impedance of this transmission line is about 7 . This impedance level, coupled with the value of calculated above, produces a lossy transmission line section ideal for distributed mixing. The transmission line is terminated by a third SIS junction. This last junction not only contributes to the distributed mixing, it also provides a low impedance termination to the line. Since the distributed -network is roughly a quarter-wave section, this results in an input impedance somewhat higher than 7. . This impedance is further matched to the source impedance ( 40 real) using a quarter-wave transformer section. The 3-dB IF bandwidth of an SIS mixer is given by the RC time constant given by the load resistance and the total IF output capacitance . -

This equation shows that the real part of is reduced by a factor of and since the product is inversely proportional to the critical current density, a higher current density is desirable. This requirement can be relaxed if we reduce the characteristics impedance of the equivalent transmission line, , formed by the circuit section embedding J1

(2)

In our design, the IF output capacitance is in turn dictated largely by the capacitance because is in series with all the junction capacitances, and therefore makes little contribution to the total IF output capacitance. We estimate that the value of , is about 0.25 pF, which yields an IF bandwidth of 13 GHz

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Fig. 3. Current–voltage characteristics of the series distributed mixer in the presence of magnetic field (black solid). When LO power at 224 GHz is applied, a photon step appears between bias voltages of 5.8 and 8.5 mV (brown dashed). Also shown are the receiver IF power outputs over the photon step in response to black bodies placed at the receiver input at an IF of 6 GHz. A maximum Y-factor of 2.97 was obtained at a bias voltage of 7.8 mV.

for a 50- load. This bandwidth can be further increased by minimizing the output capacitance. IV. LABORATORY RECEIVER PERFORMANCE The new 3-junction series distributed mixer devices have been fabricated both at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), Taiwan, and National Astronomical Observatory of Japan (NAOJ), Japan. Both facilities have produced high quality devices. The mixer chips produced by ASIAA provide the best frequency tuning for our application and they have been tested extensively in both the laboratories in Cambridge, MA, USA, and in Taipei, Taiwan. In order to exploit the wide IF bandwidth offered by the new distributed mixer, we have placed a wideband cryogenic isolator at the IF port of the mixer, followed by a wideband low noise amplifier. The isolator is usable between 4 and 14 GHz with maximum insertion loss of 1.2 dB.1 It is an advanced version of the 4–12 GHz cryogenic isolator used in some of the ALMA bands. The low noise amplifier used is the CITCROYO1-12 MMIC amplifier.2 Select units of this model of amplifier demonstrate noise temperatures below 10 K up to 14 GHz. The receiver was characterized in a wet cryostat laboratory bench. Details of the setup are described elsewhere [8]. Fig. 3 shows the I–V curves of the mixer as well as its response to ambient and liquid nitrogen cooled loads placed at the receiver input ports. It is noted that the device demonstrates low sub-gap leakage. The total junction quality factor, defined as the ratio of the bias current at 12 mV bias to that at 6 mV divided by 2, is greater than 20. The maximum observed Y-factor is 2.97, from which a double-side-band receiver noise temperature of 30 K, or equivalently 2.8 , is deduced. This sensitivity is comparable to that of the double-side-band (DSB) mixer, which is at the heart of the ALMA Band-6 sideband separating receiver [5], [9]. 1“Isolator

CWJ1014.” Quinstar Corporation, Torrance, CA, USA.

2“CIT-CRYO1-12.”

California Institute of Technology, Pasadena, CA, USA.

Fig. 4. Measured double-side-band noise temperature of the receiver as a function of IF for various LO frequencies. The noise temperature is calculated directly from the measured Y-factors for ambient (294 K) and liquid nitrogen cooled (79 K) loads in the Rayleigh–Jean limit. No correction is applied.

The receiver noise temperatures as a function of IF at various LO frequencies are plotted in Fig. 4. It can be seen that the measured noise temperatures remain close to 30 K for IF below 10 GHz and for LO frequencies ranging from 208 to 232 GHz. This means that the receiver works well over a signal bandwidth in excess of 40 GHz once the 2 sidebands are taken into consideration. In terms of the IF bandwidth, the noise temperature is below 50 K up to 11 GHz. In order to understand the noise performance as a function of IF, we have computed the DSB conversion gain of the mixer from the receiver power output in response to ambient and liquid nitrogen cooled loads, scaled by the gain and noise temperature of the entire amplifier chain. At low IF, the mixer is operating at a DSB conversion gain of 3 dB. This gain is reduced to 2 dB at an IF of 12 GHz and to 0 dB at 14 GHz IF. Since the conversion gain does not roll off as fast as the sensitivity, we infer that the higher receiver noise above 10 GHz has to be introduced by the IF components. We are currently investigating a newer generation of wideband cryogenic isolator and low-noise amplifier to take full advantage of the wide IF bandwidth offered by these new receivers. V. FIELD DEPLOYMENT These new series-connected distributed mixers have been installed in the 220 GHz receiver inserts at the Submillimeter Array (SMA), an 8-element radio interferometer on Mauna Kea, Hawaii, USA. The new mixer chip replaces the original endloaded SIS stub mixer [10] which has a lower IF bandwidth because of its larger output capacitance. Although the SMA signal processing capacity is still limited in bandwidth by its correlator, we have made changes to the analog signal processor in each antenna to enable access to the higher IF region offered by these new receivers. By introducing an additional intermediate LO, the higher IF region can be mapped onto the SMA backend in a tunable 2 GHz wide sub-band. In order to validate this new configuration, we have performed on-sky test observations towards Orion BN/KL using

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Fig. 5. Upper- and Lower-side-band spectra taken towards Orion BN/KL at an LO frequency of 226.2 GHz, with IF spanning 4–12 GHz. The higher IF portion of the spectra were measured sequentially in 2-GHz segments. Known molecular transition lines are labeled in the spectra.

an LO frequency of 226.2 GHz. Fig. 5 gives the spectra observed using 2 receivers from 2 telescopes, operating in an interferometric mode. Since the receivers have DSB mixers, the sidebands are separated in the correlator by differential phase switching the Local Oscillators in the time domain. The resultant system noise temperature is somewhat higher than that from a sideband separating receiver. In the spectral regions of 236.2–238.2 GHz and 214.2–216.2 GHz, which correspond to the highest IF of 10–12 GHz, the spectrum baselines in Fig. 5 show slightly higher noise. Despite this, these observations demonstrate that the receiver is still functioning well up to the highest IF. As this goes to press, all 8 antennas of the SMA are equipped with these wideband receivers based on the series distributed array. These receivers are currently being used in routine astronomical observations. The new receivers have improved the stability, linearity and sensitivity of observations at the SMA, confirming the validity of the design. A dedicated correlator is being built to exploit the full capabilities of these wideband receivers. VI. CONCLUSION We have demonstrated that the series-distributed SIS mixer circuitry has the potential to provide low noise mixing over very wide IF bandwidth. A 220 GHz receiver based on a 3-junction design has been tested. The best double-side-band noise temperature measured for this receiver is about 30 K, or 2.8 . This level of quantum-limited sensitivity is also achieved over an input signal bandwidth of more than 40 GHz. The IF bandwidth is somewhat limited by the available IF system but very good sensitivity is obtained up to 11 GHz. A number of these wideband receivers have been installed in the Submillimeter Array and are being used in routine observations in the 220 GHz. REFERENCES [1] C.-Y. E. Tong, R. Blundell, B. Bumble, J. A. Stern, and H. G. LeDuc, “Quantum limited heterodyne detection in superconducting non-linear transmission lines at sub-millimeter wavelengths,” Appl. Phys. Lett., vol. 67, pp. 1304–1306, Aug. 1995.

[2] S.-C. Shi, T. Noguchi, J. Inatani, Y. Irimajiri, and T. Saito, “Characterization of the bandwidth performance distributed junction arrays,” IEEE Trans. Appl. Supercond., vol. 9, pp. 3777–3779, June 1999. [3] C.-Y. E. Tong, R. Blundell, K. G. Megerian, J. A. Stern, S.-K. Pan, and M. Popieszalski, “A distributed lumped-element SIS mixer with very wide instantaneous bandwidth,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 490–494, Jun. 2005. [4] S. Rudner, M. J. Feldman, E. Kollberg, and T. Claeson, “Superconductor-insulator-superconductor mixing with arrays at millimeter-wave frequencies,” J. Appl. Phys., vol. 52, no. 10, pp. 6366–6376, Oct. 1981. [5] A. R. Kerr, S.-K. Pan, A. W. Lichtenberger, and H. H. Huang, “A tunerless SIS mixer for 200–280 GHz with low output capacitance and inductance,” in Proc. 9th Int. Symp. Space THz Symp., Mar. 1998, pp. 195–203. [6] M. J. Feldman and L. R. D’Addario, “Saturation of the SIS detector and the SIS mixer,” IEEE Trans. Magn., vol. 23, pp. 1254–1258, 1987. [7] M. J. Wang, H. W. Cheng, P. K. Chuang, S. L. Wu, and C. C. Chi, “New AlOx thickness control process for SIS tunnel junctions fabrication,” IEEE Trans. Appl. Supercond., vol. 13, pp. 1101–1103, June 2003. [8] C.-C. Han, M.-J. Wang, T.-J. Chen, W.-C. Lu, and C.-Y. E. Tong, “A 220 GHz low noise superconducting receiver with wide instantaneous operating bandwidth,” in 2012 Asia-Pacific Microw. Conf. Proc. (APMC), Dec. 2012, pp. 427–429. [9] A. R. Kerr, S.-K. Pan, E. F. Lauria, A. W. Lichtenberger, J. Zhang, M. W. Pospieszalski, N. Horner, G. A. Ediss, J. E. Effland, and R. L. Groves, “The ALMA band 6 (211–275 GHz) sideband-separating SIS mixer-preamplfier,” in Proc. 15th Int. Symp. Space THz Tech., 2004, pp. 55–61. [10] R. Blundell, C.-Y. E. Tong, D. C. Papa, R. L. Leombruno, X. Zhang, S. Paine, J. A. Stern, H. G. LeDuc, and B. Bumble, “A wideband fixed-tuned SIS receiver for 200-GHz operation,” IEEE Trans. Microw. Theory Techn., vol. 43, no. 4, pp. 933–937, Apr. 1995. Cheukyu Edward Tong (M’89) received the B.Sc. (Eng.) degree from the University of Hong Kong, Hong Kong, in 1983. From 1985 to 1988, he was with the Institut de Radio Astronomie Millimétrique (IRAM), Grenoble, France, where he studied low-noise superconducting receivers for millimeter wavelengths. He received the Ph.D. degree in physics from the Université de Joseph Fourier, Grenoble, France, in 1988. From 1989 to 1991, he was a Post-Doctoral Fellow with the Communications Research Laboratory, Koganei, Tokyo, Japan. Since 1991, he has been with the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, where he is a staff member responsible for the development of ultra-sensitive superconducting receivers for submillimeter

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waves. His research interests include superconducting devices and their applications to THz frequencies, low-noise heterodyne instrumentation, and near-field beam measurement at submillimeter wavelengths.

Paul Grimes received the B.A. (Hons.) and M.Sci. degrees in experimental and theoretical physics from King’s College, University of Cambridge, U.K., in 2001 and the Ph.D. degree in astrophysics from the Cavendish Laboratory, University of Cambridge, U.K., in 2006. From 2005 to 2011 he was a Post-Doctoral Researcher with Oxford Astrophysics, University of Oxford, U.K.,, developing astronomical instrumentation for radio, millimeter and submillimeter wave astronomy and cosmology, also carrying out research on superconducting bolometers and heterodyne mixers. In 2011 he joined the Harvard–Smithsonian Center for Astrophysics as a Physicist. His main research interest is the development of superconducting mixers and other instrumentation for submillimeter-wave astronomical telescopes.

Raymond Blundell received the B.S. and Ph.D. degrees in physics and electrical engineering from the University of Leeds, Leeds, U.K., and spent a brief spell in industry before taking up an appointment at the Institut de Radio Astronomie Millimétrique, where he was head of the Receiver Development Group. He then moved to the Smithsonian Astrophysical Observatory to head a similar group with the prime responsibility of developing sophisticated receiver instrumentation for the Submillimeter Array. His main technical interests include super-

conducting mixer technology, low noise microwave amplifier technology, and cryogenic and quasi-optical systems. He is currently Director of the SMA and is frequently called upon to review technical articles for publication, grant proposals to a number of funding agencies, and to provide guidance as a panel member for a variety of astronomical programs, and visiting committee member to a number of radio observatories.

Ming-Jye Wang received the B.S. and Ph.D. degrees from the Physics Department of National Tsing-Hua University at Taiwan, in 1989 and 1994, respectively. He then joined the Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), Taiwan, as a Post-Doctoral researcher and became an Assistant Research Fellow in 1999. Currently, he is a Research Fellow of ASIAA. He has worked on superconductors and their applications for more than two decades. His current interests are on superconducting devices for THz detection.

Takashi Noguchi was born in Saitama, Japan, in 1952. He received the B.S., M.S. and Ph.D. degrees in applied physics from Tohoku University, Sendai, Japan, in 1976, 1978, and 1981, respectively. In 1981, he joined the Central Research Laboratory, Mitsubishi Electric Corporation, where he was engaged in the research and development of superconducting devices for analog applications. He left Mitsubishi Electric Corporation in 1991 to join the National Astronomical Observatory of Japan, where he worked on research and development of low-noise SIS mixers for millimeterand submillimeter-wave receivers. Now he is in charge of the development and production of SIS junctions for millimeter- and submillimeter-wave receivers of ALMA. His current research interest is the development of high-sensitivity superconductive detectors at submillimeter wavelengths.