Low noise SIS receiver performance using Pb alloy tunnel junctions

Low noise SIS receiver performance using Pb alloy tunnel junctions

S.R. Davies L.T. Little

Indexing terms: Lead alloy junction, SIS heterodyne receiver, Notse temperature, Conversion loss, Scaled modelling experiments

Abstract: A receiver noise temperature of 61 K (DSB) has been measured for a superconductor- insulator-superconductor (SIS) heterodyne receiver operating at 232GHz, the lowest value reported to date for such a receiver employing a lead alloy junction. Analysis of the losses of the receiver components yields mixer noise temperature and conversion loss, T, = 24K and L, = 2.ldB (DSB). These are compared with predictions based on 3-port quantum mixer theory, made using the measured characteristics of the junction and an equivalent circuit for the waveguide mixer mount deduced from scaled modelling experiments.

1 Introduction

The most sensitive heterodyne receivers at millimetre wavelengths employ superconductor-insulator--superconductor (SIS) tunnel junctions as the mixer elements. Early SIS receiver work used Pb alloy junctions almost exclusively because of their ease of fabrication. Junc- tions made from Nb have superior durability and han- dling properties compared to Pb-based junctions, although their fabrication is far more complicated and expensive. Some very low receiver noise temperatures have been reported for Nb SIS mixers: T,,, < 40K (DSB) over the range 180-250GHz [l], TI,, 11 113K (DSB) at 490GHz [2] and Tr,, = 230K (DSB) at 612GHz [2]. Pb alloy SIS mixers can produce noise performance comparable with niobium mixers. Meas- urements of mixer noise temperatures approximately two times the quantum limit, hvik, are described, achieved with Pb alloy SIS mixers at 232, 268 and 345GHz.

2 Receiver configuration

The measurements described below were made using the 205-285 GHz SIS heterodyne receiver designated Receiver A2, operating as a facility instrument on the James Clerk Maxwell Telescope, Hawaii [3]. The mix- 0 IEE, 1998 IEE Proceedings online no. 19982251 Paper received 23rd January 1998 S.R. Davies is with the Department of Physics, University of Bath, Bath BA2 IAY, UK L.T. Little is with the Electronic Engineering Laboratory, University of Kent, Canterbury, Kent CT2 7NT, UK

ing element was a Pb-In-Au alloy SIS junction having normal-state resistance RN - 60Q and area - 0 . 6 ~ ~ . It was mounted across a 4: 1 reduced-height rectangular waveguide, fed via a corrugated conical feedhorn. Tun- ing was accomplished with a single, adjustable, choked, noncontacting backshort. The mixer output was fed through an impedance matching circuit and an isolator to a cooled HEMT IF amplifier with a centre frequency of 1.5 GHz and an instantaneous bandwidth of 5OOMHz The mixer and IF amplifier were cooled in a hybrid liquid helium cryostat. The mixer block and isolator were mounted on the 4K cold plate of the cryostat; the HEMT amplifier on the 18K radiation shield. A superconducting coil magnet suppressed unwanted Josephson currents. The combined signal and LO power passed through the cryostat window (50 pn thick mylar film) followed by IR filters mounted on the radiation shields: a crystalline quartz plus polyethylene window in the 80K shield and a fluorogold window in the 18K shield. Before entering the cryostat, the signal and LO power were spatially combined using a thin piece of mylar film as a dielectric beamsplitter. The lay- out of the receiver is illustrated in Fig. 1. The receiver performance for three different beamspliffer thicknesses was measured; the contributions to total receiver noise are analysed in the Sections that follow.

3 RF and IF loss components

The conversion loss of the complete receiver is given by

Boltzmann’s constant, B is the instantaneous IF bandwidth and Phot and Pcold are the IF output powers due to hot and cold loads at temperatures Tho* and Tcold. A heterodyne receiver has a noise temperature, referred to its input, given by T,,, = TlnP -t- L,, Tnl -t L,,, TIF , where T,, is the mixer noise temperature, TIF is the noise temperature of the IF amplification chain and Tlnp is that arising from the quasioptical input losses, LlFIp. The conversion loss of the entire receiver is L,,, = L,, L,, where L, is the mixer conversion loss. The total gain of the IF amplification chain was estimated to be G,F = 71.7 2 1.3dB. Table 1 summarises the results of experiments to investigate the effect of beamsplitter thickness on receiver performance. All other tuning parameters were left unchanged. The final col- umn of Table 1 shows the difference between the two values of L,,, obtained for each experiment. For comparison, the computed transmissions of mylar as a function of thickness and incidence angle were estimated using formulae given by Goldsmith [4] (see Table 2). The values for the difference in insertion loss

LWC (Phor - Pcold)l(kB(Thot - Tcold)GIF) where k is

315 IEE Proc.-iWicron. Antennas Propug., Vol. 145, No. 5, October 1998

beamsplitter

backshort tuner phase lock electronics

isolator IF LNA

Gunn bias supply

beamsplitter

backshort tuner phase lock electronics

isolator IF LNA

Gunn bias supply

to 1 spectrometer

Fig. 1 Schematic l q " ~ t of Receiver A2

for different beamsplitters are very consistent between Table 1 and Table 2. The 50wn thick cryostat window introduced an insertion loss of 0.21dB at 232GHz. This window, apart from radiating some energy into the mixer, would have reflected radiation emanating from the 80K shield. Its effective temperature was estimated to be -250K. The quartz and fluorogold filters, mounted on the XOK and 18K radiation shields, were estimated to add insertion losses of -0.2dB and 0.1 dB, respectively. The loss of the corrugated feedhorii and was estimated to be 0.2dB.

Table 1: Effect of beamsplitter thickness on receiver performance

Frequency, Beamsplitter T,,, K GHz thickness, pm (DSB)

232 50 108 23 77

232 23 72 12 61

268 23 77 12 69

L,,, dB Difference IDSB) in L,,,, dB

4.57 0.35 4.22

3.76 0.08 3.68

4.69 0.10 4.59

The 1F output of the mixer was fed through an isolator on the 4K surface of the cryostat to the HEMT amplifier on the 18K shield via a length of thin, stain- less steel semi-rigid coaxial cable (measured electrical

loss 0.4dB). The isolator and the cable connecting it to the mixer output port were estimated to have a loss of 0.4dB. An IF impedance transformer was used to increase the load impedance, presented to the mixer of the IF amplification chain, to 200Q. The HEMT amplifier operated with an equivalent noise temperature of 5.1 K, averaged over the instantaneous IF bandwidth of 500MHz, defined by a bandpass filter in the I F chain. An independent gauge of the noise contribution of the I F amplification chain was obtained by measuring the amplified IF output power, with no LO power applied to the SIS mixer, biased well above the energy gap voltage (the SIS junction acting as a resis- tive load on the input to the IF chain). Including the thermal noise power reflected from the IF output port of the mixer, resulting from the impedance mismatch between the SIS junction and the IF chain, TIF = 7.1 K. The noise contributions of components of the IF chain subsequent to the HEMT amplifier are negligible. The amplified output power represents thermal noise from elements in front of the mixer, thermal and shot noise in the junction plus the noise in the IF chain itself. Subtracting the computed thermal and shot noise power from the output power leaves the noise power due to the IF chain, P,, = kBT,,. From the measured value of amplified I F output power, 3.72pW at 7mV bias voltage, then T,, = 8.3K.

Table 2: Computed transmission of mylar film

232 GHz 268GHz

Mylar thickness, Incidence Fraction Insertion Fraction Insertion p m angle, deg. transmitted loss, dB transmitted loss, dB

50 45 0.91 1 0.41 0.887 0.52

23 45 0.979 0.09 0.973 0.12

12 45 0.994 0.03 0.992 0.04

50 0 0.954 0.21 0.941 0.26

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From the losses and temperatures for each component, summarised in Fig. 2, values of T, and L,, for the mixer can be computed. A breakdown of the noise contributions from each component of the receiver is shown in Table 3 for the measurements made at 232GHz using a 12pn thick mylar beamsplitter. TIl17y,, represents the physical temperature of each component, T,? the contribution of each component to TLnp, including losses due to any preceding components. From Table 3, we find that at 232GHz, Ti, = 24.5K, and L, = 2.14dB (DSB). A similar analysis at 268GHz yields Ti?, = 25.1K, and L,?, = 2.99dB. Independent measurements were made on a similar Pb-In-Au alloy SIS junction mounted in a 320-380GHz receiver system comparable to the one described above. At a frequency of 34SGHz, T,.,, (DSB) = 65K was measured [SI, at which frequency the quantum noise limit is hvlk = 15.7K.

Mixer noise temperature and loss

beamsplitter ryostat quartz figold wiguide mixer cable IF amp.

295K t250H /dOK 118K I '"7 Ill// 18H 1 window filter filter

..--I 0.21 dB I 0.lOdB I-=?+- 0.20 dB 5K

0.03 dB 0.20 dB 0.20 dB 0.50 dB Fig. 2 Sclzemulic luvoul of' Reeiver AZJiont-md

Table 3: Contributions to receiver noise temperature

232GHz dip w i n qtz flg wvg mix is0 calb amp

Tphys,K 295 250 80 18 4 4 4 11 18

Loss, dB 0.03 0.21 0.20 0.1 0.2 2.14 0.4 0.4 3.68

Tn,K 2.0 12.5 3.7 0.3 0.1 29.1 0.2 1.2 11.9

5 Theoretical noise analysis

The expected conversion loss and mixer noise temperature at key frequencies were computed for comparison with the experimental measurements described above. These computations were made using Tucker's quantum theory of mixing [6], applied to the measured DC I-V curve of the SIS mixer junction. A theoretical model for the electrical equivalent circuit of the waveguide mixer mount was formulated with the aid of scaled modelling experiments [7]. This equivalent circuit is shown in Fig. 3. The junction consists of the nonlinear tunnelling element with a shunt capacitance resulting from its overlapping structure. The backshort can be represented by a real and imaginary pair of shunt impedances, the values of which depend on the VSWR of the backshort and its position in the waveguide. A backshort VSWR - SO was estimated [8]. The series inductance arises from the SIS junction met- allisation across the waveguide; it has a reaclace of j45Q at 232GHz. Computations were made using the 3-port formulation of the quantum theory of mixing applied to the §IS junction (defined by its 1-V curve), mounted in the equivalent circuit of Fig. 3. An estimate of the admittance of the S1S mixer embedding circuit was made using the DC and LO pumped I-V' curves,

IEE Pvoc.-Mir.roiv Aniciirziis Proi~ug.. Vol. 145, No. 5, Ocfohcr 1998

following the method of Skalare [9]. This was in rea- sonable agreement with the admittance predicted from the equivalent circuit. The embedding admittances were determined to be Y = 1.39 - j0.47 (normalised to the waveguide admittance U126 a) at 232GHz, and Y = 1.51 - j0.13 (normalised to 1/115Q) at 268GHz. These values include the susceptance due to the junction capacitance. The results of the theoretical modelling are summarised in Table 4, along with the values deduced previously from experimental data. There is good agreement between the two sets of values.

iwL

t waveguide backshort junction

Fig.3 Equiiulent c i i a u t of iiuvcqude inmi mount

Table 4: Mixer noise performance at 232 and 268GHz

Theory Experiment

Frequency L, (DSB) T, (DSB) L, (DSB) T, (DSB)

232GHz 2.05dB 22.7K 2.14dB 24.5K

268GHz 2.41dB 25.3K 2.99dB 25.1 K

6 Conclusions

Low noise mixer performance is obtainable with small area lead alloy SIS junctions. At both 232 and 268GHz, we have measured DSB receiver noise temperatures of roughly six times the quantum limit, Izvlk. From these measurements, we have computed DSB mixer noise temperatures of roughly two times the quantum limit. The best results for SIS receivers using niobium junctions have been achieved using on-sub- strate tuning stubs which provide a parallel inductance to cancel junction capacitance [ l , 21. Such stubs were not used for the receiver described here. Despite this, these results show that very low noise performance is possible with lead alloy junctions. The latter tend to be smaller in area and have less specific capacitance than niobium junctions so that at these frequencies the need for integrated tuning is much reduced. At much higher frequencies, lead alloy junctions may possess advan- tages over all-niobium junctions, including ease of fabrication and higher energy gap. Due to the manu- facturing process, tuning stubs integrated with niobium junctions tend to be made from niobium or aluminium, which suffer from frequency limitation problems. Our fabrication process also permits tuning stubs made from silver, copper or gold to be integrated with lead alloy junctions, which may prove advantageous at higher frequencies (> 750GHz).

7 References

1 BLUNDkLL, I<, TONG, C -Y b , PAPA, D C , LEOM- BRUNO, R L , Z H A N C , X , lPAINE, S , STERN, J A , LEDUC. H G , and BUMBLE, B A widebdnd fixed-tuned STS receiver Tor 200GHz operation'. ICE17 Trunc, 1995, MTT-43, pp 9 3 3-937

717

2 ZMUIDZINAS. J., LEDUC, H.G., STERN, J.A., and CYPHER, S.R.: ‘Two-junction tuning circuits for submillimeter SIS mixers’, IEEE Trans, 1994, MTT-42, pp. 698-706

3 DAVIES, S.R., CUNNINGHAM, C.T., LITTLE, L.T., and MATHESON, D.N.: ‘A 210-280 GHz SIS heterodyne receiver for the James Clerk Maxwell Telescope. Pt. I : Design and performance’, Int. J. Infrared Millim. Waves, 1992, 13, pp. 647-658 GOLDSMITH, P.F.: ‘Quasioptical techniques at millimeter and submillimeter wavelengths’, Infrared Millim. Waves, 1982, 6 , pp. 277-343

4

5 CUNNINGHAM, C.T.: (private communication)

378

6 TUCKER, J.R., and FELDMAN, M.J.: ‘Quantum detection at millimeter wavelengths’, Rev. Mod. Phys., 1985. 57, pp. 1055- 1113

7 DAVIES, S.R., and LITTLE, L.T.: ‘The effect of mixer mount loss on the performance of SIS receivers’, lnt. J . Infiared Millim. Waves, 1990, 11, pp. 89-100

8 ELLISON, B.N., LITTLE, L.T., MA”, C.M., and MATH- ESON, D.N.: ‘The quality and performance of tuneable waveguide backshorts’, Electron. Lett., 1991, 27, pp. 139-141 SKALARE, A.: ‘Determining embedding circuit parameters rrom DC measurements on quasiparticle mixers’, Int. J. Injrared Mil- lim. Waves, 1989, 10, pp. 1339-1353

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Low noise SIS receiver performance using Pb alloy tunnel junctions