We summarize the development and the delivery of two SIS mixers for the 1.1-1.25 THz band of the heterodyne spectrometer of Herschel Observatory (HSO). The quasi-optical SIS mixer has two Nb/AlN/NbTiN junctions with the area of 0.25 um^2. The Josephson critical current density in the junction is 30-50 kA/cm2. The tuning circuit integrated with SIS junction has the base electrode of Nb and a gold wire layer. With the new SIS mixers the test receiver maximum Y factor is 1.41. The minimum receiver uncorrected DSB noise temperature is 450 K. The SIS receiver noise corrected for the loss in the optics is 350-450 K across the 1100-1250 GHz band. The receiver has a uniform sensitivity in the full IF range of 4-8 GHz. The sub-micron sized SIS junction shape is optimized to ease the suppression of the Josephson current, and the receiver operation is stable. The measured mixer beam pattern is symmetrical and, in a good agreement with the requirements, has the f/d =4.25 at the central frequency of the operation band. The minimum DSB SIS receiver noise is close to 6 hv/k, the lowest value achieved thus far in the THz frequencies range

Bumble, B.

Karpov, A.

LeDuc, H. G.

Miller, D.

Rice, F.

Stern, J. A.

Zmuidzinas, J.

Caltech Authors - Main

PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Development of 1.25 THz SIS mixer
for Herschel Space Observatory
A.  Karpov, D.  Miller, F.  Rice, J. A. Stern, B.  Bumble, et
al.
A.  Karpov, D.  Miller, F.  Rice, J. A. Stern, B.  Bumble, H. G. LeDuc, J.
Zmuidzinas, "Development of 1.25 THz SIS mixer for Herschel Space
Observatory," Proc. SPIE 6275, Millimeter and Submillimeter Detectors and
Instrumentation for Astronomy III, 62751U (6 July 2006); doi:
10.1117/12.684534
Event: SPIE Astronomical Telescopes + Instrumentation, 2006, Orlando,
Florida , United States
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Development of 1.25 THz SIS mixer for Herschel Space Observatory 
A. Karpova, D. Millera, F. Ricea, J. A. Sternb, B. Bumbleb, H. G. LeDucb, J. Zmuidzinasa
a California Institute of Technology, Pasadena, CA 91125, USA 
b MDL, Jet Propulsion Laboratory, Pasadena, CA 91109, USA 
ABSTRACT 
We summarize the development and the delivery of two SIS mixers for the 1.1-1.25 THz band of the heterodyne 
spectrometer of Herschel Observatory (HSO). The quasi-optical SIS mixer has two Nb/AlN/NbTiN junctions with 
the area of 0.25 um2. The Josephson critical current density in the junction is 30-50 kA/cm2. The tuning circuit 
integrated with SIS junction has the base electrode of Nb and a gold wire layer. 
With the new SIS mixers the test receiver maximum Y factor is 1.41. The minimum receiver uncorrected DSB 
noise temperature is 450 K. The SIS receiver noise corrected for the loss in the optics is 350-450 K across the 
1100-1250 GHz band. The receiver has a uniform sensitivity in the full IF range of 4-8 GHz. The sub-micron 
sized SIS junction shape is optimized to ease the suppression of the Josephson current, and the receiver operation 
is stable. The measured mixer beam pattern is symmetrical and, in a good agreement with the requirements, has 
the f/d =4.25 at the central frequency of the operation band. The minimum DSB SIS receiver noise is close to 
6 hv/k, the lowest value achieved thus far in the THz frequencies range. 
Keywords: SIS mixer, THz frequency, far infrared, low noise, Herschel space observatory, radio astronomy. 
1. INTRODUCTION 
We developed and delivered for further integration the two SIS mixers for the 1.25 THz band of the Heterodyne 
Instrument for Far Infrared (HIFI) 1 of the Herschel Observatory (HSO) 2. The goal of the HIFI project is to supply 
HSO with a heterodyne spectrometer with high resolution (up to 106-7), covering up to 1.910 THz to measure the C 
II line in the Milky Way and nearby galaxies. The HIFI should have a large instantaneous bandwidth to detect the 
spectral lines in the extra-galactic objects and to allow for the fast spectral surveys. The HIFI should use the state-of-
the art mixers to detect faint (isotopic) lines 1.
Our contribution for the HIFI is the development of the mixers for the Band 5 of the spectrometer covering 1.12 – 
1.25 THz with a mixer noise preferably in the 200-300 K range and with a broad 4-8 GHz Intermediate Frequency 
band. The other verifiable specifications are concerning the maximum mass of the mixer, the maximum current 
available for the electrical magnet, the beam width at – 11 dB and the ability to sustain the vibrations at the launch.  
The SIS receiver technology is a relatively new approach to the problems of the molecular spectroscopy in a Far 
Infrared part of the electro magnetic spectrum. Until the development of the NbTiN based SIS devices, other 
technologies, as Hot Electron Bolometer or Schottky mixers, were considered as a sole competitive approach to 
heterodyne spectroscopy in the 1-2 THz band. Our goal is to use the Nb/AlN/NbTiN SIS junction technology 
advance 3 to bring in THz range the possibility to build receivers with sensitivity close to the quantum limit, already 
achieved with SIS mixers at the frequencies below 1 THz 4.
Below we give a short description of the mixer structure. Then we describe the test installation and the mixer 
performance on the delivery for integration in HIFI.  
2. SIS MIXER  
In this work we are using a quasi-optical SIS mixer structure, similar to one described in 5. At the mixer chip we are 
using a double slot antenna with a two SIS junction array attached to the back side of a Silicon lens. For the signal 
mixing are used the two Nb/AlN/NbTiN junctions with the area of 0.25 ?m2 each. The Josephson critical current 
density in the junction is 30-50 kA/cm2. The tuning circuit integrated with SIS junctions has the base electrode made 
of Nb and a gold wire layer. For more details on the mixer chip layout one can see a paper 5. The SIS mixer general 
Millimeter and Submillimeter Detectors and Instrumentation for Astronomy III, 
edited by Jonas Zmuidzinas, Wayne S. Holland, Stafford Withington, William D. Duncan, 
Proc. of SPIE Vol. 6275, 62751U, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.684534
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n-6: a__
:1
luiJv
view is presented in Fig.1. The mixer housing with a circular hole in the middle of it front side serves to connect the 
Silicon lens and the mixer IF/DC board.  
Fig. 1. A general view of the mixer delivered for integration in HIFI of the Herschel Observatory. The mixer 
structure is solid and sustains well the vibrations levels possible at launch. 
3. TEST RECEIVER 
The receiver used at Caltech for the functional test of the 1.25 THz SIS mixers consists of an Infrared Laboratories 
LH-3 cryostat, of the local oscillator, and of the bias electronics. The cryostat vacuum window is in Mylar 12 ?m
thick. An infrared filter made of Zitex is located at the 77 K stage of the cryostat. The local oscillator power is 
coupled to the mixer beam using a Mylar beam splitter 5 or 12 micron thick. The intermediate frequency range is 4 
GHz – 8 GHz and the IF amplifier noise is about 3 K. During the test the physical temperature at the mixer block 
was about 2 K. 
The mixer beam pattern has been measured using the heterodyne detection of a hot black body (a heater) of a small 
size mounted on a two-dimensional translation stage. The black body signal was modulated with a chopper and 
detected using a lock-in amplifier.  
4. SIS MIXER PERFORMANCE IN THE TEST RECEIVER 
The test receiver noise temperature was measured by Y factor method. In the measurements we used liquid nitrogen 
cooled and ambient temperature calibration loads. The signal from calibration load was passing about 5 cm in open 
air before the test cryostat vacuum window. The receiver IF power was detected in entire 4-8 GHz band. The 
minimum uncorrected noise measured with a 5 micrometer thick beam splitter is about 450 K (Y= 1.41). The test 
receiver noise corrected for the optics loss is presented in fig. 2. The performance of the two mixers is almost 
identical. The receiver noise versus LO frequency has a nearly linear dependence, except some residual spikes 
related to the effect of the laboratory atmosphere. The most visible double maximum in the receiver noise is centered 
at the frequency of absorption line of water at 1163 GHz, and is due to the absorption of the cold load signal in the 
laboratory air. The twin structure of the maximum is related to the consecutive obscuration by absorption line in the 
upper and lower sidebands of the test receiver. It will not be affecting the receiver when the mixer will be used in 
vacuum at the HSO. In more details the role of the absorption in the laboratory air is demonstrated below in Fig. 3. 
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0100
200
300
400
500
600
700
1100 1120 1140 1160 1180 1200 1220 1240 1260
LO Frequency (GHz)
D
SB
 R
ec
ei
ve
r 
no
is
e 
(K
)
Mixer # 04
Mixer # 06
Fig. 2. Measured performance of the test receiver with the two different SIS mixers. The receiver noise is corrected 
for the beam splitter and for the cryostat window loss. The corrected receiver noise is below 500 K in entire 1.1 – 
1.25 THz band. The minimum corrected receiver noise is 330 K, about 6 h?/k. Note a double maximum in the noise 
centered at the frequency of an absorption line of water vapor at 1163 GHz. Some smaller spikes in the 1.2-1.24 THz 
band may be also related to the absorption in the laboratory atmosphere (Fig. 3) 
The estimated loss of the signal at the distance of 5 cm in the laboratory air is outlined in the Fig. 3 (dotted line). The 
Atmospheric Transmission at Microwave (ATM) model 6 developed by J.R. Prado et al was used for this 
calculations. When detecting with a double side band heterodyne receiver, the signal loss is averaged over the two 
sidebands. The solid line shows the transmission coefficient averaged over the 4-8 GHz IF side bands of a 
heterodyne receiver at each LO frequency. The shape and location of these minimums of transmission corresponds 
well to the locations of the maximums in the excess noise of the test receiver (Fig. 2). 
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1100 1120 1140 1160 1180 1200 1220 1240 1260
LO  frequency (GHz)
Tr
an
sm
iss
io
n 
 
Fig. 3 Absorption in the 5 cm of the laboratory air may affect the test data. The dotted line is the transmission as a 
function of frequency calculated using ATM model6 (courtesy of J.R. Prado). The solid line gives the transmission 
of the signal for a double side band heterodyne receiver, averaging the transmission over upper and lower sidebands 
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for each LO frequency. The receiver IF band is 4-8 GHz. One can mention a similarity in the shape and in location 
of the measured receiver noise spikes (Fig. 2) and the minimums in calculated averaged transmission (solid line). 
Uncorrected receiver noise  at LO  frequency 1128 
0
100
200
300
400
500
600
700
800
900
1000
4 5 6 7 8
IF (GHz)
D
SB
 R
ec
ei
ve
r 
no
ise
 (K
)
Test receiver DSB Noise temperature F lo = 1128 GHz
0
100
200
300
400
500
600
700
800
900
1000
4 5 6 7 8
IF (GHz)
D
SB
 N
oi
se
 T
em
pe
ra
tu
re
 (K
)
Fig. 4. The test receiver noise temperature versus Intermediate Frequency measured wit the two different SIS 
mixers. No corrections applied. The SIS mixer response is uniform over the entire 4 to 8 GHz IF band. Note the 
measured noise is below 500 K over the full IF band. 
SIS test receiver beam at 1180 GHz
Mixer # FM-06-09  and FM-04-1
at -11 dB the beam width  13.4 deg, f/d=4.25
-15
-13
-11
-9
-7
-5
-3
-1
-15 -10 -5 0 5 10 15
Angle (deg.)
A
m
pl
itu
de
 (d
B
)
Fig. 5. The receiver beam vertical and horizontal scans measured with the two different mixers. 
All the four measured beam profiles are nearly identical and fit well with the Gaussian approximation of the beam 
(solid line). 
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The uniformity of the receiver sensitivity over the full IF band has been verified using a spectrum analyzer in the Y 
factor measurements. The receiver noise measured at the LO frequency 1128 GHz for the two SIS mixers is 
presented in the fig. 4. The average receiver noise is close to the 450 K level, as mentioned before. The receiver 
response is uniform over the entire 4-8 GHz IF band. 
The mixer beam pattern has been measured using the heterodyne detection of a hot black body (a heater) of a small 
size mounted at an X-Y translation stage. The black body signal was modulated with a chopper and detected using a 
lock-in amplifier. The beam shape was measured in vertical and horizontal planes. The data obtained for the two SIS 
mixer at the LO frequency of 1180 GHz are presented in the Fig. 5. All the measured beam profiles are identical 
within the precision of the measurements and fit well with a Gaussian approximation (solid line without dots). The 
measured beam is symmetrical. At the –11 dB level the beam f/d ratio is about 4.2. 
5. SUMMARY 
We developed and delivered two low noise SIS 1.1 – 1.25 THz band mixers for installation in a Heterodyne 
Instrument for Far Infrared (HIFI) at the Herschel Observatory (HSO) 
On delivery mixer performance has been found to meet specifications for the use at HSO (mass, beam pattern, 4-8 
GHz IF band, RF band, noise, LO power limits, Josephson current suppression). 
The minimum test receiver DSB noise is 450 K ( 8 h?/k), the receiver noise corrected for the loss in the test cryostat 
optics is 330 K ( 6 hv/k), and the minimum mixer noise is 260 K ( 5 hv/k) are the lowest values achieved thus far in 
the THz frequencies range 
REFERENCES 
1. T. de Graauw and F. P. Helmich, “Herschel-HIFI: the Heterodyne Instrument for the Far-Infrared”, Proc. 
Symposium The promise of the Herschel  Observatory, 12-15 December 2000, Toledo, Spain, ESA SP-460, July 
2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, and R. Harris, pp 45 – 51, 2001. 
2. G. L. Pilbratt, “Herschel mission: status and observing opportunities”, in Optical, Infrared and Millimeter  
Telescopes, J.C. Mather, ed, Proc. SPIE 5487, 401 (2004) 
3. B. Bumble, H. G. LeDuc, J. A. Stern, and K. G. Megerian “Fabrication of Nb/Al-Nx/NbTiN Junctions for SIS 
mixer applications,  IEEE Trans. on Applied Superconductivity, Vol. 11, No. 1, pp. 76-79, 2001. 
4. J.Zmuidzinas, P.L. Richards, “Superconducting detectors and mixers for millimeter and submillimeter 
astrophysics”, Proceedings of the IEEE, Vol. 92, No 10, pp 1597-1616, 2004. 
5. A. Karpov, D. Miller, F. Rice, J. A. Stern, B. Bumble, H. G. LeDuc, J. Zmuidzinas, “Low Noise SIS Mixer for 
Far Infrared Radio Astronomy”,  in Millimeter and Submillimeter Detectors for Astronomy II, Proceedings of SPIE,
Vol. 5498, October 2004, pp. 616-621. 
6. J. R. Pardo, J. Cernicharo, and E. Serabyn, "Atmospheric Transmission at Microwaves (ATM): An Improved 
Model for mm/submm applications", IEEE Trans. on Antennas and Propagation, 49/12, 1683-1694 (2001) 
ACKNOWLEDGMENT 
We would like to thank Tom J. Phillips for useful discussions, Gary S. Parks, James M. Newell, and 
Margaret A. Frerking for the support of this work, Lorraine V. Valenzuela and Nancy Cowardin for the 
quality assurance work, and Ricardo A. Paniagua from Caltech mechanical workshop for the work on the 
prototype of the mixer housing and for production of the critical mechanical parts of the mixer delivered 
for installation at Hershel Space Telescope.  
This work was supported by a NASA grant. 
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Development of 1.25 THz SIS mixer for Herschel Space Observatory

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Development of 1.25 THz SIS mixer for Herschel Space Observatory

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