A planar, quasi-optical SIS receiver operating at 230 GHz is described. The receiver consists of a 2 x 5 array of half wave dipole antennas with ten niobium-aluminum oxide-niobium SIS junctions on a quartz dielectric-filled parabola. The 1.4 GHz intermediate frequency is coupled from the mixer via coplanar strip transmission lines and 4:1 balun transformers. The receiver is operated at 4.2 K in a liquid helium immersion cryostat. We report accurate measurements of the performance of single receiver elements. A mixer noise temperature of 89 K DSB, receiver noise temperature of 156 K DSB, and conversion loss of 3 dB into a matched load have been obtained

Dengler, Robert J.

Leduc, Henry G.

Siegel, Peter H.

Stimson, Philip A.

English

NASA Technical Reports Server

Third International Symposium on Space Terahertz Technology Page 235
' ' N9
A PLANAR QUASI-OPTICAL SIS RECEIVER ~
FOR ARRAY APPLICATIONS ' ~
Philip A. Stimson, Robert J. Dengler, Peter H. Siegel and Henry G. LeDuc
Jet Propulsion Laboratory, Pasadena, CA, 91109
Abstract—A novel planar, quasi-optical SIS receiver operating at 230 GHz is
described. The receiver consists of a 2x5 array of half wave dipole antennas
with ten niobium—aluminum oxide—niobium SIS junctions on a quartz dielec-
tric-filled parabola. The 1.4 GHz intermediate frequency is coupled from the
mixer via coplanar strip transmission lines and 4:1 balun transformers. The
receiver is operated at 4.2 K in a liquid helium immersion cryostat. We report
here accurate measurements of the performance of single receiver elements.
A mixer noise temperature of 89 K DSB, receiver noise temperature of 156 K
DSB and conversion loss of 8 dB into a matched load have been obtained.
INTRODUCTION
The quasiparticle superconductor-insulator-superconductor (SIS) mixer is the most
sensitive detector in the millimeter-wave region and forms the basis of most high quality
receivers for millimeter-wave astronomy [l]. The quantum limit for noise temperature (in
a SSB mixer) has essentially been reached at 100 GHz [2] [6], but at higher frequencies the
available performance is poorer, with 10 times the quantum limit being a more realistic
goal. This figure has recently been reported from the best waveguide mixers around
200 GHz [3] [4] [5]. The major cause of the performance reduction at high frequency is
the SIS junction capacitance, which presents a smaller parallel reactance and shunts
the quasiparticle response. Tuning structures can, in principle, alleviate this limitation
but are not yet well understood at higher frequencies [8]. The approach most often
used, and that used here, is to fabricate high current density junctions with exceedingly
small areas (< 1 /Jm2) to reduce the capacitance. Another serious problem is control of
Josephson currents in the junction. Noise temperatures obtained with broadband hot
and cold loads may be highly inaccurate in the presence of Josephson currents. These
effects become more important as the frequency and/or bandwidth is increased. Other
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Page 236 Third International Symposium on Space Terahertz Technology
problems include losses in conductors and dielectrics, the fabrication difficulties of small
waveguide components and difficulties in obtaining convenient local oscillators.
Quasi-optical receivers with planar circuit mixers are an attractive approach for sys-
tems at frequencies in the neighborhood of 1 THz [7] [8] [9]. They suffer the disadvantage
of being fixed tuned but provide the advantage of convenient monolithic fabrication.
Planar configurations are also a desirable approach to realizing array receivers.
In this paper, we report accurate measurements on a quasi-optical array-type receiver
at 230 GHz. We have been able to suppress Josephson currents almost completely, and our
intermediate frequency versus bias voltage curve exhibits the smooth oscillatory behavior
of the best waveguide mixers [3]. Our configuration is designed to allow an array of mixers
to be measured during one cool down cycle. We report here the performance of a single
array element. We will report on complete array performance in a separate paper. The
SIS junctions used for these experiments were nominally identical to those used in recent
waveguide receivers [3] [4], with which our results may be compared.
SIS JUNCTION FABRICATION
The junction wafer used for this receiver carries a 2x5 array of resonant dipole an-
tennas with 0.4x0.4//m niobium-aluminum oxide-niobium SIS junctions at the termi-
nals. The junctions were fabricated using a self aligned lift-off trilayer process. The
niobium-aluminum oxide-niobium trilayer was sputtered onto the 0.25mm thick, 17mm
diameter quartz substrate through a photoresist stencil. The trilayer remaining af-
Fig. 1. The mixer block with the upper half removed. The central dielectric-filled parabola (dark),
containing 10 antenna and mixer elements, is surrounded by 10 IF baluns (light) and SSMA connectors
at the edge of the block.
Third International Symposium on Space Terahertz Technology Page 237
ter lift-off formed half of each antenna and the ten coplanar strip transmission lines
used for the. IF. The junction mesa was patterned using electron beam lithography on
1200 A thick PMMA over a 4000 A thick polyimide layer, followed by evaporation of
500 A of chromium metal and lift-off. The chromium stencil was transferred to the poly-
imide underlayer by reactive ion etching in an oxygen plasma. The contact regions of the
trilayer were then protected with a resist stencil and the chrornium/polyimide mask was
used to etch the junction. Thermal SiO was deposited using the same stencil to provide
electrical isolation of the base electrode and to provide dielectric for two RF blocking
capacitors located one quarter and three quarter wavelengths away from the junction
down the coplanar strips. The polyimide was then removed with dichloromethane. The
second half of the antennas was made by deposition of niobium and reactive ion etching.
RECEIVER DESIGN
The mixer block, shown in Figure 1, consists of the junction/antenna wafer, a quartz
reflector, and IF baluns and connectors mounted in a brass housing. The wafer is held
on the flat face of a quartz parabolic lens, whose rear surface is metalized. Incoming
radiation is reflected by the metal surface and focussed onto the antenna elements at
the center of the wafer. The configuration, called a Dielectric-Filled Parabola (DFP), is
analogous to a conventional parabolic dish antenna. The IF signals are coupled from the
wafer via coplanar strip transmission lines. Monolithic IF baluns transform the 200 fi
characteristic impedance of the coplanar strips to that of 50fl coaxial transmission line.
Details of this design, including extensive low frequency modeling, are described by Siegel
et al. [10] [16]. A superconducting magnetic field coil is mounted on the block to suppress
Josephson currents in the junctions.
chopper -. mixer block 12 IF cobles
bias
to flange
20 dB coupler isolator
otten
to flange
atten
IF amplifier
38dB
Voltage tuned
oscillator
variable temp
IF load
Fig. 2. Schematic diagram of the IF system of the array receiver. The entire system is immersed in
liquid helium except for the 77 K load which is bolted to the liquid nitrogen shield of the cryostat.
Page 238 Third International Symposium on Space Terahertz Technology
The IF system shown in Figure 2, consists of ten IF cables routed through two 6-
position coaxial switches and one 2-position switch to a single amplifier chain. The
remaining two switch positions are used to connect a short and a variable temperature
IF load to the amplifier input. The load consists of a resistor terminating a stainless
steel coax cable on a thermally isolated plate which contains a heater resistor and diode
thermometer. The structure is enclosed in an indium sealed can. This permits accurate
calibration of the IF system and very accurate mixer measurements [ll]. An isolator
is used to reduce the SWR at the amplifier input and a directional coupler with cooled
attenuators allows signals to be injected into the IF system to measure the mixer reflection
coefficient. After removal from the cryostat the IF signal is further amplified and passed
through a variable center frequency 50 MHz wide filter and fed to a power detector. The
IF system noise temperature is approximately 7K at 1.4 GHz.
The optical system consists of a chopper mounted directly in front of the mixer, and
the hot and cold loads. When the chopper blade is closed the input beam is directed
onto a 4 K (cold) load mounted on the receiver plate; when it is open the beam passes
through a quartz window to a 77 K (hot) load mounted on the liquid nitrogen shield of the
cryostat. The loads are pyramidal absorbers manufactured from Eccosorb CR-110, which
is known to provide high absorption and low reflection at this frequency. Reflection from
a flat plate of CR-110 has been measured at less than -10 dB in this frequency range [12].
The window is exactly five wavelengths thick and passes almost all the incident 230 GHz
radiation. The theoretical transmittance is 0.999; we measured a transmittance of over
0.95. Local oscillator radiation is produced by a Gunn diode and Schottky diode doubler
and is injected through the back of the mixer block. No diplexer is required.
The entire receiver is immersed in liquid helium which eliminates heat sinking prob-
lems. The dielectric constant of the helium is 1.048 [15]. The switches, thermometers,
Voltage (mV)
Fig. 3. Pumped and unpumped IV curves for a typical Nb-AlOr-Nb SIS junction used in the planar
receiver.
Third International Symposium on Space Terahertz Technology Page 239
liquid level meter...etc., and all data aquisition is controlled by a computer.
MEASUREMENT TECHNIQUE
We use a variation of the technique of McGrath et al. [ll], to obtain mixer gain and
noise temperature. First, the IF system is calibrated by plotting the temperature of
the IF load as a function of the IF output power. This measures the IF system noise
temperature TIP- The receiver noise temperature TR is measured using the hot and cold
loads (TH and TC), the ratio of the IF output powers Y = PIFH/PIFC and Equation 1.
TR = TH - YTCY - l
J_
LM
R
TIFH
TH
(1-P)
-TipcV
T J I
— lc / V
'1
1
-72
_ p
(1)
(2)
(3)
Next, the temperatures of the IF load, TIFH and TiFC) which produce output powers
PIFH and PIFC are calculated from the calibration, and the effective bath temperature
TS determined by measuring the power output from the IF system with a shorted input.
The IF reflection coefficient of the mixer F (and of the load 7) is measured by injecting
a signal from a voltage tuned oscillator through the coupler and recording the difference
in reflection between the mixer and the short. The loss into a matched load and noise
temperature are then calculated from Equations 2 and 3.
pa -21
V -22
O
CU
t. -23
- 4 - 2 0 2 4
Voltage (mV)
Fig. 4. IF output power as a function of bias voltage for hot and cold load inputs. The curve exhibits a
smooth oscillatory behavior similar to that expected from theory with no sharp spikes or discontinuities
indicating excellent control of Josephson currents.
Page 240 Third International Symposium on Space Terahertz Technology
RESULTS
Typical pumped and unpumped IV characteristics are shown in Figure 3, and IF
output power as a function of bias voltage for hot and cold load inputs is shown in
Figure 4. A superconducting magnet was used to suppress Josephson currents. The curve
exhibits a smooth oscillatory behavior similar to that expected from theory [13] [14] with
no sharp spikes or discontinuities. The IF output power is expected to decline towards
zero bias; the fact that there is some power output at zero bias indicates some remaining
Josephson currents which were not fully suppressed. These remain visible on the IF curve
even though the IV curve appears smooth. Nevertheless, we believe that this is the best
IF behaviour reported from a planar quasi-optical SIS receiver.
The most recent experiments performed with this receiver used junctions with an
area of 0.2/«n2. The normal state resistance was 56 fi, the critical current density was
15kAcm~2 and the u>RC product was approximately 1.3. The mixer and receiver noise
temperatures and mixer conversion loss are plotted as a function of IF frequency in Fig-
ure 5. The LO frequency was 230 GHz. The best results are obtained at 1.35 GHz where
a TM of 89 K DSB, a TR of 156 K DSB and conversion losses of 8dB (into a matched
load) were measured. The IF mismatch is approximately 1 dB across the IF band. Esti-
mated uncertainties in the noise temperatures are ±5K, and in the loss, ±0.5dB. These
values neglect any uncertainty due to RF load reflections or beam spillover. The largest
Y-factor was obtained on the first quasiparticle step below the energy gap, at a bias
voltage of approximately 2.3 mV. An inferior Y-factor was noted on the second step. The
500
400
cd^
300
200
0)
ai
'o 100
Loss-
Mixer
12
10
1.1 1.2 1.3 1.4 1.5 1.6
Frequency (GHz)
1.7
PQ
-d
CO
CO
o
OJ
*
Fig. 5. Mixer and receiver noise temperatures and mixer loss as a function of IF frequency. The best
results are obtained at 1.35 GHz where a TM of 89 K DSB, a TR of 156 K DSB and conversion losses of
8 dB were measured.
Third International Symposium on Space Terahertz Technology Page 241
mixer noise temperature and conversion loss are seen to be essentially constant across
the IF band. Mixer noise temperature is referred to the optically coupled loads at the
system input and includes the effects of all components through to the IF connectors at
the output of the balun transformers. The receiver noise temperature follows the noise
behavior of the IF amplifier.
At each data point on the curves, the change in IF reflection coefficient, and the
change in bias point, caused by switching between the hot and cold loads was measured.
This is necessary to ensure that the observed Y-factor is not produced by different LO
pumping conditions, or change in bias point when observing the hot and cold loads.
Different pumping would be expected to change the junction output impedance and the
IV curve shape. The reflection coefficient change was verified to be less than 1 %, and
the change in bias voltage less than 0.02 mV. This indicates that the observed Y-factor
has no appreciable component due to these factors.
Recent results from waveguide mixers at similar frequencies using junctions with
similar specifications from the same fabrication process [3] [4] give mixer temperatures of
48 K DSB and 60 K SSB and conversion losses of 2dB. Our noise temperature results,
although a factor of two higher, are consistent with these values given the lack of tuning
capability inherent in our planar circuit.
CONCLUSION
We have demonstrated a planar quasi-optical SIS mixer and low noise receiver which
is suitable for array applications. Best performance of an individual element at 230 GHz
was a mixer noise temperature of 89 K DSB, a receiver temperature of 156 K DSB and a
conversion loss of 8 dB. The IF output shows a smooth variation with bias, indicating good
control of Josephson currents. The noise results are consistent with recent measurements
using similar junctions in waveguide receivers, and are only a factor of two higher. The
conversion loss is rather large, but consistent with other planar mixer values. We will
report on array performance in a future publication.
ACKNOWLEDGEMENT
We are extremely grateful for the constant assistance and encouragement of Dr. W.R. McGrath,
without whom this work could not have been completed. We also thank Dr. H.H.S. Javadi and Dr.
M.A. Frerking of JPL, and Dr. A.R. Kerr and Dr. S.K. Pan of NRAO for useful advice and discussions.
We acknowledge the support of Mr. B. Bumble, Dr. J. Stern and Mr. S.R. Cypher on junction
fabrication, Mr. H. Moham for fabricating the array mount, and Mr. R. McMillan for fabricating the
quartz parabola. This work was carried out at the Jet Propulsion Laboratory, California Institute of
Technology under contract with the National Aeronautics and Space Administration.
Page 242 Third International Symposium on Space Terahertz Technology
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A planar quasi-optical SIS receiver for array applications

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