A cryogenically cooled Ka-band (33.6-GHz), seven-element front-end array for the DSN was built and tested. This system uses seven high electron mobility transistor (HEMT) low-noise amplifiers cooled by a two-stage closed-cycle refrigerator. All system components from the polarizers to the output isolators are cooled to a physical temperature between 18 and 35 K. The noise temperatures of the individual elements range from 64 to 84 K over a 2.75-GHz bandwidth

Bowen, J.

Neff, D.

English

NASA Technical Reports Server

TDA ProgressRepod 42-114
J -22_
, //0 AugustiS, 1993
N94-14375
A Cryogenic Seven-Element HEMT Front End for DSS 13
J. Bowen and D. Neff
RadioFrequencyand MicrowaveSubsystemsSection
A cryogenically cooled Ka-band (33.6-GHz), seven-element front-end array for
the DSN has been built and tested. This system uses seven high-electron-mobility-
transistor (HEMT) low-noise ampliIiers cooled by a two-stage closed-cycle refriger-
ator. AH system components from the polarizers to the output isolators are cooled
to a physical temperature between 18 and 35 K. The noise temperatures of the
individual elements range from 64 to 84 K over a 2.75-GHz bandwidth.
/
I. Introduction
There has been a desire to increase the operating fre-
quency of the DSN for the purpose of improving telemetry
capabilities to reduce spacecraft costs for a given perfor-
mance. Although improvements in gain and bandwidth
are achievable, problems associated with antenna defor-
mations and focusing become significant. This currently
limits the maximum operating frequency of the 70-m an-
tennas to 8.4 GHz [1]. The use of an array of feeds has
been suggested [2] and studied [3] as a method for correct-
ing these problems. Digital signal combining techniques
can then be utilized to recover the power losses associated
with the antenna deformations [4].
This article describes an array front end built to demon-
strate this technique at 32 GHz. The front end is described
and its performance characteristics are documented.
II. System Description
The input to the system consists of seven 22-dB-gain,
smooth dual-mode feedhorns arranged in a triangular grid.
The feedhorns with associated mode generators, waveguide
tapers, and vacuum windows bolt directly to the top plate
of the vacuum jacket (Fig. 1 shows a block diagram of a
single channel). Inside the jacket, a round-to-square wave-
guide transition is located between the feedhorn and the
hybrid polarizer. The left circularly polarized (LCP) port
of the polarizer is terminated while the right circularly po-
larized (RCP) port connects to a waveguide isolator that
provides input isolation to the low-noise amplifier (LNA).
At the output of the LNA is an output isolator similar
to the isolator at the input. The output waveguide sec-
tion is connected to the bottom plate of the vacuum vessel
and serves as the connection to an external downconverter
assembly, which contains follow-up filters and amplifiers.
These components were assembled in a single vacuum
jacket, 35.5 cm in diameter and 91.4 cm long. Cryogenic
cooling was provided by a single CTI model 1020 closed-
cycle refrigerator (CCR) (CTI-Cryogenics, Waltham,
MA). The amplifier bodies were cooled to less than 20 K
by thermally connecting them to the CCR cold station
with multiple layers of copper shim stock.
51
https://ntrs.nasa.gov/search.jsp?R=19940009902 2020-06-16T21:35:46+00:00Z
Thermal insulation at the input is provided by wave-
guide sections having a very high thermal resistance but
low microwave loss (see Appendix A). The input round-
to-square transition is a copper waveguide section sus-
pended in a fiberglass tube. The room-temperature end
of the transition is a brass flange bolted to the fiberglass
tube. The cold end is a one-piece copper flange and wave-
guide that has no direct physical connection to the room-
temperature flange of the top plate. The RF connection
is achieved with a choked circular waveguide flange at the
top. Additional heat straps are connected to the polarizers
to bring their temperature down to 35 K. With the polar-
izers at 35 K and the amplifiers at 18 K, the remainder of
the circuit components, waveguides, and isolators will all
be within the range of 35 to 18 K.
At the output section, thermal insulation is provided by
stainless-steel waveguide sections that have a thin copper
plating on their interior surfaces. In each output channel
are two sections, one that connects the room-temperature
flange to the first stage of the refrigerator (70 K) and
another that connects the first stage to the LNA output
waveguide. A short piece of flexible waveguide is incorpo-
rated in each channel to relieve mechanical stress caused
by thermal contraction.
A noise diode was provided for system calibration. The
output of the noise diode was split by an eight-way power
divider to each element of the array and coupled to each
LNA by a 20-dB coupler. The noise diode has an excess-
noise ratio (ENR) of 23 dB with 32 dB of coupling loss
between the diode and each LNA input.
Figure 2 shows the completed front end with the vac-
uum vessel and radiation shield removed. Instrumentation
is also shown.
III. LNA Performance
The high-electron mobility transistor (HEMT) LNA's
for the array were obtained from the National Radio As-
tronomy Observatory (NRAO), Charlottesville, Virginia
[5]. These amplifiers are a four-stage single-ended design
utilizing Fujitsu (FHR10X) HEMT transistors mounted in
a gold-plated brass chassis with microstrip matching cir-
cuits on CuFlon. The LNA input and output waveguide
transitions are E-plane probes coupled to a K-connector
bead which then couples to the microstrip circuit. The
LNA DC bias is supplied by a servo power supply exter-
nally located to the amplifier and vacuum vessel. This
supply permits adjustment of the individual stage drain
voltage and regulated drain current but requires individ-
ual leads for drain and gate voltages to the amplifier.
At a physical temperature of 20 K, this LNA design
provides a gain of 22 to 33 dB in the band of 26 to 36 GHz
(26 to 29 dB at 32 GHz). The noise temperature varies be-
tween 33 and 63 K in the band of 26 to 36 GHz (52 to 59 K
at 32 GHz). Figures 3 and 4 show the maximum, average,
and minimum gain, respectively, and noise temperatures
for the seven HEMT LNA's.
It should be noted that because of the large number of
LNA's required for this project, there was no attempt to
optimize the performance of each amplifier. Therefore, the
large variation in performance is indicative of the kind of
results to be expected in mass production.
IV. System Performance
A. Measurements
Noise-temperature and gain measurements were per-
formed on the system prior to installation in a 34-m beam-
waveguide antenna. The method of measurement con-
sisted of hot and cold sources alternately placed over each
feedhorn and measurement of the Y-factor (the ratio of
the two output noise powers). The hot load consisted of
a piece of microwave absorbing pad, the temperature of
which was monitored with a mercury thermometer; the
cold load was a section of the same type of absorbing pad
immersed in liquid nitrogen. A waveguide filter with a
passband of 31.20 to 33.95 GHz was placed at the output.
The receiver consisted of a follow-up amplifier, a preci-
sion adjustable attenuator, and a second follow-up ampli-
fier. The output of the second follow-up amplifier was sent
to the power meter directly without frequency conversion.
The attenuator maintained the power level within the op-
timum range of the power-meter sensor.
The first follow-up amplifier was of the same NRAO
design [5] as each of the array amplifiers. This provided a
low-noise temperature (285 K at room temperature) and
sufficiently broad bandwidth to cover the frequency band
of the output filter.
Measurements were also performed to determine the
gain of each channel and effective noise input provided to
each channel by the noise diode. The gain was determined
by placing an absorbing pad at room temperature at the
feedhorn aperture and sampling the total output power
(subtracting the power of the noise contribution by the
52
amplifieritself). Noise-diode temperature was determined
by again placing an absorbing pad at the feedhorn aperture
and sampling the power with the diode on and off. A Y-
factor was determined and used, in turn, to calculate the
noise temperature of the diode as seen at the LNA input.
B. Results
Table 1 lists the values determined for effective input
noise temperature as referenced at the feedhorn aperture,
Te, and the gain of each channel. The values for noise
contribution by the front end, Tie, were determined by
subtracting the receiver temperature (Trcvr = 285 K) di-
vided by the gain for that channel:
Trc_r
TI_ = T_ gain
C. Error Analysis
The equation for noise temperature is
Th - Y(T_)
T,= Y-1
where Te is the equivalent noise temperature of the front
end as referenced at the feedhorn aperture, Th is the phys-
ical temperature of the room-temperature pad, and T¢ is
the physical temperature of the liquid-nitrogen pad. The
error in Te can be characterized by [6]
/I cgTe dE, _ aTe _ OT¢
The noise diode added 35.5 K 4-1.8 K at the front end. or, in this case,
Y- 1 _]7:_-_)2 [crY]
Here gTh is the error in the hot-load physical temperature,
gT_ is the error in the cold-load physical temperature, and
gY is the error in the Y factor. The hot-load physical
temperature is measured by a mercury thermometer in-
serted into the pad. The ability to accurately read the
thermometer is limited to about 0.5 K, but the emissivity
of the pad can vary by as much as 1 percent, which leads
to an error at room temperature of as much as 3 K. At
the liquid-nitrogen temperature, this 1-percent error will
be much less significant, but the error in the temperature
sensor will be on the order of 1 K.
Here Ap is the difference between the power resulting from
the hot load, Ph, and the power resulting from the cold
load, Pc, in dB. The error in the Y-factor is
[ln(lO)l [lO[ae/l°]] (gAp)or = L--_--J
where gAP is then the RMS error generated by the stan-
dard deviations in the individual power measurements [6]:
Since the Y-factor is a ratio of tile power measurements,
its source of error rests in the errors of the power measure-
ment. The Y-factor is calculated by
Y = 10AP/1°
gAp = ¢crP_ + crP_
As a result, the error in the noise-temperature measure-
ments is 3.0 K.
53
V. Summary
A cryogenic seven-element Ka-band array front end has
been assembled and tested. The system noise-temperature
performance from 31 to 34 GHz varied from 64 to 84 K as
referenced to the feedhorns.
This system can be applied to any antenna that suffers
performance degradation due to surface deformations. Its
relatively wide bandwidth should be attractive to radio as-
tronomy applications as well as applications in deep-space
telecommunications.
Acknowledgments
The authors thank Marion Pospieszalski, Bill Lakatosh, and Ron Harris of Na-
tional Radio Astronomy Observatory for development of LNA's and thermal iso-
lators. Also, the authors thank Gary Bury, Juan Garnica, and Willard Smith for
operational support at DSS 13, and Gary Glass for assistance with error analysis.
References
[1] P. W. Cramer, "Initial Studies of Array Feeds for the 70-Meter Antenna at
32 GHz," TDA Progress Report 42-104, vol. October-December 1990, :Jet Propul-
sion Laboratory, Pasadena, California, pp. 50-67, February 15, 1991.
[2] P. D. Potter, "64-Meter Antenna Operation at Ka-Band," TDA Progress Report
4P-57, vol. March-April 1980, Jet Propulsion Laboratory, Pasadena, California,
pp. 65-70, June 15, 1980.
[3] S. :j. Blank and W. A. Imbriale, "Array Feed Synthesis for Correction of Reflector
Distortion and Vernier Beamsteering," IEEE Trans. Antennas Propagation, vol.
AP-36, no. 10, pp. 1351-1358, October 1988.
[4] V. A. Vilnrotter, E. R. Rodemich, S. J. Dolinar, Jr., '¢Real-Time Combining
of Residual Carrier Array Signals Using ML Weight Estimates," IEEE Trans.
Communications, pp. 604-615, March 1992.
[5] M. W. Pospieszalski, :j. D. Gallego and W. J. Lakatosh, "Broadband, Low-Noise,
Cryogenically Coolable Amplifiers in 1 to 40 GHz Range," 1990 IEEE MTT-S
International Microwave Symposium Digest, ppl 1253-1256, 1990.
[6] J. R. Taylor, An Introduction to Error Analysis, The Study of Uncertainties in
Physical Measurements, Mill Valley, California: University Science Books, 1982.
54
Appendix
Waveguide Thermal Insulators for Vacuum Systems
I. Introduction
Waveguide thermal insulators are used in the vari-
ous frequency bands to connect room-temperature signal
waveguides to 15-K ItEMT's or 4.2-K maser amplifiers.
New devices have been constructed for use at X-, K-, and
Ka-bands.
II. Description
A thermal insulator typically consists (see Fig. A-l) of
a room-temperature flange, a shim providing an 0.08-mm
gap, and a second cooled flange. The assembly is held
and supported by an outer shell of G-10 fiberglass tube
with a wall thickness of 0.76 mm. The gap dimension can
be varied by simply changing the shim to the thickness
desired; however, the 0.08-mm shim has proven optimum
(at all frequencies) for the RF and thermal characteristics,
providing a proper gap for room-temperature operation or
when cooled below 15 K.
III. Performance
The performance at room temperature or cooled is typ-
ically less than -30-dB return loss and the insertion loss
equivalent to a straight waveguide of the same length.
Having an abrupt change in the temperature gradient
(hot to cold) simplifies the calculation of noise temperature
for the various waveguide components as well as providing
the maximum amount of cooled signal path.
The heat leak is difficult to measure; however, it ap-
pears to be no worse at 32 GHz than a stainless-steel
waveguide of the same length with a thin copper-plated
inside diameter (three skin depths).
IV. History
Transitions of this type have been constructed by a
number of different organizations for different frequencies
and different applications. This adaptation was originally
undertaken by Ron lftarris and Sander Wienreb of National
Radio Astronomy Observatory. Their design used a unit
with a fixed gap; setting the gap dimension depended on
the skill of the machinist. Setting the gap for use at the
higher frequencies becomes much more difficult. The new
JPL design makes it much easier to control the gap as-
sembly at any frequency. An effect noticed after repeated
cold cycling was the apparent growth of the G-10 fiberglass
(from 2.54 to 5.08 pm). This was corrected by simply re-
lapping the flange faces.
The fiberglass is held both with small screws for me-
chanical strength and an epoxy adhesive (Armstrong A-
12) to reduce movement in the joint with repeated cooling
cycles.
The latest unit developed for 32 GHz allows an 8.71-
mm diameter round waveguide from an antenna horn to
be connected--in only 50.8 mm of length--to a 5.64-mm
square waveguide feeding a polarizer assembly (the device
is constructed using the wire EDM technique). The polar-
izer output then goes to an HEMT amplifier at 15 K. A
typical 4.5-K refrigerator system would require a second
thermal isolator between 70 K and 4.2 K.
This design has been used in the 32-GHz reflected wave
maser system, for the first 32-GHz R&D HEMT feed pack-
age at DSS 13 and the KABLE HEMT 33.6-GHz package
recently delivered to DSS 13, in the WR-42 waveguide for
the K-band (18- to 26-GHz) masers at the three 70-m sites,
and for X-band HEMT testing in a 15-K CCR dewar.
V. Summary
A new component is now available for use with the
higher frequency DSN CCR (vacuum) receiver systems;
the component offers reliability, low thermal heat leak, and
low microwave insertion loss.
55
Tabte 1. Noise-temperature and gain values for each elemenL
Noise Front - end
Element temperature Gain, dB noise
with receiver, temperature
Te, K TI,_, K
1 71.4 25.3 70.6
2 64.2 26.6 63.6
3 77.0 25.3 76.2
4 65.7 26.3 65.0
5 85.2 24.6 84.2
6 80.9 27.1 80.3
7 68.9 28.1 68.5
56
MODE GENERATOR
VACUUM WINDOW
THERMAL
VACUUM INSULATOR/
VESSEL ROUND-TO-SQUARE
TRANSITION
I
HYBRID POLARIZER
CALIBRATION IN q _ J20.dBCOUPLE R
J ISOLATOR J
I
V
L
I v,cuu_,.oo_I
BANDPASS
FILTER
Fig. 1. System block diagram of a single channel.
57
ORIGINAL PAGE
dL_,SK AND WHITE PHOTOC, R_,F_-_
o
Fig. 2. The front end with vacuum vessel and Instrumentation.
58
mz"
c_
32
3o
28
26
24
22
29
' I ' I ' I ' I ' I
0 MAXIMUM
[] AVERAGE
_ A MINIMUM
L I , I , I _ I , I I
30 31 32 33 34 35
FREQUENCY, GHz
Fig. 3. The maximum, average, and minimum gain of the seven
amplifiers as a function of frequency.
L6 60
n"
:D
I,--
nr
LU
CL
ILl
I,--
LLI
50
O
z
7o ' I ' I ' I ' I ' I '
0 MAXIMUM
[] AVERAGE
A MINIMUM
40 I I I I , I , I , I J
29 30 31 32 33 34 35
FREQUENCY, GHz
Fig. 4. The maxlmum, average, and minimum noise temperatures
of the seven amplifiers as a function of frequency.
59
Fig.A-1.Thermalinsulatorfor32GHz.
6O


A cryogenic seven-element HEMT front end for DSS 13

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