The development of an X-band exciter, for use in the X-Band Uplink Subsystem, was completed. The exciter generates the drive signal for the X-band transmitter and also generates coherent test signals for the S- and X-band Block 3 translator and a Doppler reference signal for the Doppler extractor system. In addition to the above, the exciter generates other reference signals that are described. Also presented is an overview of the exciter design and some test data taken on the prototype. A brief discussion of the Block 3 Doppler extractor is presented

Johns, C. E.

English

NASA Technical Reports Server

N81-23838 
\A TDA Progress Report 42-89 January-March 1987 
Dual-Polarization 8.45 GHz Traveling-Wave Maser 
R .  6. Quinn 
Radio Frequency and Microwave Subsystems Section 
An 8.45 GHz dual-channel, dual-polarization traveling-wave maser (TWM) amplifier has 
been installed in the XKR solar system radar cone a t  DSS 14. The TWM is based on the 
Blk IIA 8.45 GHz maser structure, with two of the four maser stages being used for each 
channel, and each maser hay  then followed by a high-performance GaAs FETamplifier to 
achieve the desired net gain. A new shortened low-noise input waveguide and an orthogonal- 
mode junction which is cooled to 4.5 K feeds each amplifier chain. The rotation of an 
external polarizer permits the polarization of each channel to be defined as either linear or 
circular. A new circular waveguide switch was also developed to provide for noise calibra- 
tion and to protect the maser from incident transmitter power. 
1. Introduction 
The need by radio astronomers to receive simultaneous 
8.495 GHz radar signals of right- and left-hand circular polari- 
zation prompted the design and installation into the XKR 
cone at DSS 14 of an 8.45 GHz dual-channel, dual-polarization 
traveling-wave maser/closed-cycle refrigerator system (TWM/ 
CCR) which replaced the original single channel TWM/CCR. 
The new system has the following characteristics: (1) two 
channels with a center frequency of 8.45 GHz, a net gain 
of 245 dB, and an instantaneous bandwidth of 100 MHz, 
( 2 )  polarization of either channel selectable to right-circular 
polarization (RCP), left-circular polarization (LCP), or linear 
polarization, (3) system noise temperature of 20 K or less, and 
(4) use of the existing feedhorn and rotary polarizing section. 
II. Description 
The block diagram of the new TWM/CCR system developed 
for this project is shown in Fig. 1. It was determined that the 
requirements could be best met by dividing a Blk I1 8.45 GHz 
maser into two channels of two maser channels each, and 
incorporating an orthomode junction into the cryogenic pack- 
age, along with the maser amplifier structure. Therefore, only 
one waveguide input and vacuum window through the CCR 
interface would be required. The microwave feed system in 
the center position of the XKR cone provides separation of 
RCP and LCP by means of a rotating quarter-wave polarizing 
section and an orthomode junction. By indexing the polariz- 
ing section 90 degrees, the channel through which each signal 
(RCP or LCP) is amplified can be changed. A new and smaller 
orthomode junction was purchased from Altantic Microwave 
Corporation. Because of the reduced size of this device, it  
was possible to place it inside the CCR, where it would operate 
at 4.5 K, thus reducing its noise contribution. The existing 
feedhorn interface and polarizing section are 3.48 cm diameter 
circular waveguide. A smaller waveguide diameter was used in 
the design of the waveguide switch, vacuum window, and low- 
noise input structure in order to minimize the probability of 
RF mode conversion that might occur if the larger waveguide 
was used. An adapter was designed and supplied to join these 
two different size waveguides. The signal outputs come out 
through a pair of 0.36 cm diameter coaxial cables to hermetic- 
ally sealed SMA connectors at the CCR vacuum interface. Two 
94 
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other SMA connectors are available to couple noise calibration 
signals or test signals into the maser inputs via cryogenically 
cooled -30 dB directional couplers. A high-performance, low- 
noise GaAs FET post amplifier follows each maser channel to 
achieve the desired net gain. 
A. Traveling Wave Maser Assembly 
The Blk IIA maser structure [ l ]  is made up of four inde- 
pendent stages operating in a single superconducting magnet 
assembly and joined in series by coaxial semirigid cables. It 
was thus possible to divide the maser into two separate maser 
channels of two adjacent stages each. The maser gain and 
bandwidth of each channel is adjusted as a single unit by shap- 
ing the applied magnetic field to achieve an average gain of 8 
to 10 dB per stage and a -3 dB bandwidth of >lo0 MHz. 
6. Closed-Cycle Refregerator Assembly 
The entire maser/magnet assembly is cooled by 4.5 K by 
the same closed-cycle helium refrigerator (CCR) that is used in 
the Blk IIA systems. Modifications of the assembly were made 
to provide for maser output and noise calibration input signals 
and for the inclusion of the orthomode junction. The modifi- 
cation consisted of replacing the Blk IIA output waveguide 
and the 4-K station support with two tubular support assem- 
blies, through which the four coaxes enter the CCR. The radia- 
tion shields were redesigned to provide room for the added 
components and a heat strap was added for conduction cool- 
ing of the orthomode junction to 4.5 K. As was the case for 
the Blk IIA maser, the radiation shields were nickel-plated to 
protect the copper surfaces and reduce the radiant heat load 
on the CCR. When the refrigeration capacity measured lower 
than expected on this dual-channel system, it was discovered 
that electroless nickel rather than electroplated nickel had 
been applied to the shields. The result was a surface having 
poor infrared reflectivity. The infrared reflectivity was im- 
proved and the refrigeration capacity increased by gold-plating 
the shields. Reserve capacity measurements were made before 
and after gold-plating and with and without the low-noise 
input waveguide structure and orthomode combination. The 
results are shown in Table 1. 
Cryogenic temperature sensors from Lake Shore Cryotronics 
were installed in the CCR to measure the three refrigerator 
stage temperatures. Presently, there is no provision to monitor 
these sensors in the control rack, but the stage temperatures 
can be measured locally during maintenance. 
C. FET Post-Amplifiers 
Two high performance GaAs FET amplifiers were pur- 
chased from Berkshire Technologies, Inc. The FETs were opti- 
mized for operation at room temperature and have an ambient 
noise temperature of <125 K .  Each amplifier has an isolator 
on the input and output connectors to provide unconditional 
stability and to improve the VSWR. The amplifiers are mount- 
ed on the TWM/CCR mounting brackets on each side of the 
CCR. The power supplies for the amplifiers are built into a 
common enclosure that is mounted to the side of the maser 
package. 
The original system design called for cryogenic cooling 
of the post-amplifiers, but the performance of the maser 
exceeded that of the average Blk IIA maser, making the use of 
post-amplifier cooling unnecessary. Placing the post-amplifiers 
outside the CCR resulted in a higher reserve refrigeration 
capacity and hence a more reliable maser system. 
D. Orthomode Junction 
A special orthomode junction was made for JPL by Atlan- 
tic Microwave Corporation since the DSN’s primary frequency 
range of 8.4 GHz to 8.5 GHz fell between their standard cata- 
log models. The assembly is copper-plated for low electrical 
loss. The device provides isolation between cross-polarized 
components of >40 dB and a VSWR of better than 1.15: 1. 
E. Low-Noise Input Waveguide Assembly 
A new shortened input waveguide assembly was designed to 
allow for the added length of the orthomode junction within 
the same CCR vacuum housing. A cross-section diagram of the 
new assembly is shown in Fig. 2, and a photograph of the 
assembly with the cooled orthomode attached is shown in 
Fig. 3. Figure 3 also shows the existing Blk IIA low-noise 
input waveguide assembly for comparison. While the new 
design is shorter by >SO%, it provides an improvement in ther- 
mal isolation over the low-noise input assembly in the Blk IIA 
maser. This was achieved by replacing stainless steel construc- 
tion with (2-10 CR fiberglass as a rigid structural material. The 
normally poor infrared reflectivity of the fiberglass material 
was improved by vacuum-depositing approximately 1500 ang- 
stroms of gold. This assembly is able to maintain precise axial 
relationships between waveguide sections and maintain correct 
spacing of the RF choke joint gaps while minimizing the heat 
leak into the CCR through the structural material. As seen in 
Table 1 ,  this input waveguide assembly (and orthomode junc- 
tion) contributed 100 mW of heat load to the CCR. 
F. Output Coax/Support Assembly 
A pair of special coax/support structures was designed and 
installed in the CCR. These structures furnish structural sup- 
port for the maser/magnet assembly while providing an enclo- 
sure and feed through location for the four coaxial cables and 
connectors required for signal output and calibration signal 
95 
input. The 0.36 cm diameter coaxial cable used for output 
lines and noise source input lines was especially chosen for 
cryogenic applications. It has an outer conductor of stainless 
steel and an inner conductor of silver-plated beryllium copper. 
This type of coax is desirable in cryogenic applications because 
of its low microwave insertion loss and relatively low thermal 
conductivity. 
G. Waveguide Switch Assembly 
A waveguide switch is required between the feedhorn and 
the maser to protect the maser and receiver from incident 
transmitted power. The waveguide switch is also used to 
switch the maser to an ambient termination for system noise 
calibration. In a conventional waveguide switch, such as those 
used in DSN receiving systems, a rotating member is used be- 
tween the waveguide ports. This type of switching mechanism 
was not usable because of the mechanical and microwave per- 
formance difficulties associated with circular waveguide bends 
in a switch rotor. Therefore, a new kind of linear waveguide 
switch for circular waveguide was designed and built (shown in 
Fig. 4). A DSN-type drive mechanism (for compatibility with 
DSN control wiring) is used to move a sliding shuttle on linear 
bearings. At one end of the 5.08-cm travel, a through-path 
exists, while at the other end, the shuttle contains a tapered 
circular ambient termination (looking toward the TWM), and a 
short circuit (looking toward the feedhorn). 
H. Wide Tuning Range Pump Source 
The new TWM replaced a Blk 18.45 GHz maser that had 
been used on occasion at frequencies outside those normally 
used by the DSN. The tuning range of the maser within the 
particular structure bandpass is determined by the tuning 
range of the pump source, which is an electronically tunable 
klystron or Gunn effect oscillator. The source used to pump 
the Blk IIA TWM uses a pair of Gunn effect oscillaors designed 
to support maser operation from 8.4 GHz to 8.5 GHz. The 
oscillators are tuned by means of a negative biased varactor 
diode. The Radio Astronomy Group furnished funding for the 
purchase of specially designed Gunn effect oscillators that 
would allow the TWM to be tuned to the usual Blk IIA fre- 
quencies and also to 8.67 GHz. This higher frequency is re- 
quired for the Interstellar Microwave Spectroscopy task. The 
new oscillators, made by Central Microwave Corporation, are 
unlike the Blk IIA oscillators in that they require a positive 
tuning varactor voltage. The bias voltage change was accom- 
plished by making wiring changes in the maser pump source 
and in the solid-state pump controller and by replacing the vol- 
tage protection/modulation assemblies in the pump source. 
Because of these changes, the pump source and controller for 
this TWM are now a matched pair and are not interchangeable 
with similar components in Blk IIA maser systems. This new 
pump source allows TWM operation from 8.4 GHz to 8.7 GHz, 
with approximately 100 MHz of bandwidth. 
111. Performance 
The gain/bandwidth curves and noise temperature measure- 
ments shown in Figs. 5 and 6 were measured in the laboratory 
prior to replacement of the cryogenically optimized post- 
amplifiers with those optimized for room temperature opera- 
tion. The total gain is now about 4 dB less than shown, but the 
system noise temperature on the antenna was improved by 
0.5 K ,  for a total system temperature of approximately 20 K.  
The isolation between the two polarization channels was 
measured on the antenna with the use of a test probe mounted 
over the feedhorn. The isolation between RCP and LCP chan- 
nels exceeded 17 dB. 
IV. Conclusions 
This new dual-channel, dual-polarization TWM system 
demonstrates the ability to incorporate more than one maser 
amplifier into one CCR and also to lower the total system tem- 
perature by the cryogenic cooling of components that had pre- 
viously operated at room temperature. This accomplishment 
was due to improvements in FET post-amplifier noise perfor- 
mance, the availability of a compact orthomode junction, the 
design of a new cryogenic low noise input waveguide structure, 
and the development of a suitable waveguide switch in circular 
waveguide. 
References 
[ I ]  D.L. Trowbridge, “Block IIA Traveling Wave Maser,” TDA Progress Report 42-87, 
V O ~ .  July-September 1986, pp. 158-164, Jet Propulsion Laboratory, Pasadena, CA, 
Sept. 15, 1986. 
96 
Table 1. Reserve CCR heat capacity 
With 
low-noise input 
assembly and 
orthomode junction 
installed 
Without 
low-noise input 
assembly and 
orthomode junction 
Electroless 
444 MW nickel plating on copper 
radiation shields 
After gold 
plating of 690 MW 
radiation shields 
340 MW 
590 MW 
97 
ROTATING POLARIZING 
SECTION 
E- 
l 
WAVEGUIDE SWITCH 
CIRCULAR WAVEGUIDE 
VACUUM WINDOW 
LOW NOISE INPUT 
WAVEGUIDE 
ASSEMBLY 
ORTHOMODE JUNCTION 
WAVEGUIDE - 
COAX ADAPTORS ( 2 )  
-30 dB DIRECTIONAL 
COUPLERS ( 2 )  
TWM ( 2 )  
CALIBRATION INPUTS ( 2 )  
GaAs FET POST 
AMPLIFIERS ( 2 )  
OUTPUTS TO 
RECEIVER ( 2 )  
Fig. 1. Block diagram of 8.45 GHz dual polarization TWMlCCR 
70 KELVIN RADIATION SHIELD INTERFACE 
RF CHOKE GROOVE 
ATLANTIC MICROWAVE POLARIZER, 
CONDUCTION COOLED TO 4.5 KELVIN 
Fig. 2. Cross section of low noise input waveguide assembly 
99 
ORK##AL PAGE Is 
OF POOR QUAUTY 
60 70i 
m 50 
i 
-0 
< 
40 
30 
20 
45dB GAIN REF LINE ,-<\ 
- 
I 
8300 8400 8500 8600 
FREQUENCY, MHz 
Fig. 5. Gainlbandwidth curves, TWM tuned to 8.495 GHZ 
Fig. 3. New low noise input waveguide assembly and orthomode 
compared with Blk IIA low noise input waveguide assembly 
SWITCH SHUTTLE 
DRIVE MECHANISM 
WAVEGUIDE 
CHOKE JOINTS 
2 
: 31 8420 8440 8460 8480 8500 
8400 
TEST FREQUENCY, MHz 
Fig. 6. Equivalent input noise temperature vs frequency curves 
with TWM tuned to 8.45 GHz 
AMBIENT LOAD J LINEAR BEARING GUIDE 
Fig. 4. Cross section of circular waveguide switch 
100 


Block 3 X-band receiver-exciter

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