TRANSPONDERS, TWTAs, and SSPAs

Introduction

In a communication satellite serving the earth, the transponder transforms the received signals into forms appropriate for the transmission from space to earth. The transponder may be simply a repeater (a "bent pipe") that merely amplifies and frequency shifts the signals, or it may be much more complex, performing additional functions including signal detection, demodulation, demultiplexing, remodulation and message routing. In this section, the technologies of the major transponder elements are presented, with major emphasis on the transmitters and amplifying devices, i.e., the traveling wave tube amplifiers (TWTAs) and the solid state power amplifiers (SSPAs).

The information presents a "snapshot" in time, based upon visits to the major European and Japanese satellite communications entities (both governmental and private) during June and October 1992, respectively. The information gathered then and the subsequent review of the technical literature are synopsized to provide indications of the current technology levels in Europe and Japan.

Transmitters and Amplifiers

One of the key elements of any spacecraft transponder is the output transmitter, consisting of a high power amplifier (HPA) and the associated power supply. The amplifier is operated at or near saturation (maximum output power level) to attain a high overall efficiency of converting dc energy from the power supply into useful radio frequency (RF) energy that carries information. The transmitter is required to amplify the wanted signal without distortions and without other impairments which would decrease the usefulness of the signal. Two types of amplifiers (transmitters) are in common use, electron beam devices (commonly TWTAs) and solid state power amplifiers (SSPAs). For the TWTAs, the electrical power supply is often required to supply a number of high voltages (in the multikilovolt range), which presents a series of technology and design challenges.

Electron Beam Amplifiers (TWTAs). There remain only three major manufacturers of space qualified TWTs in the world, AEG and Thomson in Europe and Hughes in the United States. Both AEG and Thomson were visited. In Japan, NEC and Toshiba have built TWTs for space use, and both were visited.

Thomson is first in volume in the space TWT business. From the tube business of 115 million FFr/year, 20% of the revenue is applied to support R&D. The delivered output is produced by about 45 "hands on" employees (engineers, scientists and technicians) out of a workforce of 60. The staff is well experienced and has state-of-the-art laboratory facilities for development and evaluation.

Thomson's success is based on an emphasis on fundamentals. The design approach is modular construction, with one tube envelope used for a range of specific designs. Only several variables occur in designs (helix pitch, for example). New designs are small perturbations of past proven performance. A 14-point test program assures performance of the product. M-type cathodes are used to provide long life.

The specific performance levels achieved by Thomson are shown in Table 2.1 (provided by Thomson). In general, efficiencies are increasing (52% to 62% over time), mass is decreasing (down to typically 600 gm from 900 gm, previously). The HISPASAT TWT achieved 58% efficiency with a three-stage collector. Performance details on 32 and 60 GHz TWTs were not provided.

Future performance improvements are predicted by Thomson to be: 5% to 7% increase in efficiency using dual-taper helix designs, mass reduction of 100 gm, and linearity improvements.

In addition to being the leading manufacturer of space tubes, Thomson also produces high power tubes for earth station transmitters. Often the design for space tubes is based upon prior designs and technologies used for terrestrial tubes.

AEG has a long history of providing TWTs for space use and a progression of technology developments, as shown in Table 2.2 (provided by AEG). The Microwave and High Vacuum Devices Subdivision employs 400 to 500 employees and has revenues of 110 million DM/year. Technology development is mostly internally funded with some additional ESA and government of Germany funding. Products include tubes for radars, electronic countermeasures, ground stations, and satellites. 385 TWTs built by AEG have been put into orbit. The reported developments of AEG are shown in Table 2.3.

The success of AEG is based on developed techniques and proprietary methods for building tubes well. "All competing companies have access to the same basic technologies. Each can apply certain technologies better than the others."

To achieve increasing efficiency, long life, linear performance, and other desirable operating characteristics AEG uses several techniques. These include multistage depressed collectors, up to five stages, with magnetic focussing, anti-symmetric design, potential-free collector for better heat dissipation; and where applicable, double comb delay line, for higher power tubes than achievable with helix tubes. AEG is the only manufacturer in production with the feature of double comb delay line. AEG has its own in-house, unique, analysis software used for its designs. They manufacture their own cathodes. These mixed-metal cathodes "have 500 k hour life" and "no build failures due to cathode in last 2 1/2 years," with 1 amp/cm(superscript 2) loading, 930 to 990 degrees centigrade operating temperature. Heater power has been reduced from 3 W to 2 W.

Table 2.1
TWT Development Status at Thomson

Table 2.2
Design Heritage of AEG Low and Medium Power Space TWTs

Table 2.3
TWT Development Status at AEG

AEG uses improved helix taper designs to improve circuit efficiency. They have progressed from step taper to soft taper to HELOS which produces linear phase. In linearization, the AEG approach is to develop the best combination of TWT and external linearizer.

Planned future improvements by AEG to increase efficiency include improved collector design, removal of harmonics, improved magnetic and electrostatic lenses, minimization of conduction losses (by use of copper, better plating and improved heat transfer, reduced secondary emission) and reduced backstreaming electrons.

On the basis of information gained from European site visits and after comparisons to the U.S. space communications TWT manufacturer (Hughes), it is concluded that there is a strong, balanced competition between the European and U.S. TWT manufacturers.

In Japan, TWTA development has been strongly directed towards broadcast satellite applications. NHK, as Japan's public broadcaster, has supported NEC and Toshiba in the development of TWTAs for space use. NHK has in-house capabilities for modelling, design, and tests. The tubes are built by NEC and Toshiba. Japanese manufacturers have also built tubes for non-broadcast applications. Characteristics of several of the TWTAs developed by Japan that were discussed are listed in Table 2.4. SCR is carrying out additional TWT and TWTA R&D supported by Toshiba and NEC.

Table 2.4
Several TWTAs Developed in Japan

Actual hardware delivered into space shows the Japanese TWT industry to be trailing both Europe an U.S. However, the Japanese space TWT industry is developing quickly and is particularly strong in broadcast satellite applications.

TWT Transmitter Packages. The major European manufacturer of electronic power conditioners (EPCs) for TWT amplifiers is ANT. They combine their own EPC with, typically, an AEG or Thomson TWT to provide the complete amplifier or transmitter package. ANT was formerly a part of AEG, and close ties remain. Technology developments by ANT are approximately 50% internally funded and 50% outside funded. About 15% of annual Space Division sales of $80 million are spent for R&D. The successful business approach by ANT has been to apply well understood principles and techniques to their product line and to introduce new technology in small steps. The approach produces a highly reliable EPC with "no failures in orbit during cumulative mission life of 1.7 million hours."

As with the space TWTs, high efficiency is a prime objective for the EPCs. Typically ANT achieves an EPC efficiency (DC input to high voltage output) of 89%. A patented transformer design and special shaping of the 20 kHz waveform contribute to the high efficiency. The EPC high-reliability design and performance take advantage of open, high voltage construction.

ANT has built the EPCs for 260 W TWTs at Ku-band for direct broadcast satellite applications. The current approach to high power at Ku-band is to parallel two 110 W TWTs, powered by either one or two separate EPCs.

ANT has targeted EPC efficiency improvement to 93%. New technologies to be incorporated include application specific integrated circuits (ASICs) for the digital electronics and use of MOS-FETs instead of bipolar transistors. These targets are reached in the latest flight programs.

In Japan, NHK reports EPC efficiencies of 90% at 12 GHz and 89% at 22 GHz.

SSPAs. For lower frequencies and lower power applications SSPAs are often used as the final amplifiers in the spacecraft transponder. Also, for phased array antenna applications where the power gain is distributed over a large number of elements, SSPAs are used sometimes in the form of MMICs.

The status of SSPAs reported by the several manufacturers visited in Europe is given in Table 2.5. Amplifiers for space use have been developed from L-band through Ka-band. Typical power levels are 10 W (as needed for the applications) with efficiencies at 30% and above at the lower frequencies. At Ka-band, the achievable power levels are near 1 W unless device combining is used.

Table 2.5
SSPA Status Reported in Europe

Table 2.6 lists SSPA developments reported in Japan. Frequencies range from 2.5 GHz to 38 GHz, with both extremes being found on the ETS- VI satellite. A high power S-band amplifier has 42% efficiency. At 38 GHz, approximately 5% efficiency has been achieved.

Forecasts by European manufacturers for SSPA developments indicate that by the year 2000, SSPAs will be used "at frequencies up to 20 GHz and for RF powers up to 30 W," and "in active phased array applications" they will be used "at higher frequencies."

Transponders

Transponder complexity varies from the simple "bent pipe" approach to on-board processing (OBP) and on-board switching (OBS) transponders. Common elements include receivers, mixers, oscillators, channel amplifiers, and RF switches. OBP transponders may include additional elements of demodulators, demultiplexers, remodulators, and baseband switches. This section of the report covers the common transponder elements. OBP is covered in another section of the report.!#flag

Alcatel has built MMIC spacecraft receivers at several frequencies. At Ku-band, characteristics are noise figure (NF) less than 2.5 dB, gain of 69 dB, and mass less than 0.7 kg. At 30 GHz, using HEMT devices, characteristics are NF of 3 dB and 27 dB gain. Matra-Marconi showed a 14 GHz LNA of hybrid construction and with an NF of 1 to 1.5 dB.

Fujitsu has developed the LNAs using HEMT technology which provides an NF less than 1.8 dB and 15 dB gain at 24 to 26 GHz, and an NF less than 3 dB and 25 to 30 dB gain at 47 and 50 GHz.

For N-Star, MELCO is providing a 30 GHz HEMT front end receiver with 3 dB NF. For ETS-VI, NEC has built a 43 GHz HEMT receiver with an overall NF of 5.1 dB.

Input and output multiplexers for spacecraft transponders were displayed by Alcatel and ANT. ANT described an output multiplexer with 12 contiguous channels of 50 W each at 12 GHz. Input and output multiplexers shown were relatively large and heavy components with no evidence of miniaturization.

NHK has developed output multiplexers for broadcast satellite use. Those shown included an 8-channel, 200 W/channel multiplexer with 0.7 dB insertion loss, for contiguous channels; and a 4-channel multiplexer, 400 W/channel with 0.5 dB insertion loss, for non-contiguous channels. Both multiplexers were at 12 GHz.

Table 2.6
SSPA Developments Reported in Japan

Alcatel described programmable filters with changeable center frequencies and bandwidths from 400 kHz to 5.6 MHz. This component fits with the concept of changeable transponders where the characteristics are programmable in orbit.

Matra-Marconi provided the frequency generator for Inmarsat 3. It generates 30 frequencies in a five kg package, has low phase noise, and is described by the manufacturer as a "world beater." ANT has developed oscillators at 36.5 and 44.5 GHz using "whispering gallery mode" dielectric resonators with commercially available HEMTs. Their output is +7 dBm at 10% efficiency.

For IF switching on the spacecraft, ANT is developing a 5.2 GHz, 8 x 8 matrix switch with 20 dB gain and 60 dB isolation.

MELCO, in Japan, is developing a variable EIRP transponder concept for use at 22 GHz to combat the effects of rain attenuation. A combination of TWT power level changes and switching in an additional TWT is used to provide a power range of 60 to 320 W.

Thomson offered a view of future transponders that foresees regenerative/ reprocessing transponders, autonomous operations, many functions that are now on the ground incorporated into the satellite, and active array antennas used for both transmit and receive.

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Published: July 1993; WTEC Hyper-Librarian