PROGRESS IN TWTS AND EPCS

Introduction

A principal element of any spacecraft payload is the transmitter, consisting of a power amplifier and its associated power supply. This amplifier is usually operated close to saturation (maximum output power level) in order to attain high efficiency of converting dc energy from the solar arrays into useful radio frequency (rf) energy that carries the communications. An efficient transmitter produces by nature signal distortions and other impairments that decrease the communications capacity. The best compromise between output power and distortion is a function of the communications signals being amplified.

Two types of transmitters are used in commercial satellites, traveling wave tube amplifiers (TWTAs) and solid state power amplifiers (SSPAs). In TWTAs, the power supply, often called the electronic power conditioner (EPC), supplies a number of high voltages (usually several kilovolts), which presents some design challenges.

Traveling Wave Tube Amplifiers

There remain only two major manufacturers of space qualified TWTs in the world, Thomson (including AERG, Ulm, Germany) in Europe and Hughes EDD in the United States. In Japan, both NEC and Toshiba have built TWTs for space use with NEC having a larger product line, however, these manufacturers do not currently have a substantial market share.

Market

Currently the worldwide TWT market (including commercial and military ground and space applications) is on the order of $500 million; the U.S. market alone is about $250 million. The worldwide market in commercial space TWTs was estimated at approximately $140 million in 1996. Including military applications, the total space TWT market might amount to as much as $250 million. Customers for space TWTs are the satellite manufacturers, and in particular payload manufacturers. These are not just located in the United States but are now distributed worldwide and include Russia. Worldwide production of space TWTs is currently about 1,200 tubes per year, the majority split between Hughes and Thomson. Most of these TWTs have a lifetime in excess of 15 years. Both these manufacturers have a complete product line from L-band to above 30 GHz with just about any rf power level desired.

All TWT manufacturers have the capability to build EPCs and integrate them into TWTAs. At Hughes the emphasis is on TWTs, EPCs, and integration of those components into TWTAs. There are several ways in which the TWT and the EPC can be integrated to form a TWTA. Today many times the TWTAs are integrated by the TWT manufacturer (Hughes EDD and NEC); frequently the satellite payload manufacturer procures the TWTs and integrates them with its own EPC (Hughes, Lockheed Martin) or uses a third party EPCs. In Europe the major manufacturer of EPCs is Bosch Telecom GmbH, formerly ANT. Each integrates its own EPC with, typically, Thomson (or AERG) TWTs to provide the complete transmitter package to a spacecraft builder. As with the space TWTs, high efficiency is a prime objective for the EPCs. Today's best EPCs achieve efficiencies of over 90%. (Hughes has demonstrated efficiencies as high as 94%.) Further development work (a switching speed increase to 150 kHz) is expected to lower the EPC mass of a Ku-band EPC (currently approximately 1,300 g) to less than 900 g.

In Japan both NEC and Toshiba have developed TWTAs for space use. For a 22 GHz broadcast application Toshiba developed a 230 w coupled cavity radiation cooled TWT, and NEC developed a helix TWT (1.1 kg) with variable power from 80 to 230 w (adjustable by ground command) and 54.4% peak efficiency. This TWT does not require a radiator for cooling thanks to diamond rod helix supports. The EPC for both tubes runs around 12 kV and both will fly on the COMETS satellite. NEC has developed a product line of tubes from S-band to 44 GHz, as listed listed in Table 3.3.

Table 3.3
NEC TWT Product Line

Frequency (GHz)

2.5

4

12

20

22

26

30

44

Rf - Power (W)

120

5

20-170*

2-30

80-230

20

20

20

* >250 W under development

On the basis of information gained from site visits the WTEC study team concludes that there is a balanced competition between the European and U.S. TWT manufacturers. Actual hardware delivered into space shows the Japanese TWT industry to be trailing both Europe and the United States.

R & D Activities

AERG provided an informative graph (Figure 3.8) showing the improvement of TWT efficiency over time and projections up to the year 2004.


Fig. 3.8. TWT efficiency vs. time (Thompson).

Improvement in TWT efficiency over the last few years is due in large part to very sophisticated software modeling and optimization using proprietary computer programs. The availability of new techniques and software to perform 3-D electromagnetics calculations has allowed designers to model TWTs much more accurately and has helped substantially in the design and optimization effort. Further developments of TWT technology will continue to improve the performance. Over the next few years TWT efficiency is expected to improve gradually; no major breakthroughs in technology are expected. For instance, diamond helix supports are expected to bring a small improvement in efficiency. Adding another collector is likely to increase the efficiency by another 2% while the EPC changes to accommodate a 5th collector are minimal. For all manufacturers, increased efficiency, reduced mass, and improvements in producibility are important goals.

In the future the top TWT efficiency will climb over 70%, operating frequencies and power levels will increase and the mass of both TWTs and EPCs will further decrease. Table 3.4 provides the current status of space TWTs at HEDD.

Table 3.4
Current Status of HAC EDD TWT Performance

Frequency band

Current production

Demonstrated

S-band

Rf output (W)

120

150

Efficiency (%)

62

64

Mass (g)*

1200

1200

C-band

Rf output (W)

120

140

Efficiency (%)

59

62

Mass (g)*

800

800

Model#

8556

8556#50

Ku-band

Rf output (W)

135

170

Efficiency (%)

65

70

Mass (g)*

850

700

Model#

8898

8815

Ka-band

Rf output (W)

70

140

Efficiency (%)

55

60

Mass (g)*

850

TBD

Model#

966H

9130H



Ka-band TWTs have 3 GHz bandwidth.
* add 350 g for radiation cooled option

The current status and forecast for Thomson/AERG space TWTs is listed in Table 3.5. The current performance of NEC space TWTs is listed in Table 3.6.

Table 3.5
Current Status of Thomson/AERG TWT Performance

Frequency band

Current Laboratory

Forecast in 2000

L-band

Rf output (W)

Efficiency (%)

52

60

C-band

Rf output (W)

60

120

Efficiency (%)

60

67

Ku-band

Rf output (W)

140

220

Efficiency (%)

72

78

Ka-band

Rf output (W)

75

100

Efficiency (%)

63

72



The efficiency of an S-band TWT would fall between L-band and C-band.

Table 3.6
Current Status of NEC TWT Performance

Frequency band

Current performance

S-band

Rf output (W)

120

Efficiency (%)

52

Ku-band

Rf output (W)

170

Efficiency (%)

66

Ka-band (21 GHz)

Rf output (W)

230

Efficiency (%)

55

V-band (44 GHz)

Rf output (W)

35

Efficiency (%)

41




Published: December 1998; WTEC Hyper-Librarian