Figure 6.33
Driver-limiter-amplifier block diagram.
The device can be switched to an automatic gain control (AGC) mode, which
provides a constant drive as the uplink varies over a range of 10 dB or so. A
constant drive level is only appropriate for single carrier operation, however. AGC
also can be provided with a limiter that effectively clips the input signal and holds
it constant. A consideration with a limiter is that the input must not have significant
amplitude variations. That is because the limiter is a highly nonlinear device that
would produce undesirable intermodulation products or sidebands. The AGC can
be substituted with a commandable fixed gain step for use in multicarrier trans-
ponder mode.
The service quality of a bent-pipe transponder can be improved through the
introduction of a linearizer. By designing the circuit of the driver amplifier to
compensate for the nonlinear property of the SSPA or TWTA, it is possible to
provide a more linear overall service for applications involving digital transmission
or FDMA. An example of the improvement in IMD from a typical linearizer is
shown in Figure 6.34. The linearizer must be properly adjusted to cancel a portion
of the nonlinearity and, at the same time, to provide good overall performance for
any type of signal. Because that may not be feasible under all situations, the operator
would have the option of switching the linearizer out of the path.
Figure 6.34
Ka-band transponder linearizer IM. (
From:
[4]. Reprinted by permission of the American
Institute of Aeronautics and Astronautics, Inc.)
236
Spacecraft and Repeater
We have described the elements of the repeater section of typical communication
payloads. These devices support analog and digital services and can be used with
or without a digital processor. In Chapter 7, we review the design and performance
of the remaining payload element, the spacecraft antenna system.
References
[1]
Lyons, R. G.,
Understanding Digital Signal Processing
, Reading, MA: Addison-Wesley
Longman, 1997.
[2]
Craig, A. D., and F. A. Petz, ‘‘Payload Digital Processor Hardware Demonstration for
Future Mobile and Personal Communication Systems,’’ in E. Biglieri and M. Luise, (eds.),
Signal Processing in Telecommunications
, London: Springer-Verlag, 1996.
[3]
Bever, M., et al., ‘‘Broadband Payloads for the Emerging Ka-Band Market,’’ TRW Space
and Electronics.
[4]
Katz, A., et al., ‘‘Wideband TWTA Linearized Driver Amplifier for Ka-Band Satellites,’’
AIAA 2000 ICSSC.
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