Page 238 - Satellite Communications, Fourth Edition

218 Chapter Seven

in 14/12-GHz transponders. With advances in field-effect transistor (FET)
technology, FET amplifiers, which offer equal or better performance, are
now available for both bands. Diode mixer stages are used. The amplifier
following the mixer may utilize bipolar junction transistors (BJTs) at
4 GHz and FETs at 12 GHz, or FETs may in fact be used in both bands.

7.7.2 The input demultiplexer
The input demultiplexer separates the broadband input, covering the
frequency range 3.7 to 4.2 GHz, into the transponder frequency chan-
nels. Referring to Fig. 7.13, for example, the separate channels labeled
1 through 12 are shown in detail in Fig. 7.15. The channels are usually
arranged in even-numbered and odd-numbered groups. This provides
greater frequency separation between adjacent channels in a group,
which reduces adjacent channel interference. The output from the
receiver is fed to a power splitter, which in turn feeds the two separate
chains of circulators. The full broadband signal is transmitted along each
chain, and the channelizing is achieved by means of channel filters con-
nected to each circulator, as shown in Fig. 7.15. The channel numbers
correspond to those shown in Fig. 7.13. Each filter has a bandwidth of
36 MHz and is tuned to the appropriate center frequency, as shown in
Fig. 7.13. Although there are considerable losses in the demultiplexer,
these are easily made up in the overall gain for the transponder channels.

7.7.3 The power amplifier
A separate power amplifier provides the output power for each transpon-
der channel. As shown in Fig. 7.16, each power amplifier is preceded by
an input attenuator. This is necessary to permit the input drive to each
power amplifier to be adjusted to the desired level. The attenuator has
a fixed section and a variable section. The fixed attenuation is needed
to balance out variations in the input attenuation so that each transpon-
der channel has the same nominal attenuation, the necessary adjust-
ments being made during assembly. The variable attenuation is needed
to set the level as required for different types of service (an example
being the requirement for input power backoff discussed later). Because
this variable attenuator adjustment is an operational requirement, it
must be under the control of the ground TT&C station.
Traveling-wave tube amplifiers (TWTAs) are widely used in transpon-
ders to provide the final output power required to the transmit antenna.
Figure 7.17 shows the schematic of a traveling wave tube (TWT) and its
power supplies. In the TWT, an electron-beam gun assembly consisting
of a heater, a cathode, and focusing electrodes is used to form an elec-
tron beam. A magnetic field is required to confine the beam to travel
along the inside of a wire helix. For high-power tubes, such as might be