Page 237 - Satellite Communications, Fourth Edition

The Space Segment 217

Figure 7.14 Satellite wideband receiver. (Courtesy of CCIR, CCIR Fixed Satellite Services
Handbook, final draft 1984.)

involving noise, it is usually more convenient to refer all noise levels to
the LNA input, where the total receiver noise may be expressed in terms
of an equivalent noise temperature. In a well-designed receiver, the
equivalent noise temperature referred to the LNA input is basically
that of the LNA alone. The overall noise temperature must take into
account the noise added from the antenna, and these calculations are
presented in detail in Chap. 12. The equivalent noise temperature of a
satellite receiver may be on the order of a few hundred kelvins.
The LNA feeds into a mixer stage, which also requires a local oscil-
lator (LO) signal for the frequency-conversion process. The power drive
from the LO to the mixer input is about 10 dBm. The oscillator fre-
quency must be highly stable and have low-phase noise. A second ampli-
fier follows the mixer stage to provide an overall receiver gain of about
60 dB. The signal levels in decibels referred to the input are shown in
Fig. 7.14 (CCIR, 1984). Splitting the gain between the preamplifier at
6 GHz and the second amplifier at 4 GHz prevents oscillation, which
might occur if all the gain were to be provided at the same frequency.
The wideband receiver utilizes only solid-state active devices. In some
designs, tunnel-diode amplifiers have been used for the preamplifier at
6 GHz in 6/4-GHz transponders and for the parametric amplifiers at 14 GHz