Page 141 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
Demodulation. Demodulation methods are similar to those described earli-
er for analog signals in their use of envelope, frequency, and phase detectors.
For digital demodulation, the demodulator must only detect the presence of
a transmitted “one” or “zero” which is relatively more simple than recover-
ing the actual analog baseband signal from the received signal (plus noise).
This is one of the advantages of digital communications over analog systems,
as it allows a successful link with lower minimum signal-to-noise ratios.
An important factor for successful use of digital signals for communi-
cations is ensuring that everyone on the link is synchronized in time. Mod-
ification of the carrier wave by the transmitter modulator only occurs each
bit time (which, as we’ve seen, can be measured in nanoseconds), and dig-
ital demodulators are looking to detect changes in the received signal at
these times only as well.
COMMUNICATIONS SYSTEMS

Figure 5- 15 displays the components of a typical communication system.
In the transmitter, the source represents the originator of the information
signal (FLtl). This may be the microphone of a radio or telephone; the
video receivers of a television camera; the antenna of a remote sensor; or
whatever is picking up or originating the desired information, turning it
into electromagnetic signals suitable for transmission. If the signal is to be
digitized, an An> converter would be part of the preparatory electronics
that make a signal ready for modulation onto the carrier wave which
occurs within the modulator section of the transmitter. The anzpZ$er

TRANSMITTER

MODULATOR H AMPLIFIER H ANTENNAF CHANNEL+ x(t)

RECEIVER

Figure 5-1 5. Communications systems. Many of the common components
of a typical communications system are depicted.