Page 131 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
CARRIER
WAVE

PHASE
MODUMTED
SIGNAL
Figure 5-8. Phase modulation. The phase of the carrier wave is changed in a
way to represent the information to be transmitted.

modulation is not used much in conventional (analog) communications
systems, but is widely used in digital communications schemes employed
by many satellite systems.

Demodulation

Demodulation is defined as the recovering of the baseband signal from the
received modulated signal. The basic process used is simply the opposite of
that used to modulate the carrier signal in the first place. For AM signals, a
device called an envelope detector senses the change in amplitude of the
received signal and reconstructs the baseband signal from this. Similar meth-
ods are used by FM demodulators (frequency detectors) and PM systems
(phase detectors). It is very important that the receiver be able to reproduce
the carrier wave exactly, as this is used by the demodulator to “subtract” this
component from the received signal (when you “tune” in a station you are
adjusting the receiver’s oscillator to match the desired carrier frequency).
Though demodulation may sound simple, the procedure is complicated
by the fact that the received signal is usually very weak, with the infor-
mation-carrying portion of the signal (PR) possibly weaker than the noise
signal (No). Also, due to its transmission through the atmosphere (and per-
haps the ionosphere), the signal may be significantly corrupted (changed
by interaction with the elements) which may affect proper recovery of the
information signal.

DIGITAL COMMUNICATIONS

Digital communications offers some benefits over analog communica-
tions, mainly in terms of the transmission, reception, and handling tech-
niques of the digitized information. Since more and more systems are