Page 129 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecra# Applications
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tics. For example, if communications with a satellite in orbit is desired,
the signal frequency must be above the critical frequency of the ionos-
phere or the signal will not pass through into space.
As important as its propagation characteristics, carrier waves are also
chosen according to the amount of baseband information that must be trans-
mitted. Only so much information can be impressed upon a carrier wave
and still be recovered when the signal is received (and demodulated) by the
receiver. As a rough rule of thumb, it can be considered that a carrier wave
can only be impressed with basebandfiequencies up to about 10% of the
carrier wavefrequency. So, to transmit the human voice (around 4 kHz) a
minimum carrier frequency of about 40 kHz would be required.
As mentioned above, the baseband information must be impressed onto
the carrier wave for propagation between stations. In all cases, some char-
acteristic of the carrier wave, such as amplitude or frequency is modified
to represent the baseband information whether it is in analog or digital
form. The following sections describe the general modulation techniques
using analog baseband signals. Digital communication techniques are
very similar to these analog techniques and will be presented later in this
chapter after digital signals themselves are more fully described.

Amplitude Modulation (AM)

In amplitude modulation schemes, the baseband signal is used to
change the amplitude of the carrier wave in such a way as to represent the
baseband information. Figure 5-6 shows how an analog baseband signal
modifies a carrier wave to produce an AM signal.
Notice that the frequency of the carrier wave has not changed. Only the
amplitude varies and it does so in exactly the same manner as the base-
band signal. The AM signal will propagate between stations with the char-
acteristics of the carrier wave, but if we only look at its amplitude, we can
see the characteristics of the information impressed upon the signal.

Frequency Modulation (FM)

In frequency modulation, the frequency of the carrier signal is modified
by the baseband signal, as illustrated in Figure 5-7. Note that the ampli-
tude of the signal remains constant. The frequency variation from the
known carrier frequency is directly proportional to the frequencies of the
baseband (information) signal.