mechanism, and the relative levels of the desired and
b. Analog Signals.
(1) In an analog system. a continuously
(2) Since both the desired and interfering
variable waveform is transferred. Any interference
signals are present in the system output signal, the
entering the system is added to that waveform. resulting
output signal can be monitored with an oscilloscope or
in a composite of both the desired analog signal and the
other appropriate display device to indicate the
interfering signal: this is shown in figure 3-2. As the
presence and type of interference being experienced.
level of the interfering signal increases. the amount of
'With proper equipment and experience, an operator can
degradation in system performance encountered can .be
usually identify the type of interference (noise, digital.
gradual and the overall system performance will depend
radar, jammer, etc.) and thus determine the nature of
upon the types of interfering signals. the interference
Figure 3-2. Example of analog signal with random noise interference.
is detected and converted back to the original analog
c. Digital Signals.
form. Interference is not additive as in the analog case,
(1) Digital communications systems do not
but changes the relationship of the "ones" and "zeros"
transmit an actual analog input signal. Instead, the
for the particular sample; this is shown in figure 3-4.
initial signal is transmitted digitally. For example, the
Therefore, the interference does not appear directly in
analog signal may be sampled at some specific rate and
the output signal but causes a change in the amplitude
each sample converted to a digital signal representing
or some other characteristics of the reconstructed
the amplitude of the analog signal at the time it was
analog signal for the particular sample or samples.
sampled. Figure 3-3 shows a binary digital signal
consisting of "ones" and "zeros." This digital signal is
transmitted to the receiver where it