14.3 INTERFERENCE AND NOISE 649
x 1
x 2
x 3
x 1
x 1 s
x 2
x 2 s
x 3
x 3 s
fs fs
Transmission
medium
Input
signals
(a)
(b)
Output
signals
LPF
LPF
LPF
Commutator
LPF
LPF
LPF
Commutator
x 1 (t)
t
Figure 14.2.9(a)Basic TDM system.(b)Multiplexed waveform.
14.3 Interference and Noise
An information-bearing signal often becomes contaminated by externally generated interference
and noise and/or by internally generated noise. The demodulated message signal is generally
degraded to some extent by the presence of these distortions (attenuation, interference, and
noise) in the received signal. The fidelity of the received message signal is then a function
of the type of modulation, the strength of the additive noise, the type and strength of any
other additive interference, and the type of any nonadditive interference or noise. This section
only introduces some of the major causes of interference and noise, and touches upon some
methods of dealing with their effects in order to minimize the interference and suppress the
noise.
Interference may take several forms: ac hum, higher frequency pulses and “whistles,” or
erratic waveforms commonly known as static. Interfering signals can be seen to enter the system
primarily through the following mechanisms:
- Capacitive coupling,because of the stray capacitance between the system and an external
voltage
- Magnetic coupling,because of the mutual inductance between the system and an external
current
- Radiative coupling,because of electromagnetic radiation impinging on the system, partic-
ularly in the channel
- Ground-loop coupling,because of the currents flowing between different ground points
To minimize coupling from the inevitable sources, all exposed elements are usually enclosed
within conductingshields, which offer low-resistance paths to ground. When held at a common
potential, these shields are quite effective in reducing most types of interference. However, low-
frequency magnetic coupling can induce unwanted current flow through the shields themselves.
Then the shield connection has to be interrupted to avoid a closed-loop current path. An additional
