Q. Several forms of interference?

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, particularly 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 conducting shields, 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 layer of special magnetic shielding material may become necessary sometimes in extreme cases of magnetic-coupling interference. The grounding terminals, the equipment cases, and the shields are generally tied together at a single system ground point so as to prevent ground-loop current coupling interference.