Electrostatic And Electromagnetic Interference
Interference by electromagnetic radiation becomes important at cable lengths greater than 1/7 of the wavelength of the signals. At frequencies beyond 30Mhz, most of the interference occurs by e.m. radiation
Charged persons and objects can store electrical charges of up to several micro- Coulombs, which means voltages of some 10kV in respect to ground. Dry air, artificial fabrics and friction favour these conditions.When touching grounded equipment, an instantaneous discharge produces arcing with short, high current pulses and associated strong changes of the e.m. field.
There are a number of methods to prevent interference. But all of them only reduce
the interference and never fully prevent it. This means there will never be a system which is 100% safe from interference. Because the efforts and the cost will rise with the degree of reduction of interference, a compromise has to be found between the effort and the result.
The requirement for the reduction of interference will depend on:
- The strength of the interference source
- The sensitivity of the interference sink
- The problems caused by interference
- The costs of the equipment
We will discuss ways of preventing interference, their effect, and the main aspects for the optimum efficiency of each method.