Classification of sources of interference

The following classification for sources of interference can be helpful:

• Natural and technical sources

• Sources having narrow-band and broad-band frequency spectrums

• Sources for conductor and radiated disturbance variables

• Power supply as source of interference

• Regular and unintended (leakage) sources

• Continuous and intermittent sources

Natural and technical sources of interference

We differentiate between natural and technical sources of interference:

Narrow-band interference source

Narrow-band sources of interference are sources having signals with discrete frequencies such as:

• Radio and amateur radio transmitters

• Transmitter receiver devices

• Radar stations

• Industrial HF generators

• Microwave devices

• Energy currents

• Welding machines

• Sound or FX receivers

• Ultrasonic devices

• Power converter circuits

These can generate substantial electromagnetic fields, primarily in the immediate vicinity.

Broad-band interference sources

Broad-band sources of interference of conducted and radiated disturbance variables are feared disruptors in electronic automation systems, since they have very high frequencies in addition to a wide frequency spectrum.

The following belong to the broad-band sources of interference:

• Motors

• Discharge lamps

• Circuit breakers (power switches)

• Isolating switches in energy supplies

• Noise

• Controller circuits with semi-conductors

• Switching devices (relay, contact)

• Electrostatic discharge

• Atmospheric discharge

• Corona

• Nuclear discharge

Sources of conducted interference, power supply (mains)

Conducted influences run through metal conductors (wires or conductive structures), transformers, coils and capacitors. Since conductors effectively work as antennas as well, the interference can also be converted into a radiated disturbance or vise versa.

Examples: Frequency spectrum of conducted disturbances:

Many of the previously mentioned interference sources are connected to the main supply. The respective disturbance variable is sent out onto the supply network and passed on from there. Therefore, the power supply network can itself be the source of continuous and intermittent interference.

Radiated sources Interference

If the dimensions of the components are small compared to the wave length of the disturbance, then the radiated influence can be monitored separately over the electrical and magnetic fields.

With higher frequencies, the electromagnetic field must be monitored as a whole. This means that all devices, in which higher frequencies are generated and on which components deliberately or accidentally work as antennas, are to be considered as potential sources of interference.

Example: Frequency spectrum of radiated disturbances:

Regular and unintended (leakage) sources

The differences between regular and unintended sources can be helpful in EMC work when monitoring frequency ranges for devices, in taking the appropriate measures for decreasing interference or in searching for unknown interference sources. The emission values of regular sources must be taken into account as part of the planning procedure.

Continuous or intermittent Sources

Differentiating between continuous or intermittent interference is required if for example, a certain influence should be shut off for timed operations of interference sources and receivers.

Example: Switching off receivers for weather