Interference and Screening

Interference filters are used to attenuate unwanted signals on signal lines. These may serve to attenuate signals leaving and entering the equipment.

In principle these filters are low pass filters which are designed to produce minimum damping for signals, but maximum damping for interference.

The following diagram shows the path for symmetrical and asymmetrical interference.

The path for symmetrical and asymmetrical interference.

With coils and chokes, interference currents can be attenuated on the signal lines.

Basically, there are two types of chokes used for interference filters: bar-core and ring-core chokes.

In symmetrical lines, two chokes are required for each conductor. This provides good attenuation for symmetrical and asymmetrical interference. The open core of these chokes will prevent saturation. However the inductance is low and for low frequencies and high currents the chokes are very bulky.

Interference filter with bar-core chokes.

Ring-core chokes:

These are used in symmetrical lines only. The closed core has a high permeability, which results in small dimensions of the coils. Both signal current directions are conducted through the coils. Therefore the symmetrical currents compensate, so that the coil will not be saturated, even by a high signal current. This type of choke will not provide attenuation of symmetrical interference on the line. This must be provided by symmetrical capacitors between the lines and ground.

Interference filter with ring-core chokes.

Examples of bar-core and ring-core chokes.

A wide variety of interference filters are available. For improvement or modification of a circuit these filters are a suitable choice. Designing them oneself is difficult and requires a lot of experience.

Different types of interference suppressers.

The main criterion for the selection of an interference filter is the frequency range of the signal and the frequency range of the interference. The signal should not be affected by the filter, while for the interference the attenuation should be as high as possible. This will only work, if the two frequency ranges are well separated.

Often the interference occurs in the form of pulses. Then the duration or the slew rate of the pulses becomes important.

If interference is caused by pulses, the duration of the slew rate of the pulses must be considered.

This graph shows, which attenuation is achieved for filters of a given corner frequency fg for different interference pulse lengths dt.

When mounting interference filters, the following aspects must be considered:

Interference and Screening