Interference and Screening

Interference is one of the most serious as well as most common problems in audio electronics. We encounter interference when it produces effects like noise, hiss, hum or cross-talk. If a radio engineer faces such problems, good theoretical knowledge as well as experience is required to overcome them.

However, it should be considered, that interference is always present. All technical remedies only aim at reducing the effect of interference to such a degree, that it is neither audible nor disturbing. This is mainly achieved by different ways of screening.

This paper will explain the technical background of interference and provides some common rules and hints which may help you to reduce the problems.

Theoretically, the effects and mechanism of a single interference can well be calculated. But in practice, the complex coupling systems between pieces of equipment prevent precise prediction of interference. The following picture shows the different types of interference coupling.

The different types of interference between the components of an electric system.

If we consider all possible coupling paths in the diagram above we will find 10 different paths. This means a variety of 1024 different combinations. It should be noted, that not only the number of paths, but also their intensity is important.

Having a closer look at the interference of cable, we find that hf-interference currents cause measurable levels on signal (audio) lines and on supply lines.

An ground-free interference source would produce signals on a cable which spread along the line. These voltages and currents can be called symmetrical interference. In practice this rarely occurs.

Symmetrical and asymmetrical interference.

Through interference, asymmetrical signals are produced in respect to the ground. The asymmetrical interference current flows along the two wires of the symmetrical line to the sink and via the ground back to the source. These interference signals are cancelled at the symmetrical input.

Galvanic coupling of interference occurs if the source and the sink of interference are coupled by a conductive path.

As can be seen from the equivalent circuit diagram, the source impedance of the interference consists of the resistance R_C and the inductance L_C of the conductor, which are common to the two parts of the circuit. From these elements the interference source voltage can be calculated.

The following diagram shows the inductivity of copper wires of different diameter and length.

Inductivity of copper wires of different diameter and length.

The capacitive coupling of interference occurs due to any capacitance between the source and sink of interference.

Principle of capacitive coupling of interference.

The current in the interference sink can be calculated as

The interference voltage in the sink is proportional to its impedance. Systems of high impedance are therefore more sensitive to interference than those of low impedance.

The coupled interference current depends on the rate of change of the interference u_i and on the coupling capacitance C_C. For typical arrangements of conductors the coupling capacitance can be calculated by the following formulas.

Capacitance of conductors.

Capacitance of parallel conductors.

Inductive coupling of interference occurs if the interference sink is in the magnetic field of the interference source (e.g. coils, cables, etc.)

Principle of the inductive coupling of interference.

The interference voltage induced by inductive coupling is

M is the mutual inductance between the two interfering parts of the circuit. The induced voltage is therefore proportional to this mutual inductance as well as to the rate of change of current in the circuits.

The following graph shows the relationship between the mutual inductance of two neighbouring wires.

Variation of the mutual inductance of two wires over a conductive surface.

Note that by the same means the capacitive as well as the inductive coupling of interference will be reduced.

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.

Principle of the coupling by e.m. interference.

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.

Interference and Screening