Interference and Screening
Interference and Screening
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.
This is one of the simplest but most efficient methods to reduce interference. Grounding can be used for three different purposes:
1. Protection Ground
Provides protection for the operators from dangerous voltages. Widely used on mains-operated equipment.
2. Function Ground
The ground is used as a conductive path for signals.
Example: in asymmetrical cables screen, which is one conductor for the signal, is connected to the ground.
3. Screening Ground
Used to provide a neutral electrical path for the interference, to prevent that the interfering voltages or currents from entering the circuit.
In this chapter we will only consider the third aspect. Grounding of equipment is often required for the cases 1 or 2 anyhow, so that the screening ground is available "free of charge".
Sometimes the grounding potential, provided by the mains connection, is very "polluted". This means that the ground potential itself already carries an interfering signal. This is especially likely if there are big power consumers in the neighbourhood or even in the same building. Using such a ground might do more harm than good.
Example of interference via a "poluted" ground line.
The quality of the ground line can be tested by measuring it with a storage scope against some other ground connection, e.g. a metal water pipe or some metal parts of the construction.
If necessary, it is advisable to use an alternative "clean" ground connection as screening ground for the equipment. If it has to serve as protection ground too, it must have good ground contact. Use a earthing-resistance-meter to measure low resistance (<5 Ohms) against ground.
Never use the Neutral (N) of the mains as ground.
It might contain strong interference, Because it carries the load current of all electrical consumers.
The grounding can be done by
single-point grounding or by multi-point grounding.
Each method has advantages which depend on the frequency range of the signal frequencies.
The principle connection for the grounding of circuits.
SINGLE- POINT GROUNDING:
All parts to be grounded are connected to one central point. This results in no "ground loops" being produced. This means the grounding conductors do not form any closed conductive path in which magnetic interference could induce currents. Furthermore, conductive lines between the equipment are avoided, which could produce galvanic coupling of interference.
Central grounding requires consistent arrangement of the grounding circuit and requires insulation of the individual parts of the circuit. This is sometimes very difficult to achieve.
A system using the single-point grounding.
MULTI-POINT GROUNDING:
In multi-point grounding all parts are connected to ground at as many points as possible. This requires that the ground potential itself is as widely spread as possible.
In practice, all conductive parts of the chassis, the cases, the shielding, the room and the installation are included in the network. The interconnection of these parts should be done at as many points as possible.
For the selection of either type the following rules should be considered:
- For systems where the signal lines (and supply lines) are shorter then 1/7 of the signal's wavelength, single-point grounding should be used.
- For systems where the signal lines (and supply lines) are much longer then the signals wavelength, multi point grounding should be used.
A system using the multi-point grounding.
- Systems, where neither of the two cases are present, produce a problem (wide band circuits, e.g. video circuits). To find out which of the two types of grounding produces the best results, a special evaluation of the system is necessary .
Alternatively, the so-called Triax-system can be used. This is a combination of single and multi-point grounding. The resulting circuit is more complex and consequently more expensive.
The principle of the Triax grounding.
When considering the effect of electrical and magnetic fields, we have to distinguished between low and high frequencies. At high frequencies the skin effect plays an important roll for the screening. The penetration 
describes the depth from the surface of the conductor, where the current density has decayed to 37% compared to the surface of the conductor.
Decay of the current density in the surface of a conductor due to the skin effect.
In practice, we can assume that at a depth of 5, the current in the conductor has decayed to zero. The following table shows the penetration for various conductor materials at different frequencies.
The penetration of current for different materials as a function of frequency.
When signal lines run close to interference sources or when the signal circuit is very sensitive to interference, screening of signal lines will give an improvement.
There are different ways of connecting the cable screen:
Three different ways of connecting the cable screen.
Cable screen not connected.
Cable screen grounded on one side only.
Cable screen grounded on either side.
When connecting cable screens the following rules should be considered:
- Outer cable screens should always to be connected to the ground on both sides. Good contact all around the screen with low resistance and inductance should be observed.
- The inner screen (if present) should be connected either on one or on both sides to the ground, depending on which type of grounding is used.
- When a cable passes through the case, it should be connected to the case.
The following rules for cabling further reduce the interference:
- Symmetrical lines with twisted pair cables should always be preferred over asymmetrical lines with coaxial cable.
- The two wires of one signal should be as close to each other as possible. If possible, they should be twisted and within one screen.
- Avoid using the cable screen for the signal path. (use symmetrical lines).
- Avoid using the case or chassis for the signal path.
- Separate sensitive signal lines (e.g. audio cables) from cables producing interference (e.g. mains cables, H.F. cables).
- Mount signal cables close to conductive surfaces (case or chassis).
- Do not mount cable across openings in the case.
- Choose single-point grounding.
- Ensure proper and careful connection of the screens.
- Use suitable plugs in connection with the cable screen.
For A.F. frequencies up to 10kHz, the normal copper screen of cable is only of limited efficiency. For difficult cases of interference screening hoists of iron screen can be used.
TWISTING OF CABLES
For symmetrical signal lines, the twisting of two signal wires alone produces already very good protection against interference. Because it produces almost no extra costs, it is at the same time a very cheap means of interference reduction.
When an interfering signal effects the twisted wires, it will produce in both wires a signal with the same direction and (almost) same intensity. From the point of view of the signal's sink, these signals will have opposing directions and will therefore cancel out.
The use of symmetrical lines with twisted pair cables is the most powerful means of avoiding interference. They should be used whenever possible, especially for signals sensitive to interference. (e.g. microphone lines)
Note that the advantages of symmetrical lines and twisted pair cables can only be used, if the signal's source as well as the sink have balanced outputs and balanced inputs respectively.
Housings and cases around electronic equipment are meant to protect the sensitive parts from interference, but also to prevent any interfering signal produced by the equipment from escaping it and disturbing others.
Electrical fields can be attenuated sufficiently by very thin layers of conductive material (e.g. plastic metallized surface).
Damping of magnetic fields, especially of low frequencies (e.g. mains frequency), requires well-conducting surfaces of sufficient thickness.
The diagram shows the damping of different materials and thickness at different frequencies.
A minimum interference damping of 60dB is recommended for cases.
Interference can also enter the case via any opening in the case. Therefore, a tightly closed case is very important for avoiding interference.
This illustration shows all methods for improving the interference attenuation of the case of an equipment.
Interference and Screening