THEORY OF INTERFERENCE.

The causes of interference vary and can be complex. To help in the solving of problems it is necessary to understand the concepts involved, which break down into several types:

1. Induced interference is caused by cables carrying radiating electromagnetic waves in close proximity to cables such as receiving cables, d.c. power supply cables or even metal parts. This is the principle on which transformers operate. Examples of unsuitable combinations would be VHF antenna cables or echo-sounder transducer cables close to the D.C. power cable to other equipment - the effects could be to transmit the clicking sound of the echo-sounder pulses into the entire D.C. system of the boat, which could affect virtually anything. This type of interference can be prevented by good installation practise involving the separation of unsuitable cables and by the use of suitable screened and earthed cable conduits to enclose sensitive cables. A major culprit in causing this type of interference is networking, where data is shared between different pieces of equipment, although manufacturers attempt to reduce it by the use of high quality screened cables. Modern microprocessor based designs contain clock oscillators as part of their design - these produce signals at specific frequencies (e.g. heard at one channel only on a VHF). Interference can be discouraged from travelling along D.C. supply cables to reach sensitive equipment by winding positive and negative supply conductors around ferrite toroids - a practise recommended by many marine electronics manufacturers. The toroids should be positioned close to the equipment rather than to the circuit breaker panel and the conducting cable wound around the outer ring of the toroid. This has the effect of allowing through a constant D.C. supply and rejecting sudden surges as in interference pulses. Where the interference is being generated at high frequency by computerised systems or networking, it can be reduced or eliminated by de-coupling the signal to ground by the use of small specialised filter capacitors - connecting them either across signal cable pairs or between cables and ground, usually at the point of input into equipment affected. This can reduce corruption on NMEA data cables by removing the unwanted 'spikes' that switch logic levels from one state into another.

2. Radiated interference is caused by the generation of electromagnetic waves which are of fairly high power being received by other circuits which act as receiving circuits. Certain types of petrol engine ignition systems use control devices known as thyristors. These can generate high voltage spikes which are then received by other circuits in the boat. Radiated interference can be received by metal rails which are not earthed and then re-radiated out to other parts of the boat. The prevention of this is by good design, which can involve fitting the vessel with a signal ground plate (when out of the water) and laying an internal connection cable or copper strap from that plate to any metalwork that could potentially carry interference. A good understanding of electolytic corrosion is required if carrying out this procedure to avoid inititating electrolytic corrosion. The same signal ground plate can be used to provide a signal earth for M.F. or H.F. radio equipment or to earth non-ferrous or screened cable trunking to prevent radiated signals being received by cables within that trunking. It is essential than no connection is made to battery negative or corrosion could develop. Alternators with worn slip-rings and brushes cause arcing and can result in radiated interference when the engine is running. Arcing brushes can sometimes be helped by fitting suppression capacitors as used to reduce interference from car alternators or heater motors - the capacitors are fitted across the positive and negative supply terminals to the motor causing the interference (e.g. water pump, alternator or windscreen wipers).

3. Electrostatic interference can be generated by moving parts that do not normally carry electrical current. An example of this would be a rotating propellor shaft which is isulated by the gearbox coupling or oil and this type of interference is only seen when under way and the shaft is spinning.

All these types of interference can become integrated and the descriptive names can vary, dependant on the exact cause.. Other terms such as R.F.I (Radio Frequency Interference) and E.M.I. (Electromagnetic Interference) are sometimes used. British Standard BS 1597 defines the permitted level of interference over a frequency of 15 KHz to 100MHz on board ships, but does not cover the more insiduous low frequency interference that frequently causes regular clicks and buzzing sounds on radios. Different construction classes apply to vessel construction methods within this standard and European legislation sets down standards that marine electronic equipment must meet to gain type approval.

THE FIRST STEP IS TO IDENTIFY THE TYPE OF INTERFERENCE. This can be involved and my suggestions are not in any particular order. To some extent it accepts that you have some means of HEARING the interference - e.g. The VHF radio is 'clicking'. The suggestions are kept as simple as possible for the non-technically qualified individual. Other methods of 'seeing' interference are possible but require special equipment such as an oscilloscope.

1. Switch off each equipment individually until the interference stops. This can identify the equipment responsible for creating the interference source.

2. Substitute a small completely separate supply battery to temporarily power the equipment on which interference is heard, keeping the cable length to a minimum. This can identify if the interference was being fed along the D.C. supply cable. Please ensure that there is no common negative ground connection as this can act as a signal ground, resulting in the reception of radiated interference which would give misleading results.

3. Wrap equipment with LCD screens etc. in aluminium foil and temporarily earth the foil. This can identify if interference is being radiated from the face of some equipment.

 

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