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Noise

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Interference and Electromagnetic Compatibility

Noise is an unwanted (often random) signal added to the wanted signal. The noise obscures the wanted signal content. In an ideal world there would be 100% signal and no noise or interference. In real life, all signals come with added noise.

Devices should be designed to perform their intended function without accidentally behaving in other ways.

Minimising Noise

Use screened co-axial connecting cables.
Place electronic circuits inside screened metal cases to prevent unwanted signals getting in or out.
Be careful with earthing. Single point earthing is good.
Use ferrite rings to reduce high frequencies on the outside of screened cables.
Use filters so only the wanted signals are passed.
Decouple circuits to reduce noise getting in or out.

Noise Sources

Here are some example of what can go wrong with examples of noise sources ...

Magnetic tape hiss caused by random variations in the magnetic material coating the tape.
Thermal noise. This is caused by random movements of electrons because they are warm. It is most obvious when a radio receiver is not tuned into any station. You can hear the hiss generated by the first amplifier stage in the receiver.
Carbon resistor noise. The noise is caused by fluctuations in contact resistances between the carbon granules.
Lightning strikes. These cause short intense bursts of radio noise. (Impulsive Noise)
Radio noise from many sources such as industrial machinery and circuits being switched on or off (Impulsive Noise).
Big Bang background radiation. This can be detected with very sensitive receivers and radio telescope dishes.
Solar / stellar noise. The sun and stars radiate random noise radio energy. This can be detected with something as simple as an analogue TV receiver. It appears as a snow storm effect of the screen. With careful measurement, it is possible to detect more noise when the TV antenna is pointing at the sun.
Fluorescent lights radiate random radio interference modulated at 100Hz (Europe) or 120Hz (Americas).
Cathode ray tubes in older TV sets and computer monitors can radiate energy that damages radio reception.
Strong radio signals can leak into more sensitive systems.
On your HiFi or sound recording equipment, you might hear mobile phones using maximum power to connect to a base station.
Microwave ovens leak radiation which can show up on TV receivers.
CB, police, fire, amateur radio and other transmissions can leak into poor quality audio equipment in a similar way.
If you live near a powerful broadcast transmitter, you might pick up the transmissions on unexpected devices like hearing aids, false teeth or even from your bath drain!
Radar transmissions sometimes show up on HiFi or audio recording equipment. This has a very short distinctive buzz repeated every time the radar dish is pointing at your electronic equipment.
Cheap audio amplifiers pick up radio transmissions such as short wave radio. These signals ought to be filtered out using low pass filters.
Parasitic oscillations. This is when an amplifier (or voltage regulator) accidentally oscillates.
Motors transmit radio noise pulses caused by the sparks where the motor brushes connect to the commutator.
Computer networks can leak radiation that damages radio reception.
The sparks from car/bike/lawn mower/etc petrol engine ignition systems can cause serious interference.
Lightning strikes radiate large amounts of radio frequency energy over a wide range of frequencies.
Power line computer network adapters can obliterate short wave radio reception. There are on-going legal battles over whether these devices should be allowed in the home.

Cables are often fitted with a ferrite sleeve near the connector. This makes the cable very inductive and it behaves as a low pass filter blocking radio frequency radiation.