Noise-reducing measures start with knowing about the different types and properties of noise.

Noise sources: natural noise, artificial noise, external noise, and internal noise

Tracking noise to its origins, we find two major noise sources: natural noise existing in the natural world and artificial noise created by human-made objects or phenomena.
Typical examples of natural noise are atmospheric static noise (lightning), solar noise, cosmic noise, etc.
Human-made objects powered by electricity usually emit noise. Typical examples of such objects include spark plugs, switches, and electric motors. Equipment we use in our daily life, such as microwave ovens, TV sets, PCs, and smartphones also emits electromagnetic noise.

A noise source analysis may be carried out by focusing on whether a noise source is external or internal. Whether a noise source is a natural or artificial one can be determined fairly easily. Determining whether a noise source is internal or external however, requires the point of reference to be the object affected by the noise. For this reason, an internal noise source may become an external noise source and vice versa depending on the situation. Natural noise mentioned above is usually external noise. Artificial noise, on the other hand, can be both external noise and internal noise.

For example, in the case of a TV set, a lightning surge from the AC line counts as natural external noise. In rare cases, a car passing near a TV set can cause a disturbance of its picture and sound, in which case radio waves from the car's spark plugs, etc., are creating the disturbance and are therefore treated as artificial external noise to the TV set. The TV set also houses a switching power supply, high-speed CPU, image processing LSI, etc., which emit switching noise and other noise characteristic of high-speed digital signals. These are treated as artificial internal noise. The TV set itself is immune to such noise because of the anti-noise measures incorporated in it. However, a radio brought closer to this TV set shows increasingly poor reception, and emits distorted sounds. In this case, the radio is affected by the artificial external noise emitted from the TV set.

The way noise propagates: conduction noise and radiation noise

Noise can be classified into two types: conduction noise and radiation noise.

Conduction noise propagates through power/signal cables and conductors such as thin-film wiring lines on a printed board. This noise travels along both power lines and signal lines. A power line transmits surge, ripple, and spike noise. A signal line transmits flicker (1/f), shot, and thermal noise from analog systems and crosstalk and reflection (ringing, over/undershoot, jitter) noise from digital systems.

Radiation noise is noise released (radiated) into the air. A line or cable carrying conduction noise, i.e., noise current, works as a sort of antenna which emits undesirable electromagnetic waves, that is, radiation noise.

EMC (Electromagnetic Compatibility)

Manufactured equipment is increasingly required to meet EMC standards. We will now brief you on EMC in connection with external and internal noise. EMC stands for Electromagnetic Compatibility, and refers to a piece of equipment's compatibility with the environment affected by electromagnetic waves or applicability to such an environment. It encompasses both concepts of EMI (Electromagnetic Interference) and EMS (Electromagnetic Susceptibility). Equipment having EMC thus offers both advantages of not making electromagnetic interference (EMI) and maintaining its performance even if subjected to electromagnetic interference. EMI includes conduction noise and radiation noise, and EMS requires resistance to both types of noise.

The following Figure will give you a visual impression of what we've described above.