Distortion, noise, interference; four types of noise

Distortion, noise and interference

• Distortion is the change in waveform caused by the imperfect response of the system itself. Example: an overdriven audio amplifier clips peaks, adding harmonics; a long cable attenuates high frequencies more than low, smearing pulses.
• Noise is random, unwanted electrical energy from natural or device sources. Example: hiss in a radio from thermal noise of the RF amplifier; static from lightning.
• Interference is contamination by unwanted signals from other man-made sources. Example: a nearby FM station heard on your channel; power-line hum (50 Hz) picked up by audio cables; crosstalk in telephone lines.

Four types of noise

1. Thermal (Johnson) noise: caused by random motion of electrons in any resistor or conductor above 0 K. Its power is $P_n=kTB$ and voltage $V_n=\sqrt{4kTBR}$; spectrum is white. It is the main internal noise in receivers.
2. Shot noise: occurs in active devices (diodes, transistors, tubes) because current is made of discrete charges arriving randomly at a junction. Mean-square current $\overline{i_n^2}=2qI_{dc}B$. Its spectrum is also nearly flat.
3. Flicker (1/f) noise: found at low frequencies (below a few kHz) in semiconductors and carbon resistors, due to surface and crystal imperfections. Its PSD varies roughly as $1/f$, so it matters in audio and DC amplifiers.
4. Transit-time (high-frequency) noise: at very high frequencies the carrier transit time through a device becomes comparable to the signal period, giving noise that rises with frequency and limits the device's upper frequency.

Other types worth naming: external noise such as atmospheric (lightning static, below about 30 MHz), extraterrestrial (solar and cosmic noise) and industrial (man-made, from ignition, motors, switching).