Noise, distortion, interference; internal and external noise

Definitions

• Noise: any unwanted, random electrical signal that adds to the wanted signal and tends to mask it, e.g. hiss in a radio receiver.
• Distortion: an unwanted change in the shape of the signal caused by the imperfect response of the system or channel (non-flat gain, non-linear phase, non-linearity). It vanishes when the signal is removed.
• Interference: contamination by unwanted signals from other sources, usually other transmitters or circuits, e.g. adjacent-channel signals, crosstalk.

External noise (generated outside the receiver)

1. Atmospheric noise (static): produced by lightning discharges and other natural electrical disturbances; heard as crackling; strong below about 30 MHz, worse in the tropics and in thunderstorm seasons.
2. Extraterrestrial noise:
   • Solar noise: radiation from the sun's hot surface; increases greatly during sunspot activity.
   • Cosmic (galactic) noise: from distant stars and galaxies; noticeable from about 8 MHz to 1.5 GHz.
3. Industrial (man-made) noise: from automobile ignition, electric motors, switchgear, fluorescent lamps, welding, high-voltage lines; caused by sparking and switching; most intense in cities, from about 1 to 600 MHz.

Internal noise (generated inside the receiver/devices)

1. Thermal (Johnson) noise: random motion of free electrons in a conductor due to temperature. Noise power $P_n = kTB$ and rms voltage $V_n = \sqrt{4kTBR}$, where $k = 1.38\times10^{-23}$ J/K. It is white (flat spectrum) and Gaussian.
2. Shot noise: random arrival of charge carriers across a junction or at an anode in diodes, transistors and tubes; $I_n = \sqrt{2qI_{DC}B}$.
3. Partition noise: random division of current between two or more electrodes (e.g. emitter current dividing into base and collector currents).
4. Flicker (1/f) noise: due to surface and material irregularities in devices; its power falls as frequency increases, important below a few kHz.
5. Transit-time noise: at very high frequencies, when carrier transit time across a device is comparable to the signal period, causing random input current; it rises with frequency.