1. NOISE
Noise is always present within electronic systems and is due to
fluctuations within the atomic structure of the components making
up the system.
Original digital signal
Original analogue signal
Original analogue signal
with noise
Original digital signal
with noise
Noise is not related to the input signal

2. COMMON TYPES of NOISE
Thermal (Johnson) Noise: Caused by heat generated due to the random
movement of atoms within the components. Has an infinite bandwidth with
equal noise power.
Flicker Noise: Caused by random variations in the diffusion of charge
carriers within devices, transistors especially and is a low frequency noise.
Shot Noise: Caused by uneven distribution of charge carriers due to the
granular nature of semiconductor materials as a result of fluctuations in the
diffusion process.
Pink Noise: A low frequency noise similar to flicker noise also known as 1/f
noise since its power spectrum is inversely proportional to frequency.
White Noise: Similar to thermal noise, having noise components at all
frequencies with equal noise power across the spectrum.

3. SIGNAL to NOISE RATIO
The signal-to-noise ratio (S/N ratio) is a quantitative method of describing the
quality of a signal in terms of its corruption by noise.
That is, the ratio of the magnitude of the signal to that of the noise, usually
expressed as the ratio of signal power (Ps) to the noise power (Pn).
Ps
S N ratio = 10 log10
Pn
dB
The signal-to-noise ratio within a given system varies with the magnitude of
the signal.
If the signal becomes very small, the relative size of the noise will increase
causing the S/N ratio to decrease, (remember the noise level is independent
of signal).
Manufacturers of audio equipment often like to quote the signal-to-noise ratio
as a selling point for their equipment.