Electronics/Noise in electronic circuits


 * Electrical Noise: any unwanted form of energy tending to interfere with the proper and easy reception and reproduction of wanted signals.

Based on Origin

 * 1) External noise
 * 2) Atmospheric
 * 3) Extraterrestrial
 * 4) solar
 * 5) Cosmic
 * 6) Industrial
 * 7) Internal noise
 * 8) Thermal Agitation Noise
 * 9) Shot Noise
 * 10) Transit Time Noise
 * 11) Flicker Noise
 * 12) Miscellaneous Sources

Thermal noise
$P_n \propto\ T\;\delta\!f = k\,T\;\delta\!f$ where k = Boltzmann's constant = 1.38x10-23J/K
 * Thermal Agitation Noise: Also known as Johnson noise or White noise.
 * T = absolute temperature, K = 273 + °C
 * &delta; f = bandwidth of interest
 * Pn = maximum noise power output of a resistor

$P_n = \frac{V^2}{R_L} = \frac{\left(\frac{V_n}{2}\right)^2}{R} = \frac{V_n^2}{4R}$ $V_n^2 = 4RP_n = 4RkT\;\delta\!f$ $V_n = \sqrt{4kT\;\delta\!f\;R}$

Shot Noise
$i_n = \sqrt{2ei_p\;\delta\!f}$ where in = r.m.s. shot-noise current
 * e = charge of an electron = 1.6x10-19C
 * ip = direct diode current
 * &delta; f = bandwidth of system

Addition due to several sources
noise voltages: $V_n1 = \sqrt{4kT\,\delta\!f\,R_1}$, $V_n2 = \sqrt{4kT\,\delta\!f\,R_2}$...and so on, then $V_{n,tot} = \sqrt{{V_n1^2}+{V_n2^2}+...} = \sqrt{4kT\,\delta\!f\,R_{tot}}$ where Rtot = R1+R2+...

Addition due to Cascaded Amplifier stages
Req = R1+R'2 $$R_{eq} = R_1 + \frac{R_2}{A_1^2} + \frac{R_3}$$

Differential signaling
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analogue signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, PCI Express and USB.

Good grounding
An ideal signal ground maintains zero voltage regardless of how much electrical current flows into ground or out of ground.

When low-level signals travel near high currents, their return currents shouldn't be allowed to flow in the same conductor. Otherwise, noise such as AC ripple on the high current will modulate the low-level signal.