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People often talk about noise in circuits. Cheap op-amps are noisy, running a motor can create noise on the supply, and a lot of analog circuits deal with the signal-to-noise ratio (ie: trying to keep the noise floor low).

My intuition is that noise is the presence of signals at frequencies we're not interested in. (This may or may not be right.) However, I don't know where this noise comes from.

How does electrical noise appear? What generates it? How do I get rid of it?

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    \$\begingroup\$ I like to divide noise into two categories: Fundamental noise. (Johnsson noise, shot noise, 1/f noise (maybe) And Technical noise. (interference, vibrations.. the list can be almost endless. ) You are pretty much stuck with the fundamental noise.. though you can do crazy things like reduce the temperature. Technical noise can be reduced with good techniques. \$\endgroup\$ – George Herold Jan 20 '15 at 19:20
  • \$\begingroup\$ @GeorgeHerold Why the 'maybe' on flicker noise? \$\endgroup\$ – Spehro Pefhany Jan 20 '15 at 19:43
  • \$\begingroup\$ @SpehroPefhany, Well 1/f, flicker, pop corn noise seems to me to be intermediate. As an IC user I can't do much about it, but with better technique the chip makers can make (some) of it better. So it's kinda technical noise for IC designers. \$\endgroup\$ – George Herold Jan 20 '15 at 20:16
  • \$\begingroup\$ @GeorgeHerold I agree with your division but I think a good chip designer can do a lot of things to reduce fundamental noise. CDS or chopping for 1/f for example, filtering out thermal noise that would fold when mixing or such... \$\endgroup\$ – Vladimir Cravero Jan 20 '15 at 21:42
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    \$\begingroup\$ It's interesting to note that it's exactly this question, asked by engineers working at Bell Labs in the 60s when they're trying to eliminate all noise from their circuits and failing, that led to the discovery of cosmic background radiation. Which confirmed the big bang theory. And led to astronomers building giant radio antennas and calling them "telescopes". \$\endgroup\$ – slebetman Jan 21 '15 at 2:06
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Presence of power at frequencies you're not interested in can easily be filtered out. Presence of power at frequencies you ARE interested in is the problem, as this cannot be filtered out.

There are several main sources of noise. It depends on what context you're talking about, though - things such as interference or cross-talk can be considered noise in the context of, say, the signal-to-noise ratio, but when you build a 'low noise amplifier', this refers to intrinsic sources of noise.

One source of noise that is unavoidable is thermal noise. Any object that is not sitting at absolute zero behaves like a black body and radiates electromagnetic radiation. This is a problem for long range RF communications because the black body radiation from the ground, buildings, etc. will appear in the band of interest and put a 'floor' on the level of signal that you can receive. This noise is more or less flat up to around 80 GHz, so the noise power is simply proportional to the bandwidth and temperature. Thermal noise in electronics is called Johnson noise. Johnson noise is generated by electrons (or other charge carriers) wiggling around due to not being at absolute zero. This can be modelled as a voltage source in series or a current source in parallel with each resistor in a circuit. Johnson noise is proportional to bandwidth, temperature, and resistance.

Shot noise is a very different type of noise that occurs when charges move across a gap (vacuum tube) or through a semiconductor junction (diode, BJT). Since charge carriers are discrete (you can count them), charge must be measured in these quantized units. When a current flows, an integer number of charge carriers will move, arriving at random intervals. For large currents, the fluctuation is so small that it is basically undetectable. However, for very small currents, the current will flow in a series of 'pulses', one for each electron. As a result, shot noise becomes a large problem at low signal levels. Shot noise is white; meaning that it is independent of frequency and the overall noise power is proportional to the bandwidth.

Flicker noise, or 1/f noise, is another, different type of noise. This occurs in electronic devices, in addition to Johnson noise and shot noise. Flicker noise is called 1/f noise because the noise power is proportional to the inverse of the frequency - it is high at low frequencies and low at high frequencies. Generally flicker noise is dependent on the DC level.

Other sources of noise are a bit less common, such as avalanche noise. Avalanche noise is caused by avalanche breakdown. During avalanche breakdown, flowing electrons release more electrons and create an exponentially growing current. Devices such as avalanche photodetectors use this effect to detect small numbers of photons by biasing the device just on the edge of avalanche breakdown so a small number of photons hitting the detector will release enough electrons to trigger the breakdown. Current flow during avalanche breakdown is very noisy. In fact, it is so noisy that avalanche diodes are used as RF noise sources for testing various RF components.

Crosstalk, interference, and intermodulation are also sources of unwanted signals, but these are not technically noise. Crosstalk and interference are unwanted signals coming from external sources. Intermodulation comes from non-linearities and causes adjacent channels in the same medium to be superimposed on top of each other. This is a major problem when trying to transmit a large number of channels in parallel as they mix with each other. Generally this is 2 Fa - Fb. For example, if I transmit two channels with 1 kHz spacing on 1 MHz, then I am transmitting 1.000 MHz an 1.001 MHz. IMD means I will get some power on 2*1.000 - 1.001 = 0.999 MHz and 2*1.001 - 1.000 = 1.002 MHz, which would interfere with adjacent channels on the same spacing.

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    \$\begingroup\$ Nice, One thing about shot noise. Not all currents show shot noise. So the current from a battery and resistor will have no shot noise, though still the Johnson noise of the resistor. Stick a forward biased PN diode in the same circuit and it will show shot noise... or the current from a photodiode with light shinning on it. Shot noise happens when there is a random process in the current generation, thermal excitation in the pn diode, photo-excitation in the photodiode. Kinda weird. \$\endgroup\$ – George Herold Jan 20 '15 at 21:26
  • \$\begingroup\$ Well, electrons are quantized, so wherever there is a current flowing, you will see shot noise. But you may need a really small current - e.g. pA range. Some devices have shot noise at significantly higher currents, though. I believe it's more obvious in a diode due to the voltage drop of the junction. \$\endgroup\$ – alex.forencich Jan 20 '15 at 21:30
  • \$\begingroup\$ You might want to look up "solid state shot noise" by Rolf Landauer. It's a bit on the high end, being from a theoretician. But to your point, I've measured the shot noise from photodiodes (and found e the charge of the electron) and also looked for the same noise in the resistor situation I mentioned above. Nada. (Well there is a very little bit of excess noise in resistors with voltage across them, but it's way below the shot noise level... there's a paper from LIGO...(search for "resistor current noise") \$\endgroup\$ – George Herold Jan 20 '15 at 21:51
  • \$\begingroup\$ Ah, I see - it's the gap or semiconductor junction that creates shot noise. Without the gap, the electrons can flow more smoothly. As for excess noise in resistors, they do exhibit flicker noise, but it does depend on the type of resistor. \$\endgroup\$ – alex.forencich Jan 20 '15 at 22:41
  • \$\begingroup\$ Oh good, Yeah the flow of electricity in wires and things is a lot smoother than electrons jumping around. It's something that's hard to get a handle on, when you try and think deeply about it. The model Landauer suggests for bulk resistors, if we want to think about some individual electron transversing the entire resistor, is that each scattering event of that electrons creates and E-field pulse on the electrode of the resistor, and the shot noise is decreased by a fraction that goes as the scattering length/ resistor lenght. \$\endgroup\$ – George Herold Jan 21 '15 at 0:00

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