1/f Noise in Electronic Circuits:

Previously we have discussed thermal noise (fluctuations in the velocity of electrons through a conductro) and shot noise (fluctuations in the number of electrons through a conductor some electrons are lost).

Thermal noise is somewhat controllable by cooling electronics.

Shot noise is usually only an issue when your signal is low. This is a problem in astrophysical imaging systems, but not generally in most electronics.

Both Thermal Noise and Shot Noise are forms of "white" noise, which is the same as random noise just like in the random error examples done earlier.

Thermal noise depends on the total bandwidth of the system (which is material dependent) and on the temperature. Thermal noise is not frequency dependent.

Shot noise usually occures whenever carriers are injected into a sample volume independently of one another. Current flow in a semiconductor diode is a common example. In this case, shot noise is dependent on the total number of injected carriers (i.e. the current).

1/f noise (f stands for frequency):

Finally, one other form of noise is present in electronics. This is called 1/f noise and is frequency dependent.

1/f noise originates whenever flucations in either charge density or charge velocity are not purely random in time. This happens whenever carriers are trapped or stored in the system and released later.

Stored charge in a capacitor is a good example of this. That stored charge is released over time, giving by the time constant of the capacitor. A circuit board with many capacitors on it, would then have many locations where charge is stored and released in a non random way with time. The "decay" curve of a capacitor typically looks like this:

1/f noise is stongly dependent on frequency - decreasing as frequency is increasing. Note that this source of noise is not fully understand theoretically, its just measured to exist in all electronic systems. The important point is that 1/f noise is frequency dependent, where as thermal noise and shot noise, are not.

Recent research has revealedthat the 1/f noise in doped semiconductors, the platform for all modern electronics, originates in the random distribution of impurities and the mutual interaction of the many electrons surrounding them.

This indicates that 1/f noise is an intrinsic property of many atom systems.