What causes flicker or 1/f noise?

Question

What physical processes causes the generation of electrical and radio/EM/RF flicker or 1/f noise? Or why are those sources not generating normally distributed electrical observables (such as thermal noise in resistors).

Answer 1

Contrary to OP's belief that "those sources [are] not generating normally distributed electrical observables", in that source first studied with respect to 1/f noise -- the vacuum tube cathode's trapping sites -- an individual trapping site releases electrons according to a simple exponential relaxation law N(t) = N₀exp(-λt). If we extend the normality concept outside the Gaussian distribution conformance to just simple ubiquity of the observed law of power spectrum, the Lorentzian power spectrum N₀²n/(λ²+ω²) of relaxation law can certainly be considered a "normally distributed electrical observable". Finally, in an ensemble of trapping sites with the relaxation rate parameter λ uniformly distributed over the ensemble, the resulting noise power distribution is 1/f.

The comprehensive review of the research into the phenomena see in 1/f noise: a pedagogical review by Edoardo Milotti

A spoiler:

12. Conclusions

... do we have by now an "explanation" of the apparent universality of flicker noises? Do we understand 1/f noise? My impression is that there is no real mystery behind 1/f noise, that there is no real universality and that in most cases the observed 1/f noises have been explained by beautiful and mostly ad hoc models. ...

One thing that disturbs the imagination of those struggling to understand the 1/f noise phenomenon, is the divergence of the total energy of this noise. The spectrum diverges logarithmically both at the lower and upper frequency limits. There is no evident problem of this kind with white noise, because the white noise is always bandlimited by well known physical laws: for example, the thermal noise obeys to Planck's law of blackbody radiation and, at room temperature, exponentially decreases to zero at frequencies above few terahertz.

The cited reference calms down the divergence worries with very ingenious argument. Let there be some unknown physics under this noise's origin that permits the exact conformance to 1/f law at both low and high frequencies. Whatever are lower and upper limits in the integral over spectrum $$ \int_f^F{df\over{f}} = ln({F\over{f}}) $$ we cannot measure longer than the lifetime of the Universe, 6·10¹⁶, so f ~ 10^-17Hz. The smallest observable time, Plank's time, is 10^-43s, so F ~ 10⁴³, and the integral totals to 60/0.434. The measurements in thin-film resistors shows near perfect 1/f noise over more than 6 frequency decades and provide an experimentally measured confirmation for one-tenth of our theoretically estimated total. The divergence, due to logarithm behavior, is not that frightening, as it seems at first glance.

Answer 2

1/f noise is band limited white noise, meaning that it is like white noise but there is some frequency cutoff from a low pass filter. This is what art of electronics says about it:

Art of Electronics section 8.1.3:

Other noise-generating mechanisms often produce 1/f noise, examples being base-current noise in transistors and cathode current noise in vacuum tubes. Curiously enough, 1/f noise is present in nature in unexpected places, e.g., the speed of ocean currents, the flow of sand in an hourglass, the flow of traffic on Japanese expressways, and the yearly flow of the Nile measured over the last 2,000 years.9 If you plot the loudness of a piece of classical music versus time, you get a 1/f spectrum! No unifying principle has been found for all the 1/f noise that seems to be swirling around us, although particular sources can often be identified in each instance.

There isn't any one source, each process is different in electronic devices. Although I work a lot with measuring temperature and the sub 10Hz range (some of the products I work with take data on the weeks to months range), and I have noticed that temperature probably has a lot to do with 1/f noise.

One thing to think about are time constants because 1/f is filtered noise. In most analog electronic devices all of the time constants that could found be at the same timescale of 1/f noise (0.1 to 10Hz, or lower) are large enough to be formed by filtering caused by thermal conductivity and thermal mass (which is like an RC timeconstant where the resistor is thermal resistance and the capacitor is mass), that is where I think some of the 1/f noise comes from, thermal noise that is band limited \ filtered with low pass filters formed from thermal mass and conductivity.

Also if you've ever stared at a fire and seen glowing embers, think of the same thing going on but different areas of a chip glowing with temperature (and not as hot).