Flicker, 1/f Noise

- basics of Flicker or 1/f noise that dominates at low frequencies or low frequency offsets from oscillators.

Flicker noise is a form of noise that exhibits an inverse frequency power density curve.

Flicker noise has a 1/f characteristic, or a "pink noise" power density spectrum.

Flicker noise occurs in almost all electronic devices, and it has a variety of different causes, although these are usually related to the flow of direct current.

It is important in many areas of electronics including within oscillators used as RF sources.

Flicker Noise basics

Flicker noise occurs in virtually all electronic components (as well as in many other physical items in everyday life from the earth's rotation to undersea currents and many other items).

Often, Flicker noise is mentioned in relation to semiconductor devices such as transistors and especially MOSFET devices.

It can show up as a variety of effects, but often occurs as a resistance fluctuation.

Flicker noise can be expressed in the form:

S (f)   =   K   /   f

Flicker noise in oscillators

As flicker noise is proportional to the inverse of the frequency, in many applications of electronics components such as within RF oscillators there are regions in which the flicker noise dominates and other regions where the white noise from sources such as shot noise and thermal noise dominate.

Within the oscillator the flicker noise manifests itself as sidebands that are close to the carrier, the other forms of noise extending out from the carrier with a flatter spectrum, although decaying the greater the offset from the carrier.

The simplified noise spectrum of an oscillator showing the region of flicker or 1/f noise closer in to the carrier
Simplified noise spectrum of an oscillator showing flicker noise

In view of this, there is a corner frequency, fc, between the regions dominated by the different forms of noise.

For a system such as an oscillator it is generally found that the noise outside that where the flicker noise predominates is phase noise. This decays with increasing offset from the carrier until flat white noise predominates.

MOSFETs have a higher fc (can be in the GHz range) than JFETs or bipolar transistors which is usually below 2 kHz for the latter.

By Ian Poole

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