The Q factor or quality factor is a measure of the "quality" of a resonant system. The linewidth is defined as the reciprocal of the quality factor. Resonant systems respond to frequencies close to the natural frequency much more strongly than they respond to other frequencies. On a graph of response versus frequency, the bandwidth is defined as the 3 dB change in voltage level besides the center frequency. The definition of the bandwidth as the "full width at half maximum" or FWHM is wrong.

The Q factor is defined as the resonant frequency (center frequency) f₀ divided by the bandwidth Δf or BW:

Bandwidth BW or Δf = f₂ - f₁, where f₂ is the upper and f₁ the lower cutoff frequency.

In a tuned radio frequency receiver (TRF) the Q factor is:

where R, L and C are the resistance, inductance and capacitance of the tuned circuit, respectively.

From the expression for the resonant frequency of a tuned circuit,

the alternative formulation:

can be derived.

In optics, the optical Q factor of a resonant cavity is the ratio of energy stored to energy dissipated in the cavity. If the Q factor of a laser's cavity is abruptly changed, the laser can be induced to emit a pulse of light; this technique is known as Q-switching.

Other uses

In marketing and pop culture commentary, Q factor is sometimes used as a casual synonym for Q Score. In recumbent bicycle mechanics, Q factor refers to the width between the cranks and should be small when building a streamliner.

External link

Conversion: Quality factor Q to 'bandwidth per octaves' and 'bandwidth per octaves' N to quality factor Q