The Q factor or quality factor is a measure of the "quality" of a resonant system. 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.

External link

Conversion: Quality factor Q to 'bandwidth per octaves' and 'bandwidth per octaves' N to quality factor Q (http://www.sengpielaudio.com/calculator-bandwidth.htm)