Q meter

A Q meter is a piece of equipment used in the testing of radio frequency circuits. It has been largely replaced in professional laboratories by other types of impedance measuring device, though it is still in use among radio amateurs. It was developed at Boonton Radio Corporation in Boonton, New Jersey in 1934 by William D. Loughlin.

A Q meter measures Q, the quality factor of a circuit, which expresses how much energy is dissipated per cycle in a non-ideal reactive circuit:

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Q = 2 \pi \times \frac{\mbox{Peak Energy Stored}}{\mbox{Energy dissipated per cycle}}. \, </math> This expression applies to an RF and microwave filter, bandpass LC filter, or any resonator. It also can be applied to an inductor or capacitor at a chosen frequency. For inductors

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Q = \frac{X_L}{R} = \frac{\omega L}{R} </math> Where <math>X_L</math> is the reactance of the inductor, <math>L</math> is the inductance, <math>\omega</math> is the angular frequency and <math>R</math> is the resistance of the inductor. The resistance <math>R</math> represents the loss in the inductor, mainly due to the resistance of the wire.

For LC band pass circuits and filters:

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Q = \frac{F}{BW} </math> Where <math>F</math> is the resonant frequency (center frequency) and <math>BW</math> is the filter bandwidth. In a band pass filter using an LC resonant circuit, when the loss (resistance) of the inductor increases, its Q is reduced, and so the bandwidth of the filter is increased. In a coaxial cavity filter, there are no inductors and capacitors, but the cavity has an equivalent LC model with losses (resistance) and the Q factor can be applied as well.

See also

• LCR meter

Sources and external links

• "An experimental 'Q' meter" — article by Lloyd Butler (originally published in Amateur Radio, November 1988; revised April 2004)

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