View source for 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 [[Electrical impedance|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]]<ref>[http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0010/ Boonton Q-Meter Type 160-A, 1946] — HP Virtual Museum</ref>. 

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:
:<math>
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
:<math>
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:
:<math>
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 [[RLC circuit|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==
*[http://users.tpg.com.au/users/ldbutler/QMeter.htm "An experimental 'Q' meter"] — article by Lloyd Butler (originally published in ''Amateur Radio'', November 1988; revised April 2004)

<references/>
{{Electrical and electronic measuring equipment}}

[[Category:Electronic test equipment]]
[[Category:Radio electronics]]
[[Category:Measuring instruments]]


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