Q meter


Q meter

A Q meter is a piece of equipment used in the testing of radio frequency circuits. The Q meter has been largely replaced in professional laboratories by other types of impedance measuring device, though it is still in use among radio amateurs.

A Q meter is used to measure the Q factor (quality factor ) or Q. The Q was originally used to quantify the quality of inductors. In a more physically approach, the Q expresses how much energy is dissipated per cycle in a non ideal reactive circuit. Physically: Q = 2 * PI_Number * Peak Energy Stored per Cycle / Energy Dissipated per CycleThis expression can be applied to inductors, RF and microwave filter, LC filters, cavity filters and so on. For inductors:
Q = X / R
Where X is the inductive reactance of the inductor and R is the AC resistance of the inductor.
R represents the loss in the inductor, the resistance of the wire.

The Q factor is also applied to RF and microwave filter, bandpass LC circuits (bandpass filters). For LC band pass circuit and filters:
Q = F / BW
Where F is the ressonant frequency (center frequency) and BW is the filter bandwidth. In a band pass filter, using a LC ressonant circuit, when the loss (resistance) of the inductor increases, its Q is reduced, and so the BW of the filter.

In a coaxial cavity filter, there is no inductors and capacitors, but the cavity has an equivalent LC model with losses (resistance) and the Q factor can be applied as well.

ources and external links

* [http://www4.tpgi.com.au/users/ldbutler/QMeter.htm "An experimental 'Q' meter"] — article by Lloyd Butler (originally published in "Amateur Radio", November 1988; revised April 2004)
* [http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0010/ Boonton Q-Meter Type 160-A, 1946] — HP Virtual Museum


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