Q meter

Lore & Background

The Q meter is a test instrument for radio frequency circuits, measuring the quality factor (Q) of a circuit—how much energy is lost per cycle in a non-ideal reactive circuit. This applies to RF and microwave filters, bandpass LC filters, resonators, and individual inductors or capacitors at a chosen frequency. For an inductor, Q depends on its reactance (determined by inductance and angular frequency) divided by its resistance, which mainly comes from wire losses. For LC bandpass circuits and filters, Q equals the resonant frequency divided by the bandwidth; as inductor loss increases, Q drops and bandwidth widens. Coaxial cavity filters, though lacking discrete inductors and capacitors, have an equivalent LC model with losses, so Q applies there too.

Inside, a basic Q meter has a tunable RF generator with very low output impedance and a detector with very high input impedance. It often includes a calibrated high-Q capacitor to measure inductors alone. The generator acts in series with the tuned circuit under test; its negligible output resistance barely affects Q. The detector measures voltage across one component (usually the capacitor), and its high shunt impedance also leaves Q largely unchanged. By comparing the developed RF voltage to the applied RF current, and using the reactive impedance at resonance and the source impedance, the Q factor can be read directly from the scaled detected voltage.

Developed in 1934 at Boonton Radio Corporation in Boonton, New Jersey by William D. Loughlin, the Q meter has been largely replaced in professional labs by other impedance measuring devices, but remains in use among radio amateurs.

Reader's Guide

The Q meter has been largely replaced in professional laboratories by other types of impedance measuring devices, though it is still in use among radio amateurs. Its significance lies in its ability to directly measure the quality factor of inductors, capacitors, and resonant circuits at radio frequencies. For inductors, the Q factor is determined by the reactance divided by the resistance, where the resistance represents loss mainly due to the wire. For LC bandpass circuits and filters, Q equals the resonant frequency divided by the bandwidth; when inductor loss increases, Q is reduced and bandwidth increases. The same Q concept applies to coaxial cavity filters, which have an equivalent LC model with losses. The Q meter's legacy endures in amateur radio, where its straightforward series-resonance method remains useful for characterizing components and filters.

Did You Know?

• The Q meter was developed at Boonton Radio Corporation in Boonton, New Jersey in 1934 by William D. Loughlin.
• It works on the principle of series resonance.
• The ratio of developed RF voltage to applied RF current allows the Q factor to be directly read by scaling the detected voltage.
• It has been largely replaced in professional laboratories but is still in use among radio amateurs.