The Q-factor is called as Quality factor or storage factor. It is defined as ration of power stored in the element to the power dissipated in the element it is magnification provided by the circuit. It is defined as ratio of reactance to resistance of the reactive element. Thus for inductive reactance it is ratio of XL to R and for capacitive reactance it is XC to R. Q-meter is an instrument used to measure a electrical properties of the coil and capacitors.

Working:

The working of Q-meter is based on the characteristics of series resonant circuit. The series resonant circuit has a characteristic that the applied voltage across coil and capacitor is equal to the applied voltage time the Q-factor of the circuit.

The fixed voltage is applied to the circuit and voltage across capacitor can be calibrated to read Q-value.

In series resonant circuit when it is in resonance then XC = XL

$Q =\dfrac {X_L}{R} =\dfrac {X_C}{R} =\dfrac {E_C}{E}$

From above equation we can see that if the applied voltage E is maintained constant and at known level then a voltmeter across the capacitor can be calibrated directly interns of the circuit Q.

Following are the sources of error in Q-meter:

1. Error due to stray capacitance

The most important source of error is due to the self-capacitance of the coil. The self-capacitance is also called distributed capacitance. The presence of the distributed capacitance in a coil modifies the actual value of Q. The distributed capacitance is also called stray capacitance.

1. Error due to residual inductance

This source of error includes the residual inductance of the instrument, which is usually in the order of 0.015µH and affects the measurement of very small inductors.

1. Error due to shunt effect of voltmeter

The conductance of the Q voltmeter has a slight shunting effect on the tunings capacitor at the higher frequencies, but this effect can usually be neglected.