Q - Meter

 The instrument which measures some of the electrical properties of coils and capacitors is referred
as Q-meter. The working principle of a Q-meter depends on the characteristics of a series resonance
circuits, i.e., the voltage drop across the coil orcapacitors is equal to the applied voltage times the Q
factor of the circuit. Thus if the circuit subjected to a fixed voltage, the voltmeter connected across the
capacitor is calibrated to indicate the Q value directly. A series resonance circuit and its voltage and
current relationship at resonance conditions are illustrated in figure 8.1 (i) and fig 8.1 (ii) respectively.

XC=Capacitive reactance
XL=Inductive reactance
I =Current flowing through the circuit
E =Applied voltage
R = Resistance of the coil

The Q factor or the magnification of the circuit is defined as,

From the above equation it is clearthat if the voltage E is maintained at a fixed level, the voltmeter
across the capacitor can be calibrated in terms of Q directly. The circuit arrangement of basic and
practical Q-meter is shown below.

The oscillator is a wide range RF oscillator that supplies the oscillations whose frequency lies between
50 kHz to 50 MHz and delivers current to Rshwhich is a shunt resistance oflow value, and is typically
around 0.02Q. Therefore the Rshintroduces very negligible (almost no resistance) resistance into the
oscillator circuit. Thus it represents a voltage source of magnitude E with a very low internal resistance.
The voltage across Rshis measured using a thermocouple meter that is marked as 'multiply Q by meter.
The voltage drop across the tuning capacitor or resonating capacitor EC is measured by means of an

electronic voltmeter. The scale of this electronic voltmeter is calibrated in terms of Q values directly.
To carry out the measurement the unknown components is connected across the testterminals and the
circuit is adjusted to resonance using any one of the two methods given below.

By setting the frequency of the oscillator to a certain given value and adjusting the tuning capacitor.
By presetting the capacitor to a required value and varying the frequency of oscillator.
The Q value indicate on the output meter should be multiplied by the index setting of the 'multiply Q by’
meter to get actual, or accurate Q value.
The indicated value ofQ on the output meter is known as 'circuit Q' since it includes the losses of volt-meter tuning capacitor and insertion resistor. The effective Q value of the measured coil will be higher
than indicated Q or circuit Q. This difference is small therefore it can be neglected. However this
difference is large if the resistance of the coil issmall compared to the insertion resistor value.
The inductance of the coil can be found from the known values of c (resonating capacitance) and / (fre-quency).
                      Since XC=XL
                              2ПfL=1/2ПfC
Therefore L = 1 / ( 2Пf)square
C Henry