Q Meter Block Diagram and Working

Q-meter is an instrument to measure some of the electrical properties of inductors and capacitors. The basic principle of resonance is used in the measurement of 'Q'. We know from the concept of resonant circuits that the voltage across the tank circuit is 'Q' times the applied voltage. Therefore by applying a fixed voltage to the circuit, the voltage across the capacitor can be calibrated in terms of 'Q'. The magnification factor 'Q' of the circuit is defined as

Q = X_L/R = X_C/R = E_C/R

(a) Description of the Block Diagram:

The block schematic diagram of a 'Q' meter is shown in Figure. It consists of an oscillator which works in the frequency range of 50 kHz to 50 MHz. The oscillator drives a current through a low value of resistance R_sh. The value of the shunt resistance may be typically 0.2 Ω. The resistor represents a voltage source that can supply a voltage of magnitude E volts. A thermocouple meter is connected across the resistance to measure the voltage across it.

This meter is marked as multiply Q by. A variable capacitor is arranged in series with the test terminals as shown in the schematic diagram. An electronic voltmeter is connected across this variable capacitor also called the resonating capacitor. The electronic voltmeter's dial will be calibrated directly in 'Q' value.

(b) Measurement of ‘Q’: 

To measure the value of the Q of a coil the coil is connected to the test terminals of the Q meter. The oscillator may be tuned to resonate or the capacitor may be tuned to the frequency of the oscillator. The Q of the coil will be obtained by multiplying the value of the reading obtained on the output meter with the multiply Q by meter.

The indicated 'Q' is called the circuit 'Q' because of the loss of the resonating capacitor, voltmeter and insertion resistor will include in measuring circuit. The effective 'Q' will be greater than the indicated 'Q'. This difference can be neglected where the resistance of the coil is relatively small compared to the value of the insertion resistors. The inductance of the coil can be calculated from the knowledge of frequency and resonating capacitor value. This is because we know that X_L = X_C and therefore

L = 1 /(2πf)²C henry