Q meter basics | How to design Q meter circuit

This page covers Q meter basics. It mentions how to design Q meter circuit.

The Q meter instrument is designed and developed to measure electrical properties of coils and capacitors. The operation of Q meter is same as series resonant circuit.

As we know, voltage across coil or capacitor is equal to applied voltage times Q of the circuit. If applied voltage is fixed, voltmeter across capacitor can easily be calibrated to read Q.

series resonant circuit

The figure-1 depicts series resonant circuit. At resonance, following conditions are fulfilled.
Xc = XL
Ec = I*Xc = I * XL
E =I*R
Here, E = Applied voltage, I = Circuit Current, Ec = voltage across capacitor, Xc = capacitive reactance, XL = inductive reactance, R = Coil resistance

The magnification of the circuit is known as Q.
Where Q = XL/R = Xc/R = Ec/E

If E is maintained at constant level then voltmeter connected across capacitor can be calibrated directly in terms of circuit Q.

How to design Q meter circuit

Q meter circuit
Figure-1: Q meter circuit

The figure-2 depicts practical Q meter circuit. To make measurement, unknown coil is connected to test terminals. The curcuit can be tuned to resonance in either of the following ways.
• By setting oscillator to given frequency and varying internal resonating capacitor.
• By pre-setting capacitor to desired value and adjusting oscillator frequency.
➨The Q reading on output meter must be multiplied by index setting of "Multiply Q by" meter to obtain actual Q value.

Inductance of coil can be calculated from known values of frequency(f) and resonating capacitor (C) using following equation.
➨ L = 1 / (2*π*f)2 * C

There are three methods to connect unknown components to test terminals of the Q meter viz. direct connection, series connection and parallel connection.

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Q meter