T&M Q Meter Basics: Design and Circuit Explanation
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This page covers Q meter basics and explains how to design a Q meter circuit. A Q meter is an instrument designed to measure the electrical properties of coils and capacitors. Its operation is based on the principles of a series resonant circuit.
As we know, the voltage across a coil or capacitor at resonance is equal to the applied voltage multiplied by the Q factor of the circuit. If the applied voltage is kept constant, a voltmeter connected across the capacitor can be easily calibrated to directly read the Q value.
Figure 1: Series Resonant Circuit
Figure 1 shows a typical series resonant circuit. At resonance, the following conditions are met:
- Xc = XL
- Ec = I * Xc = I * XL
- E = I * R
Where:
- E = Applied voltage
- I = Circuit Current
- Ec = Voltage across capacitor
- Xc = Capacitive reactance
- XL = Inductive reactance
- R = Coil resistance
The quality factor, or magnification, of the circuit is known as Q.
Q = XL/R = Xc/R = Ec/E
If E (the applied voltage) is maintained at a constant level, the voltmeter connected across the capacitor can be calibrated directly in terms of the circuit’s Q value.
How to Design a Q Meter Circuit
Figure 2: Q Meter Circuit
Figure 2 illustrates a practical Q meter circuit. To make a measurement, the unknown coil is connected to the test terminals. The circuit can be tuned to resonance in one of the following ways:
- By setting the oscillator to a specific frequency and varying the internal resonating capacitor.
- By pre-setting the capacitor to a desired value and adjusting the oscillator frequency.
The Q reading on the output meter must be multiplied by the “Multiply Q by” index setting to obtain the actual Q value.
The inductance of the coil can be calculated from known values of frequency (f) and resonating capacitor (C) using the following equation:
L = 1 / ((2 * π * f)^2 * C)
There are three methods to connect unknown components to the test terminals of the Q meter: direct connection, series connection, and parallel connection.
