Working principle of a q meter, Electrical Engineering

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Q. Explain the working principle of a Q meter. Also explain the factors that cause errors during a Q factor measurement.

Ans. Working Principle of Q Meter: the q meter is an instruments designed to measure some of the electrical properties of coils and capacitors. The operations of this useful laboratory instrument is based on the familiar characteristics of a series-resonant circuit, namely, that the voltage across the coil or the capacitor is equal to the applied voltage times the Q of the circuit. If a fixed voltage is applied to the circuit, a voltmeter across the capacitor can be calibrated to read Q directly.

The voltage and current relationships of a series-resonant circuit are shown.

A practical Q-meter circuit is shown. The wide-range oscillator with a frequency range from 50 kHto 50 MHz delivers current to a low-value shunt resistance Rsh. The value of this shunt resistance is very low, typically of the order of 0.02?. It introduces almost no resistance into the oscillatory circuit and it therefore represents a voltage source of magnitude E with a very small (in most cases negligible) internal resistance. The voltage E, across the shunt, corresponding to E. is measured with a thermocouple meter marked "Multiply Q by." The voltage across the variable capacitor, corresponding to Ec . is measured with an electronic voltmeter whose scale is calibrated directly in Q values.

To make a measurements, the unknown coil is connected to the test terminals of the instrument and the circuit is tuned to resonance either by setting the oscillator to a given frequency and varying the internal resonating capacitor or by presetting the capacitor to a desired value and adjusting the frequency of the oscillator. The Q reading on the output meter must be multiplied by the index setting of the: Multiply Q by " meter to obtain the actual Q value.

The indicated Q (which is the resonant reading on the "circuit Q Meter") is called the circuit Q because the losses of the resonating capacitor, voltmeter and insetion resistor are all included in the measuring circuit. The effective Q of the measured coil will be somewhat greater than the indicated Q. This difference can generally be neglected, except in certain cases where the resistance of the coil is relatively small in comparison with the value of the insertion resistor.

The inductance of the coil can be calculated from the known values of frequency (f) and resonating capacitance (c) since

Measurement of Q Meter: The circuit for Q meter measurement is shown. The oscillator is set to the desired frequency and then  the tuning

Capacitor is adjusted for maximum value of under this condition and if the voltage is kept constant the voltmeter connected across the capacitor may be calibrated to read the value of Q directly. This measures value of Q in commonly regarded as the Q of the coil under test. There is an error. The measured value of Q is the Q of the whole circuit and not of the coil. There is error caused on account of the shunt resistance and also due to the distributed capacitance of the circuit.

Thus the measured value of Q is smaller than the true value. Now if coils of high resistance are being measured the difference between the two value may be negligible but when measurements are done on low resistance coils, the error caused on this account may be serious.


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