Working principle of a q meter, Electrical Engineering

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.

Posted Date: 7/12/2012 6:06:24 AM | Location : United States







Related Discussions:- Working principle of a q meter, Assignment Help, Ask Question on Working principle of a q meter, Get Answer, Expert's Help, Working principle of a q meter Discussions

Write discussion on Working principle of a q meter
Your posts are moderated
Related Questions
Q. Biasing the BJT? A simple method of biasing the BJT is shown in Figure. While no general biasing procedure that will work in all cases can be outlined, a reasonable approach

Look at figure below and use.  Water (50°F) flows at 250 gpm through the pipe system at the bottom of this page. All piping is four inch (4") diameter cast iron. The distance betwe

Q. Which are the three basic configurations of FET amplifiers? The three basic configurations of FET amplifiers are · Common source(CS)- It is most frequently used as it pro

Q. Sampled-data and digital control systems? These differ fromthe continuous-data systems in that the signals at one or more points of the system are in the form of either a pu

Q. Determine v, i, and the power delivered to elements in the network given in Figure. Check whether conservation of power is satisfied by the circuit.

Q. Why we Need biasing? Need for biasing : Baising is necessary to establish the quiescent operating point so that the device operates with the linear region without exceeding

Q. What do you mean by Star Topology? Star Topology: In Star topology, all user nodes are connected to central node point that interconnects all individual users' links and nod


Structure and use of PNP transistor: The figure opposite is a schematic presentation of a PNP transistor related to two voltage sources. To construct the transistor conduct ap

(a) "Pulse Code Modulation (PCM), as used in telephony, samples a signal at 8 kHz using 256 quantization levels". i. Outline how this scheme works with the help of sketches and,