What is a Q Meter : Working Principle, Circuit & Applications

Q meter was developed by William D. Loughlin at Boonton Radio Corporation in the year 1934 in Boonton, New Jersey. The Q-meter instrument has become more popular in RF impedance measurement. There are different kinds of instruments available based on system usage. These are separated into two types like low-impedance injection & high-impedance injection. This device plays a key role in testing the RF circuits and also replaced in laboratories with other impedance measuring devices, although it is still in use among radio amateurs. This article discusses an overview of the Q meter.

What is Q Meter?

Definition: A device that is used to measure the QF (quality factor) or storage factor or quality factor of the circuit at radio frequencies is called the Q-meter. In the oscillatory system, the QF is one of the essential parameters, used to illustrate the relationships among the dissipated & stored energies.

By using Q value, the overall efficiency can be evaluated for the capacitors as well as coils used in RF applications. The principle of this meter mainly depends on series resonance because the voltage drop is Q times than the applied voltage across the capacitor otherwise coil. When the fixed voltage is applied to an electric circuit, a voltmeter is used to adjust the capacitor’s Q value to read directly.

The total efficiency of capacitors & coils used for RF applications can be calculated with the help of Q value.

At resonance X_L= X_C and E_L= I_XL, E_C = I_XC, E = I R

Where ‘E’ is an applied voltage

‘EC’ is the capacitor voltage

‘EL’ is an inductive voltage

‘XL’ is the inductive reactance

‘XC’ is the capacitive reactance

‘R’ is the coil resistance

‘I’ is circuit current

Thus, Q = X_L/R= X_c/R=E_C/E

From the above ‘Q‘equation, if an applied voltage is kept stable so that the voltage across the capacitor can be calculated using a voltmeter to read ‘Q’ values directly.

Working Principle

The working principle of Q meter is series resonant because the resonant exists within the circuit once the reactance of capacitance & reactance is of the same magnitude. They induce energy to oscillate in between the fields of electric & magnetic of the inductor & capacitor respectively. This meter mainly depends on the feature of the capacitance, inductance & resistance of the resonant series circuit.

Q Meter Circuit

The circuit diagram of the ‘Q’ meter is shown below. It is designed with an oscillator that uses the frequency that ranges from 50 kHz – 50 MHz. and provides current to a shunt resistance ‘Rsh’with 0.02 ohms value.

Here thermocouple meter is used to calculate the voltage across the shunt resistance whereas an electronic voltmeter is used to calculate the voltage across the capacitor. These meters can be calibrated to read ‘Q’ directly.

In the circuit, the energy of the oscillator can be supplied to the tank circuit. This circuit can be adjusted for the resonance through unstable ‘C’ until the voltmeter reads the utmost value.

The o/p voltage of resonance is ‘E’, equivalent to ‘Ec’ is E = Q X e and Q = E/e. Because ‘e’ is known so the voltmeter is adjusted to read ‘Q’ value directly.

The coil is connected to the two test terminals of the instrument to determine the coil’s inductance

This circuit is adjusted to resonance through changing either the oscillator frequency otherwise the capacitance. Once the capacitance is changed, then the frequency of the oscillator can be adjusted to a specified frequency & resonance is attained.

If the value of capacitance is already fixed to a preferred value, then the frequency of the oscillator will be changed until resonance takes place.

The reading of ‘Q’ on the o/p meter is multiplied through the setting of an index to get the actual ‘Q’ value. The coil’s inductance is calculated from known values of the coil frequency as well as the resonating capacitor.

The specified Q is not the definite Q, as the losses of the voltmeter, inserted resistance & resonating capacitor are all incorporated in the circuit. Here, the definite ‘Q’ of the calculated coil is a bit larger than the specified Q. This dissimilarity is insignificant except wherever the coil’s resistance is relatively minute compared to the ‘Rsh’ resistance.

Applications of the Q-meter

The applications of Q-meter include the following.

• It is used to measure the quality factor of the inductor.
• By using this meter, unknown impedance can be measured using a series or shunt substitution method. If the impedance is small, the former technique is used and if it is large, then the latter technique is used.
• It is used to measure small capacitor values.
• By using this, inductance, effective resistance, self-capacitance, and bandwidth can be measured.

FAQs

1). What is a quality factor?

The quality factor is a ratio of the stored power and dissipated power in an element.

2). What is ‘Q’ meter?

Q meter is one kind of instrument, used to measure electrical properties of coils & capacitors. This instrument is also used in laboratories.

3). What is the Q meter working principle?

The working principle of this meter is a series resonance

4). A practical Q meter includes of

It includes an RF oscillator

5). What is the Q factor of a series resonant circuit?

The Q factor of a series resonant circuit is Q=XL/R = XC/R

Thus, this is all about an overview of Q-Meter or RLC meter. As the name suggests, this instrument is used for calculating the Q-factor of the inductors & self-capacitance of the coil. Here is a question for you, what are the methods for connecting unknown components for testing the Q meter terminals?