Question

Q-meter works on the principle of

• A. Parallel resonance
• B. Electromagnetic induction
• C. Series resonance
• D. Hall effect

Hint:

A "high Q" means the coil has very little resistance compared to its inductance, which is desirable in high-frequency tuning applications.

Answer 1

The Correct Option is C

The Q-meter is an instrument used to measure the "Quality Factor" ($Q$) of a coil and other electrical properties like inductance, capacitance, and dissipation factor. 1. The Principle of Series Resonance: The operation of a Q-meter is based on the characteristics of a series resonant circuit ($R-L-C$ circuit). At resonance, the inductive reactance ($X_L$) and capacitive reactance ($X_C$) are equal and opposite, leaving only the resistance ($R$) to oppose the current.

2. Voltage Magnification: The key property utilized by the Q-meter is that at resonance, the voltage across the capacitor ($V_C$) or the inductor ($V_L$) is many times larger than the applied input voltage ($V$). This "magnification factor" is exactly equal to the $Q$ of the circuit: $$Q = \frac{V_C}{V} = \frac{V_L}{V} = \frac{\omega L}{R}$$

3. Measurement Process: The Q-meter applies a known small voltage at a specific frequency and adjusts a calibrated internal capacitor until resonance is reached (indicated by a maximum voltage reading on an electronic voltmeter across the capacitor). The voltmeter is calibrated directly in terms of $Q$, allowing for a direct reading of the coil's quality.