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What is Q –Meter ?
• Q meter is design for measuring the Q factor of a
coil and for measuring inductance, capacitance and
resistance of an electric circuit at radio frequency. It
is also known as RLC meter.
• A direct-reading instrument which measures the Q
of an electric circuit at radio frequencies by
determining the ratio of inductance to resistance,
and which has also been developed to measure
many other quantities. Also known as quality-factor
• Inductors, capacitors and resisters which have to
operate at radio frequencies cannot be measured
satisfactorily at lower frequencies.
• The resonance methods are employed in which th e
unknown component may be tested at or near its
normal operating frequency.
• We know that every inductor coil has a certain amount
of resistance and the coil should have lowest possible
resistance. The ratio of the inductive reactance to the
effective resistance of the coil is called the quality
factor or Q-factor of the coil.
Q = XL / R = ωL / R
Q meter consist of the following major elements:
• Variable calibrated capacitor
• Variable –frequency ac voltage source
• Coil to be tested or measured
All elements are connected in series .
The capacitor voltage (Vc) and the source voltage ( E) are
monitored by voltmeters.
One terminal of the coil is connected to the signal
generator (Ac voltage source) while the other terminal is
connected to the variable capacitor.
Volt meter 1 shows the source voltage(E)
Volt meter 2 shows voltage across variable capacitor
Inductor and resister combine shows coil
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.
VC = VL
• First of all The oscillator is set to the desired frequency
• Then the tuning Capacitor is adjusted for maximum
value 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.
• Thus the measured value of Q is smaller than the true value.
• 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.
VC= VL and I = E/R
Q =ωL/R =1/ωCR