uav technology
TRANSCRIPT
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CONTENTS :
INTRODUCTION TO Q-METER
WHAT IS Q-FACTOR ?
OPERATING PRINCIPLE OF Q-METER
CONSTRUCTION OF Q-METER
OPERATION OF Q-METER
APPLICATIONOF Q-METER
MERIT AND DEMERIT OF Q-METER
REFERENCES CONCLUSION
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INTRODUCTION
A Q meter is a part of laboratory equipment employedin the testing of radio frequency circuits
It was invented by William D Loughlin at Boonton Radio
Corporation in Boonton,New Jersey in 1934. It measures the Q-factor(Q stands for quality) of a
circuit
Q-factor of a non-ideal circuit determines the amountof energy dissipated per cycle
It determines the Q-value of radio frequency circuitdirectly
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WHAT IS Q-FACTOR ?
Q-factor of a circuit is defined as
General: Q=2Peak energy stored/Energy dissipated per cycle
Inductors : Q=L/R
Capacitors : Q=1/CR LC band pass filters :Q=fc/BW ,where fc is the central frequency
It is a dimensionless parameter that describes how under-damped an oscillator is , or equivalently, characterizes aresonator's bandwith relative to its centerfrequency.[2] Higher Q indicates a lower rate of energy loss relativeto the stored energy of the oscillator; the oscillations die out moreslowly
http://en.wikipedia.org/wiki/Q_factorhttp://en.wikipedia.org/wiki/Q_factor -
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OPERATING PRINCIPLE
It operates on the principle of series resonance i.e. atresonate condition of an ac series circuit voltage across
the capacitor is equal to the applied voltage times of Q
of the circuit. If the voltage applied across the circuit is
kept-constant then voltmeter connected across the
capacitor can be calibrated to indicate Q directly.
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CONSTRUCTION
A Q-meter consists of a self-contained variable frequency RFoscillator.The oscillator delivers current to a low value resistance R1
Through R1,a small voltage V1 is injected into the resonant circuit ,which
in turn is measured by a thermocouple voltmeter A standard variable capacitor,C2 is employed for resonating the circuit
The coil under test is applied across the terminals T1 and T2
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OPERATIONBy definition Q-factor of the coil,
Q = XL / R
And when the circuit is under resonance condition
XL = XC
Or IXL = IXC = VC
And the voltage applied to the circuit.
V1 = IR
So, Q = XL / R = IXL /IR = V2/ V1
or, V2=Q.V1
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APPLICATION: Measurement of Q :
The value of Q is obtained directly from V2=Q.V1
Measurement of L:
L=1/4^2fc^2
Measurement of R(referred above as R2)
R=L/Qtrue
Measurement of self capacitance :
f=1/2 L(C2+C1)
f=1/2 L(C2+C1)
C1=C2-4C2/3
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MERIT It measures the Q-value of a resonating circuit directly
sans determining the value of R
Q value range could be automatically or manuallycontrolled.
DEMERIT
The value of Q measured is less than the true valueQtrue=Qmeasured(1+R1/R2)
It is inefficient ,meaning its efficiency is low
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REFERENCES:
Slide 3,4- http.://wikipedia.org/wiki/Q meter,08:10 PM
22 Feb,2012
Slide 5,7-circuitstoday.com/q-meter, meter,08:10 PM
22 Feb,2012
Slide 6,8,9-A Course in Electrical and Electronics Measurements and
Instrumentation by A.K. Sawhney,Dhanpat Rai & Co. Publication,18th
edition,ISBN:81-7700-016-0
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CONCLUSION
In fact,Q-meter had been a vital measuring instrument in the
laboratory
It has been replaced by more exotic (and more expensive)
impedance measuring devices
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Thank you