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UAV TECHNOLOGY

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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


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