Total harmonic distortion, Electrical Engineering

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Q. Explain the term total harmonic distortion. Describes the functionary of a total harmonic distortion analyzer.

Total Harmonic Distortion: Nonlinear behaviors of circuit elements introduce harmonics in the output waveform and the resultant distortion. A nonlinear system produces harmonics of an input sine wave, the harmonics consisting of sine waves with frequencies which are multiples of the fundamental of the input signal. Total harmonic distortion is measured in terms of the harmonic content of the wave, as given by equation.

     A measure of distortion represented by a particular harmonic is simply the ratio of amplitude of harmonic to that of the fundamental harmonic distortion is then represented.

 Harmonic Distortion Analyzer: a harmonic distortion analyzer which is used to measure THD. The signal source has very low distortion and this can be checked by reading its output distortion by connecting directly into the analyzer. The signal from the source is fed into the amplifier under test. The generates harmonics and the original frequency (fundamental). The fundamental frequency is removed by the notch filter.

        In the manual system of the switch SW is first placed imposition and the total content of fundamental and harmonics is measured. Then the switch is moved to position to measure just the harmonics. The value of THD is then fond from equation.

 The meter can be calibrated by putting the switch in position and adjusting the reading for full scale deflection. Wight the switch in position the meter reading is now proportional to THD.

  Shows an alternative arrangement, where the values of are read simultaneously and their ratio calculated and displayed as THD on the indicator. For good accuracy the notch filter must have excellent rejection and high pass characteristics. It should attenuate the fundamental by 100 db or more and the harmonics by less than 1dB. The filter also needs to be tuned accurately to the fundamental of the signal source. This is difficult to achieve manually and most distortion analyzer do this automatically. A common from of notch filter is Wien Bridge. The balances at one frequency only and at this frequency the output voltage at the frequency only and at this frequency the output voltage at the bridge null detector is minimum.

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