- THD analyzer
Total Harmonic Distortion (THD) Analyzers calculate the total
distortion introduced by all theharmonics of thefundamental frequency wave. In most cases THD is the amount that is required to calculate, rather than distortion caused by individual harmonics. This type of analysis is very important in systems (e.g. Audio) in which filters having an extremely smallpassband /stopband is desired, such as anotch filter .Fundamental Suppression Analyzer
This is a specific type of THD analyzer, in which basically the fundamental frequency of the input wave is "suppressed" so as to remove it from the spectra of the meters used for distortion measurement, and the total gain of all the harmonics is calculated, thus obtaining the "total distortion" caused by the harmonics.
Construction
A block diagram of a Fundamental Suppression Analyzer is shown in Fig.1. This basic construction consists of three main sections: Input section with
impedance matcher , a rejectionamplifier section and an outputmeter ing circuit. Notice thefeedback from the bridge amplifier to the pre-amp section, that enables the rejection circuit to work more accurately.Working
The applied input wave is impedance matched with the rejection circuit with the help of an
attenuator and an impedance matcher. This signal is then applied to apre-amplifier which raises the signal level to a desired value. The following section consists of aWien bridge . The bridge is tuned to the fundamental frequency by frequency control and it is balanced for zero output by adjusting the bridge controls, thus giving anotch in the frequency response of the rejection section.After the Wien Bridge, a bridge amplifier follows that simply amplifies low harmonic voltage levels to measurable higher levels. A feedback loop is formed from Bridge Amp o/p to the Pre-Amp i/p thus eliminating even the slightest effect of fundamental frequency. This filtered output is then applied to a meter amplifier which can be aninstrumentation amplifier . This amp raises the voltage levels to the compatibility of the meter scale/digital meter which follows. Thus the total voltage obtained at the meter output shows the amount of distortion present in the wave due to harmonics of fundamental.References
"Modern Electronic Instrumentation & Measurement Techniques by Albert D. Helfrick,William D. Cooper"
"Electronic Communications by Dennis Roddy, John Coolen"
See also
*
Distortion measurement
*Audio quality measurement
*ITU-R 468 noise weighting
*Loudspeaker measurement
*Alignment level
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