A total harmonic distortion analyzer calculates the total

harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...

content of a

sinewave A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in ...

with some

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...

, expressed as

total harmonic distortion The total harmonic distortion (THD or THDi) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion facto ...

(THD). A typical application is to determine the THD of an amplifier by using a very-low-distortion sinewave input and examining the output. The figure measured will include noise, and any contribution from imperfect filtering out of the

fundamental frequency The fundamental frequency, often referred to simply as the ''fundamental'', is defined as the lowest frequency of a periodic waveform. In music, the fundamental is the musical pitch of a note that is perceived as the lowest partial present. In ...

. Harmonic-by-harmonic measurement, without wideband noise, can be measured by a more complex wave analyser. Another application is measurement of the effectiveness of an

electronic filter Electronic filters are a type of signal processing filter in the form of electrical circuits. This article covers those filters consisting of lumped electronic components, as opposed to distributed-element filters. That is, using components ...

with extremely narrow

passband A passband is the range of frequencies or wavelengths that can pass through a filter. For example, a radio receiver contains a bandpass filter to select the frequency of the desired radio signal out of all the radio waves picked up by its antenn ...

, such as a

notch filter In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a ba ...

in a

parametric equalizer Equalization, or simply EQ, in sound recording and reproduction is the process of adjusting the volume of different frequency bands within an audio signal. The circuit or equipment used to achieve this is called an equalizer. Most hi-fi eq ...

Types of THD meter

There are several types of distortion analyzers: # Fundamental suppression #

Heterodyne A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is us ...

type # Tuned circuit #

Spectrum analyzer A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...

Fundamental suppression analyzer

This type of THD analyzer filters out the fundamental frequency of a signal with a

, leaving only distortion products plus noise; the ratio of this remnant to the signal amplitude is the THD.

Principles of operation

A fundamental suppression analyzer consists of three main sections: input section with impedance matcher, a

and

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...

section, and an output

meter The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its prefi ...

ing circuit.

Negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...

from the bridge amplifier to the pre-amp section may be applied to enable the rejection circuit to work more accurately.

Working of a typical unit

The input is impedance-matched with the rejection circuit with the help of an attenuator and an impedance matcher. This signal is then pre-amplified to a desired level. The following section consists of a

Wien bridge The Wien bridge is a type of bridge circuit that was developed by Max Wien in 1891. The bridge consists of four resistors and two capacitors. At the time of the Wien bridge's invention, bridge circuits were a common way of measuring component v ...

notch filter tuned to reject the fundamental frequency and balanced for minimum output by adjusting the bridge controls. The output, which is the remaining signal after the fundamental has been suppressed, is amplified to a measurable level. A feedback loop from the bridge amplifier output to the pre-amp input helps to eliminate any remaining contribution from the fundamental frequency. The output from these blocks is measured, typically using an

instrumentation amplifier An instrumentation amplifier (sometimes shorthanded as in-amp or InAmp) is a type of differential amplifier that has been outfitted with input buffer amplifiers, which eliminate the need for input impedance matching and thus make the amplifier ...

driving an analog or digital meter. The voltage at the meter is due to the harmonic distortion products plus noise.

References

* * *{{Citation , first=Dennis , last=Roddy , first2=John , last2=Coolen , title=Electronic Communications , year=1977 , publisher=Reston Publishing Co. , isbn=978-0-87909-224-5 Audio engineering Electronic test equipment

Types of THD meter

Fundamental suppression analyzer

Principles of operation

Working of a typical unit

See also

References