Total harmonic distortion analyzer

Last updated March 19, 2019

A total harmonic distortion analyzer calculates the total harmonic content of a sinewave with some distortion, expressed as total harmonic distortion (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. Harmonic-by-harmonic measurement, without wideband noise, can be measured by a more complex wave analyser.

A harmonic is any member of the harmonic series. The term is employed in various disciplines, including music, physics, acoustics, electronic power transmission, radio technology, and other fields. It is typically applied to repeating signals, such as sinusoidal waves. A harmonic of such a wave is a wave with a frequency that is a positive integer multiple of the frequency of the original wave, known as the fundamental frequency. The original wave is also called the 1st harmonic, the following harmonics are known as higher harmonics. As all harmonics are periodic at the fundamental frequency, the sum of harmonics is also periodic at that frequency. For example, if the fundamental frequency is 50 Hz, a common AC power supply frequency, the frequencies of the first three higher harmonics are 100 Hz, 150 Hz, 200 Hz and any addition of waves with these frequencies is periodic at 50 Hz.

An n^th characteristic mode, for n > 1, will have nodes that are not vibrating. For example, the 3rd characteristic mode will have nodes at $L and L, where L is the length of the string. In fact, each n th characteristic mode, for n not a multiple of 3, will not have nodes at these points. These other characteristic modes will be vibrating at the positions L and L . If the player gently touches one of these positions, then these other characteristic modes will be suppressed. The tonal harmonics from these other characteristic modes will then also be suppressed. Consequently, the tonal harmonics from the n th characteristic modes, where n is a multiple of 3, will be made relatively more prominent.$

Distortion is the alteration of the original shape of something. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal representing sound or a video signal representing images, in an electronic device or communication channel.

The total harmonic distortion (THD) 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 factor, a closely related term, is sometimes used as a synonym.

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 antenna. The passband of a receiver is the range of frequencies it can receive.

Types of THD meter

There are several types of distortion analyzers:^{[ citation needed ]}

Fundamental suppression^[1]
Heterodyne type^[2]
Tuned circuit
Spectrum analyzer

Fundamental suppression analyzer

Block Diagram of a THD Analyzer THD.jpg — Block Diagram of a THD Analyzer

This type of THD analyzer filters out the fundamental frequency of a signal with a notch filter, leaving only distortion products plus noise; the ratio of this remnant to the signal amplitude is the THD.

Principles of operation

The frequency response of a Fundamental Suppression Analyzer THD Response.jpg — The frequency response of a Fundamental Suppression Analyzer

A fundamental suppression analyzer consists of three main sections: input section with impedance matcher, a notch filter and amplifier section, and an output metering circuit. Negative feedback from the bridge amplifier to the pre-amp section may be applied to enable the rejection circuit to work more accurately.

Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied. The term complex impedance may be used interchangeably.

An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.

Negative feedback occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by other disturbances.

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

An attenuator is an electronic device that reduces the power of a signal without appreciably distorting its waveform.

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.

An instrumentationamplifier 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 particularly suitable for use in measurement and test equipment. Additional characteristics include very low DC offset, low drift, low noise, very high open-loop gain, very high common-mode rejection ratio, and very high input impedances. Instrumentation amplifiers are used where great accuracy and stability of the circuit both short and long-term are required.

Related Research Articles

In electronics, the figures of merit of an amplifier are numerical measures that characterize its properties and performance. Figures of merit can be given as a list of specifications that include properties such as gain, bandwidth, noise and linearity, among others listed in this article. Figures of merit are important for determining the suitability of a particular amplifier for an intended use.

Digital-to-analog converter device that converts a digital signal into an analog signal

In electronics, a digital-to-analog converter is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function.

A low-noise amplifier (LNA) is an electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio. An amplifier increases the power of both the signal and the noise present at its input. LNAs are designed to minimize additional noise. Designers can minimize additional noise by using low-noise components, operating points, and circuit topologies. Minimizing additional noise must balance with other goals such as power gain and impedance matching.

Audio system measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers make them to ensure equipment is still working to specification, or to ensure that the cumulative defects of an audio path are within limits considered acceptable. Some aspects of measurement and specification relate only to intended usage. Audio system measurements often accommodate psychoacoustic principles to measure the system in a way that relates to human hearing.

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 a spectrum analyzer measures is electrical; however, spectral compositions of other signals, such as acoustic pressure waves and optical light waves, can be considered through the use of an appropriate transducer. Optical spectrum analyzers also exist, which use direct optical techniques such as a monochromator to make measurements.

Preamplifier Circuit that prepares a signal for amplification

A preamplifier is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier and a loudspeaker. Without this, the final signal would be noisy or distorted. They are typically used to amplify signals from analog sensors such as microphones and pickups. Because of this, the preamplifier is often placed close to the sensor to reduce the effects of noise and interference.

An active filter is a type of analog circuit implementing an electronic filter using active components, typically an amplifier. Amplifiers included in a filter design can be used to improve the cost, performance and predictability of a filter.

An integrator in measurement and control applications is an element whose output signal is the time integral of its input signal. It accumulates the input quantity over a defined time to produce a representative output.

Linear electronic oscillator circuits, which generate a sinusoidal output signal, are composed of an amplifier and a frequency selective element, a filter. A linear oscillator circuit which uses an RC network, a combination of resistors and capacitors, for its frequency selective part is called an RC oscillator.

A function generator is usually a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. Some of the most common waveforms produced by the function generator are the sine wave, square wave, triangular wave and sawtooth shapes. These waveforms can be either repetitive or single-shot. Integrated circuits used to generate waveforms may also be described as function generator ICs.

In electronics, a frequency multiplier is an electronic circuit that generates an output signal whose output frequency is a harmonic (multiple) of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal. A subsequent bandpass filter selects the desired harmonic frequency and removes the unwanted fundamental and other harmonics from the output.

When measuring the value of an alternating current signal it is often necessary to convert the signal into a direct current signal of equivalent value which is known as the root mean square (RMS) value. Most low-cost instrumentation and signal converters carry out this conversion by filtering the signal into an average rectified value and applying a correction factor.

Noise (electronics) random fluctuation in an electrical signal

In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects.

Technical specifications and detailed information on the valve audio amplifier, including its development history.

Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.

Distortionmeter is an electronic measuring instrument which displays the amount of distortion added to the original signal by an electronic circuit.

An Audio Analyzer is a test and measurement instrument used to objectively quantify the audio performance of electronic and electro-acoustical devices. Audio quality metrics cover a wide variety of parameters, including level, gain, noise, harmonic and intermodulation distortion, frequency response, relative phase of signals, interchannel crosstalk, and more. In addition, many manufacturers have requirements for behavior and connectivity of audio devices that require specific tests and confirmations.

References

↑ Bakshi, Bakshi & Bakshi 2009, pp. 4-23 to 4-26.
↑ Bakshi, Bakshi & Bakshi 2009, pp. 4-26 to 4-??.

Bakshi, U. A.; Bakshi, A. V.; Bakshi, K. A. (2009), Electronic Measurements and Instrumentation (second revised ed.), Pune, India: Technical Publications Pune, ISBN 978-81-8431-665-0
Helfrick, Albert D.; Cooper, William D. (1989), Modern Electronic Instrumentation & Measurement Techniques, Prentice Hall, ISBN 978-0-13-593294-0
Roddy, Dennis; Coolen, John (1977), Electronic Communications, Reston Publishing Co., ISBN 978-0-87909-224-5

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[FOOTNOTEBakshiBakshiBakshi20094-23_to_4-26-1] Bakshi, Bakshi & Bakshi 2009, pp. 4-23 to 4-26.

[FOOTNOTEBakshiBakshiBakshi20094-26_to_4-??-2] Bakshi, Bakshi & Bakshi 2009, pp. 4-26 to 4-??.