|
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. THD is used to characterize the linearity of audio systems and the power quality of electric power systems. Distortion factor is a closely related term, sometimes used as a synonym. In audio systems, lower distortion means the components in a loudspeaker, amplifier or microphone or other equipment produce a more accurate reproduction of an audio recording. In radiocommunications, lower THD means pure signal emission without causing interferences to other electronic devices. Moreover, the problem of distorted and not eco-friendly radio emissions appear to be also very important in the context of spectrum sharing and spectrum sensing.〔(Iaroslav Blagouchine and Eric Moreau. ''Analytic Method for the Computation of the Total Harmonic Distortion by the Cauchy Method of Residues.'' IEEE Transactions on Communications, vol. 59, no. 9, pp. 2478—2491, September 2011. )〕 In power systems, lower THD means reduction in peak currents, heating, emissions, and core loss in motors.〔(Total Harmonic Distortion and Effects in Electrical Power Systems - Associated Power Technologies )〕 ==Definitions and examples== To understand a system with an input and an output, such as an audio amplifier, we start with an ideal system where the transfer function is linear and time-invariant. When a signal passes through a non-ideal, non-linear device, additional content is added at the harmonics of the original frequencies. THD is a measurement of the extent of that distortion. When the main performance criterion is the ″purity″ of the original sine wave (in other words, the contribution of the original frequency with respect to its harmonics), the measurement is most commonly defined as the ratio of the RMS amplitude of a set of higher harmonic frequencies to the RMS amplitude of the first harmonic, or fundamental, frequency〔〔〔(On the Definition of Total Harmonic Distortion and Its Effect on Measurement Interpretation ), Doron Shmilovitz〕〔 〕〔(THD Measurement and Conversion ) "This number indicates the RMS voltage equivalent of total harmonic distortion power, as a percentage of the total output RMS voltage."〕〔〔IEEE 519 and other standards ((draft )): "distortion factor: The ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, often expressed as a percent of the fundamental. Also referred to as total harmonic distortion."〕〔(Section 11: Power Quality Considerations Bill Brown, P.E., Square D Engineering Services )〕 : where ''Vn'' is the RMS voltage of ''n''th harmonic and ''n'' = 1 is the fundamental frequency. In practice, the THDF is commonly used in audio distortion specifications (percentage THD); however, THD is a non-standardized specification and the results between manufacturers are not easily comparable. Since individual harmonic amplitudes are measured, it is required that the manufacturer disclose the test signal frequency range, level and gain conditions, and number of measurements taken. It is possible to measure the full 20–20 kHz range using a sweep (though distortion for a fundamental above 10 kHz is inaudible). For all signal processing equipment, except microphone preamplifiers, the preferred gain setting is unity. For microphone preamplifiers, standard practice is to use maximum gain. Measurements for calculating the THD are made at the output of a device under specified conditions. The THD is usually expressed in percent or in dB relative to the fundamental as distortion attenuation. A variant definition uses the fundamental plus harmonics as the reference, though usage is discouraged:〔〔(VOLTAGE WAVE QUALITY IN LOW VOLTAGE POWER SYSTEMS José M. R. Baptista, Manuel R. Cordeiro, and A. Machado e Moura )〕〔(The Power Electronics Handbook ) edited by Timothy L. Skvarenina "This definition is used by the Canadian Standards Association and the IEC"〕 : These can be distinguished as THDF (for "fundamental"), and THDR (for "root mean square").〔(AEMC 605 User Manual ) "THDf: Total harmonic distortion with respect to the fundamental. THDr: Total harmonic distortion with respect to the true RMS value of the signal."〕〔(39/41B Power Meter Glossary )〕 THDR cannot exceed 100%. At low distortion levels, the difference between the two calculation methods is negligible. For instance, a signal with THDF of 10% has a very similar THDR of 9.95%. However, at higher distortion levels the discrepancy becomes large. For instance, a signal with THDF 266% has a THDR of 94%.〔 A pure square wave with infinite harmonics has THDF of 48.3%,〔〔(Total Harmonic Distortion Calculation by Filtering for Power Quality Monitoring )〕〔(Electric Machines By Charles A. Gross )〕 or THDR of 43.5%.〔(Calculation of harmonic amplitude sum )〕〔(Total Harmonic Distortion of a square wave )〕 Some use the term "distortion factor" as a synonym for THDR,〔(Distortion factor )〕 while others use it as a synonym for THDF.〔IEEE 519〕〔(Harmonics and IEEE 519 )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Total harmonic distortion」の詳細全文を読む スポンサード リンク
|