Calculating Total Harmonic Distortion by Measuring Sine Wave

Total harmonic distortion is an intricate and habitually confusing concept to grasp. Real-Time direct dig-ital synthesis of analog waveforms using embedded processors and digital signal processors (DSPs) connected to dig-ital-to-analog converters (DACs) is becoming ubiquitous even in the smallest systems. One method to characterize the linearity of an amplifier is to measure its Total Harmonic Distortion (THD). Total Harmonic distortion is meas-ured by utilizing a spectrally pure sine wave in a defined circuit configuration and observing the output spectrum. The amount of distortion present at the output depends on several parameters, such as:

Small and large signal nonlinearity of the amplifier being tested

Amplifier's output amplitude or power level

Frequency response of the amplifier

Load applied to the output of the amplifier

Amplifier's power supply voltage

Circuit board layout

Grounding

Thermal management, etc.

Developing waveforms for use in embedded systems or laboratory instruments can be streamlined. One can develop and analyze the waveform generation algorithm and its associated data at desktop before implementing it.

Other important points to consider include algorithm efficiency, data ROM size required, and accuracy/spectral purity of the implementation. Comparing analysis is required when performing own waveform designs. The table data and look-up algorithm alone do not determine performance in the field. Further considerations such as the accuracy and stability of the real-time clock, and digital to analog converter are also required in order to evaluate overall performance. In this paper calculation of THD (Total Harmonic Distortion) is computed & presented by measuring an approximating sine wave.