A modified DFT for improved accuracy in harmonic measurements of periodic waveforms

In many cases asynchronous sampling is more practical than synchronous sampling. In such cases when using discrete Fourier transform (DFT) methods the various “leakage” terms can give inaccurate results. By making use of an “end” correction and a modified DFT improved accuracy in the determination of both amplitude and phase components of the various harmonic terms in a periodic signal is possible. The window length used in the modified DFT is adjusted by the “end” correction to coincide with the fundamental signal period (or a multiple of it). The approach has been shown to minimize long, medium and short-range leakage errors, which results in minimizing the errors in determining the actual harmonic components. Typical application of this technique is in microprocessor/DSP based instruments where simplicity of the hardware design requires that the analog to digital (AD) conversions times are controlled by the system clock and not the period of the waveform being sampled. This approach can also be used in situations where only one period of a distorted waveform is available for harmonic component determination. The accuracy of the approach is confirmed by comparing the DFT windows with and without the modification