| Abstract |
The PWM modulation techniques, suitably employed in a.c. variable-speed drive systems, cause high levels of harmonic distortion in the waveforms of voltage and current. On the other hand, the PWM voltage waveforms exhibit large-band frequency spectra, with a slow decay of the magnitudes of harmonic components. In the paper, new DSP techniques are proposed, which allow the user to perform highly accurate measurements. A multistage decimation of the acquired data may be performed by non-recursive realization of low-pass digital filters, to achieve a frequency-zooming in fast Fourier transform (FFT) spectrum analysis. Optimized cosine windows have been designed to realize low-pass FIR filters with maximally flat amplitude response in passband. Both pass- and stopband ripple are even lower than those attained by using classical equiripple Parks-McClellan and Kaiser-window design techniques. To select the frequency-band to be analyzed with high resolution a digital multiplier is required for frequency shifting. In order to minimize the negative effects of spectral leakage caused by an asynchronous sampling, new flat-top windows have been designed and characterized, which offer negligible errors in this kind of measurements. An important advantage of these windows consists in the increase of computational speed, because of the uselessness of interpolation algorithms, otherwise necessary. |
