The task of frequency scanning involves a few data processing stages:
Low-Pass Filtering (Anti-Aliasing)
Sampling & Fourier Transform
Phase and Magnitude Error Correction
Computations are performed on-line, at each sampling instance, and are based on a sampled data window of the preceding input signal cycle. In accordance with the Nyquist Criteria, data sampling is performed at a frequency greater than double the highest harmonic frequency of interest. Sampling rates may be one of, 16, 32, 64, 128, 256 or 512 samples/cycle of base frequency, which are written to a buffer.
Since the number of samples in a window represents a period of base frequency, the dynamics of a cycle preceding a sample are captured in the computations. It should be noted that outputs of this subroutine contain valid information only if a complete data window is available for computations. Therefore, outputs for the first cycle should be discarded (unless started from a Snapshot). Linear interpolation techniques are used to compute the input at a sampling instance with minimal error.
It is important to be aware of the inherent aliasing effects due to sampling of the input signal. A low pass, anti-aliasing filter is recommended at all times, unless the input signal is guaranteed not to have any higher order harmonics. This filter is provided within the component.
The harmonic computations are based on a standard Fast Fourier Transformation (FFT) technique, used in digital signal processing. The basis function for computation of phase angle can either be a fundamental frequency cosine waveform or a sine waveform starting at time = 0.
The harmonics computed are with respect to a given constant fundamental frequency. For situations where the fundamental frequency is variable, the use of a frequency-tracking device is available to the user. The frequency-tracking unit uses the fundamental component of the input signal corresponding to the previous sampling instance (as computed by the FFT routine), to monitor small changes in the frequency of the input signal. This element is meant to monitor minor fluctuations of frequency. Frequency tracking may be enabled or disabled at users discretion.
Gibb's ringing effect, as a result of rectangular data windows, is usually not a problem with harmonics of the fundamental frequency. However, if the sampling frequency is not synchronized to the fundamental frequency of the input signal, the Gibb's effect distortions introduced on the measurement of harmonics may be significant. Therefore, use of the frequency-tracking feature may not be needed unless the fundamental component is guaranteed to be free of frequency swings.