Inverse FFT
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I have taken 'ifft' of a column matrix in frequency domain, containing complex numbers.Though I was expecting absolute values as answer in time domain,to my surprise the out put has also come as a set of complex numbers.Now I want to plot this output values vs time.Wether I have to take 'abs(OUTPUT) and then plot or as it is?.Presently when I plot as it is the max value is coming 6.8 against the expected max value of 44.6.Kindly help me .Shrinivas
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Rick Rosson
2011 年 7 月 12 日
The difference in what you have and what you want is a constant scaling factor of 2*pi. That means the shape of your result is correct regardless of the actual scale or gain. One could argue (and some often do) that a constant scale factor is generally irrelevant in most situations.
But, if you really are concerned about the actual values (instead of the shape alone), the fact that the scale factor is exactly 2*pi suggests that the ratio is a units of measure conversion. The number 2*pi is not a pure number, but rather it has units of measure: radians per cycle. If we assume that the time domain signal is in units of newtons and time itself is seconds, then the Fourier domain is in units of either newtons per hertz OR newton-seconds per radian. Since hertz is cycles per second, the conversion factor is exactly 2*pi radians per cycle.
The bottom-line really is whether you are using standard frequency (in cycles per second or hertz), or angular frequency (in radians per second) as the unit of measure for the frequency domain.
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