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iFFT of anti-alias filter transition (guard) band

Question asked by Bob on Dec 5, 2007
Latest reply on Jun 30, 2010 by tabbott
For a vector signal analyzer, an anti-alias filter is used to remove unwanted frequency before the sampling. A non-ideal filter has transition band due to roll-off. This transition band can also be used as a guardband. I've read from AN150-15 that when the analyzer computes the inverse FFT, the signals in the guardband are used to provide the most accurate time-domain results. Why is this so? Why not the passband?