Re: fourier anlysis of square wave...



Scott Seidman wrote:

Once you're in the digital domain, you can create any signal you want, and aliasing doesn't enter the picture.

Suppose you have an analog sinewave of 400Hz, the ADC/DAC operating at the sample rate of 1kHz with the perfect filtering at Nyquist 500Hz at the input and at the output and the nonlinearity y = x^2.

Consider two cases:

1. The analog sinewave is processed by x^2 nonlinearity, then lowpassed at Nyquist, digitized and then restored back to analog. Result: pure DC.

2. The analog sinewave is digitized, then the digital signal is processed by x^2 nonlinearity, then restored to analog. Result: DC + folded alias at 200 Hz.

Do you see the difference now?


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
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