stefan86's offtopic random retard thread - post away!

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Suppose you are sampling your 70MHz signal with a digitizer that has a maximum sampling rate of 100 megasamples per second (MS/s) and a bandwidth of 100MHz, this situation has been set up in Figure 2. If you are sampling at 100MS/s then your Nyquist frequency is 50MHz. Thus, the 70MHz signal that you are sampling will be aliased back and appear as an image at 30MHz (70 is 20 greater than the Nyquist frequency of 50 and so the aliased signal will occur as a mirror image at 20 less than 50, or 30). Now, by placing a high-pass, external filter of 60MHz upon the incoming signal, all real frequency components below 60MHz will be attenuated and all that will be left is the aliased image at 30MHz of the real signal at 70MHz. Because the bandwidth of the digitizer is 100MHz, the 70MHz signal and its aliased image will both have the same, slightly attenuated magnitude which should be easy to recognize.