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The bandwidth is usually specified in terms of the -3db points in the frequency response characteristic. This means that the signal at the bandwidth spec frequency is 3db down from DC, or about 1/2 of the amplitude it would be for DC (or say a 1Hz AC signal).
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At 100MHz (we are talking sine waves here) the signal that shows up on the scope would be even less than 1 division now because it would be attenuated even more at this higher frequency. If the filter has a constant -3db rolloff, then the new signal on the scope would only measure 1/2 of one division.
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