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Why Digitize Data?

    • Aliasing and speed of response

    Surf to YouTube and watch the demonstration of a strobe light illuminating a fan. If an event happens 100 times per second and we sample it at 100 Hz, we get what looks like a DC signal -- no change as a function of time. If we sample at 101 Hz, the signal changes through one cycle in one second, or the 100 Hz signal has been "aliased" to 1 Hz. This frequency shifting can be useful (in the case of NMR, where it is rare for signals to be even 0.1% shifted in frequency, a 500 MHz raw signal aliased at 499 MHz allows digitization at 1 MHz). When aliasing occurs and we are unaware of it, we get a false sense of what is happening. Sampling at the power line frequency (50 Hz in most of the world, 60 Hz in North America) aliases away noise from power supplies.

    For time-dependent signals, we need to sample the waveform many times to accurately map out the shape of what is happening. Thus, the speed with which a signal changes dictates how rapidly we must digitize.






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