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# Necessary Bandwidth to Measure a Digital Signal with a Specific Rise Time

Updated Nov 20, 2017

## Issue Details

I would like to digitize a square signal with a specific rise time, but I am not sure what the recommended bandwidth would be and what devices could achieve it.

## Solution

To satisfy the Nyquist theorem, we need to sample at least twice as fast as the maximum bandwidth of the signal (or about 10 times to have a proper shape). Digital, or square, signals have sharp edges and therefore the total bandwidth of the signal is not straight-forward to calculate.

The following is a general equation relating bandwidth and rise time:

BW = 0.35 / Tr

Where BW is bandwidth and Tr is the rise time of the signal.

For example, if you need to measure a square signal with 100 ns rise time, your bandwidth will be about 3.5 MHz (0.35 / 100E-9). Therefore, you should choose a digitizer that can sample at least twice as fast or, for shape purposes, 10 times faster: ~35 MS/s (Samples per second). For example, the NI 5112 or NI 5122 is a good choice because they have a sampling rate of 100 MS/s. Additionally, be sure that the anti-alias and noise filters on your digitizer are not enabled, as these can reduce the measurable bandwidth.