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

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.

Additional Information

For more information about the products, please refer to the High-Speed Digitizers product page in the Related Links section below.