Relationship Between Bandwidth and Rise Time Hardware: Modular Instruments>>High-Speed Digitizers (Scopes)
Problem: 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). This statement has been seen in other documents (see related links for more information) but square signals have sharp edges and therefore the total bandwidth of the signal is not straight forward to calculate. There is a general equation relating bandwidth and rise time: BW = 0.35 / Tr Where BW is bandwidth and Tr the rise time of the signal. For example, if you need to measure a square signal of about 100 ns rise time, your bandwidth will be about 3.5 MHz (0.35 / 100E-9) and 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 the example, NI 5112 or NI 5122 is a good choice because they have a sampling rate of 100 MS/s. For more information about the products refer to the High-Speed Digitizers product page in the Related Links section below. Related Links: Developer Zone Tutorial: Sampling/Digitizer Basics Products and Services: Digitizers/Oscilloscopes Knowledge Base 2D39R50A: How Do the High Speed Digitizer (scope) Boards Calculate the Rising Time of a Signal? Attachments:
Report Date: 02/14/2007 Last Updated: 11/26/2007 Document ID: 46DA70JX |
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