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3000 X-Series Oscilloscopes Advanced Training Guide
B
Oscilloscope Bandwidth Tutorial
The lowest frequency at which the input signal is attenuated by 3 dB is
considered the scope’s bandwidth (f
BW
). Signal attenuation at the - 3 dB
frequency translates into approximately - 30% amplitude error. In other
words, if you input a 1 Vp- p, 100 MHz sine wave into a 100 MHz
bandwidth oscilloscope, the measured peak- to- peak voltage using this
scope would be in the range of approximately 700 mVp- p (- 3 dB = 20 Log
[0.707/1.0]). So you can’t expect to make accurate measurements on
signals that have significant frequencies near your scope’s bandwidth.
Closely related to an oscilloscope’s bandwidth specification is its rise time
specification. Scopes with a Gaussian- type frequency response will have an
approximate rise time of 0.35/f
BW
based on a 10% to 90% criterion. But
you need to remember that a scope’s rise time is not the fastest edge
speed that the oscilloscope can accurately measure. It is the fastest edge
speed the scope can possibly produce if the input signal has a theoretical
infinitely fast rise time (0 ps). Although this theoretical specification is
impossible to test — because pulse generators don’t have infinitely fast
edges — from a practical perspective, you can test your oscilloscope’s rise
time by inputting a pulse that has edge speeds that are 5 to 10 times
faster than the scope’s rise time specification.
Required Bandwidth for Analog Applications
Years ago, most oscilloscope vendors recommended that your scope’s
bandwidth should be at least three times higher than the maximum input
signal frequency. Although this “3X” multiplying factor would not apply to
Figure 85 Oscilloscope Gaussian frequency response