The bandwidth of an oscilloscope is specified as the frequency where the
response is down 3 dB. At that point the amplitude is only attenuated by
approximately 30 percent. If you select a Gaussian filter whose 3-dB
frequency is equal to the Nyquist frequency (half the sampling rate), a
significant amount of energy will be allowed to pass through at frequencies
above the Nyquist frequency (see figure 26). This will result in significant
errors due to aliasing. A more conservative choice would be to set the 3-dB
frequency to 1/4 the sampling rate. Then the response will be down 12 dB at
the Nyquist frequency.
The bandwidth of the HP 54720 depends on the plug-in selection. You must
consider the characteristics of the signals you want to measure before
selecting the appropriate plug-in for single-shot measurements. When using
the HP 54711A or HP 54712A plug-ins, the bandwidth is greater than 1/4 the
sampling rate, so there is risk of aliasing. These plug-ins can be used for
single-shot measurements if you know the incoming signal contains no
frequencies above 1/4 the sampling rate; in other words, if the rise time of the
incoming signal is known to be greater than 2 ns.
Measurements
Time-interval measurements
13–55
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