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4000 X-Series Oscilloscopes Advanced Training Guide
B
Oscilloscope Bandwidth Tutorial
Defining Oscilloscope Bandwidth
All oscilloscopes exhibit a low-pass frequency response that rolls-off at higher
frequencies as shown in
. Most scopes with bandwidth specifications of
1 GHz and below typically have what is called a Gaussian frequency response. An
oscilloscope Gaussian frequency response approximates a single-pole low-pass
filter.
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.
Figure 104
Oscilloscope Gaussian frequency response
