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digital applications based on clock rates or edge speeds, it still applies to
analog applications, such as modulated RF. To understand where this
3- to- 1 multiplying factor comes from, let's look at an actual frequency
response of a 1 GHz bandwidth scope.
Figure 64
an Agilent 1 GHz bandwidth oscilloscope. As you can see, at exactly 1 GHz
the measured output (waveform on oscilloscope display) is attenuated by
slightly less than 3 dB (Vo/Vi > 0.7). To make accurate measurements on
analog signals, you would need to use the scope in the portion of the
frequency band where it is still relatively flat with minimal attenuation. At
approximately one- third the scope's 1 GHz bandwidth, this scope exhibits
very little attenuation (- 0.2 dB).
Figure 64 Actual frequency response of an Agilent 1 GHz bandwidth oscilloscope
Required Bandwidth for Digital Applications
Rule of Thumb
As a rule of thumb, your scope's bandwidth should be at least five times
higher than the fastest digital clock rate in your system under test. If your
scope meets this criterion, then it will be able to capture up to the fifth
harmonic with minimum signal attenuation. This component of the signal
is very important in determining the overall shape of your digital signals.
2000 X-Series Oscilloscopes Advanced Training Guide
shows a measured frequency response test (1 MHz to 2 GHz) on
f
BW
Oscilloscope Bandwidth Tutorial
>= 5 x f
clk
B
95
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