Technical data

Oscilloscope Bandwidth Tutorial B
4000 X-Series Oscilloscopes Advanced Training Guide 159
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 105 shows a measured frequency response test (1 MHz to 2 GHz)
on 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).
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.
Figure 105 Actual frequency response of an Agilent 1 GHz bandwidth oscilloscope
f
BW
>= 5 x f
clk