Acquisition System
44
4.3 Bandwidth and Sample Rate
An oscilloscope’s bandwidth is typically described as the lowest frequency at which input signal sine
waves are attenuated by 3 dB (-30% amplitude error).
At the oscilloscope bandwidth, sampling theory says the required sample rate is
𝑓
𝑆
= 2𝑓
𝐵𝑊
. However,
the theory assumes there are no frequency components above
𝑓
𝑀𝐴𝑋
(
𝑓
𝐵𝑊
in this case) and it requires
a system with an ideal brick-wall frequency response.
Figure 4.5
Brick-Wall Frequency Response
Digital signals have frequency components about the fundamental frequency (Square waves are made
up of sine waves at the fundamental frequency and an infinite number of odd harmonics), and typically,
for 500 MHz bandwidths and below, oscilloscopes have a Gaussian frequency response.
Figure 4.6
Bandwidth Limiting
In practice, an oscilloscope’s sample rate should be four or more times its bandwidth:
𝑓
𝑠
= 4𝑥𝑓
𝐵𝑊
.
Doing so causes less aliasing, and aliased frequency components have a great amount of attenuation.
Summary of Contents for 2510B Series
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