Sample Control
30
The sample rate affect the waveform in the following manner :
•
Waveform Aliasing
: Aliasing occurs when the signal is under-sampled. The signal is distorted by low frequencies
falsely being reconstructed from an insufficient number of sample points.
Figure 5.2
Low Sample Rate
5.4 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).
The sampling theory requires the sample rate to be
𝑓
𝑆
= 2 ∗ 𝑓
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 5.3
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.
