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40 User Manual
Oscilloscope 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.
0dB
-3dB
Attenuation
f
N
f
S
Frequency
However, digital signals have frequency components above 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.
f
S
/4 f
N
f
S
Frequency
Limiting oscilloscope bandwidth (f
BW
) to 1/4 the sample rate (f
S
/4)
reduces frequency components above the Nyquist frequency (f
N
).
So, in practice, an oscilloscope's sample rate should be four or more times its
bandwidth: f
S
= 4fBW. This way, there is less aliasing, and aliased frequency
components have a greater amount of attenuation.
Aliased frequency
components
Attenuation
0dB
-3dB