47
Figure 29 Bandwidth and Nyquist frequency
In practice, an oscilloscope's sampling rate should be four or more times its bandwidth: f
S
= 4f
BW
.
This way, there is less aliasing and the aliased frequency components have a greater amount of
attenuation.
4.3
Memory Depth
Memory depth refers to the number of points sampled from the waveform that the oscilloscope
can store in a single trigger sample. It directly reflects the amount of sample memory. There is
separate memory for each interleaved channel set (CH1 and CH2 interleaved, CH3 and CH4
interleaved). The oscilloscope provides up to 140 Mpts memory depth (70 Mpts if two interleaved
channels are being displayed).
To change the memory depth
1.
Press the Acquire button on the front panel.
2.
Press the Mem Depth softkey.
3.
Turn the Universal Knob to select the desired value and press the knob to confirm.
4.
Pressing the Mem Depth softkey repeatedly can also select the desired value. The current
memory depth is displayed in upper-right corner of the screen.
The following equation relates memory depth D (in samples), sampling rate R (samples per
second), and waveform length T (seconds):
𝐷 = 𝑅𝑇
Aliased
frequency
components
Attenuation
f
S
/4 f
N
f
S
-3dB
Frequency
0dB
Limiting oscilloscope bandwidth (f
BW
) to ¼ the sample rate (f
s
/4)
reduces frequency components above the Nyquist frequency (f
N
).