Oscilloscope Familiarization Labs
2
4000 X-Series Oscilloscopes Advanced Training Guide
29
Lab #4: Using Peak Detect Acquisition Mode
All DSOs and MSOs have a fixed amount of acquisition memory. This is the number
of samples that the oscilloscope can digitize for each acquisition cycle. If the
scope’s timebase is set to a relatively fast time/div setting, such as 20 ns/div, then
the scope will always have a sufficient amount of memory to capture a waveform
at that setting using the scope’s maximum specified sample rate. For example, if a
scope’s maximum specified sample rate is 4 GSa/s (250 ps between samples), and
if the scope’s timebase is set to 20 ns/div, then an acquisition memory depth of
800 points is all that is required to capture and display a complete waveform. At
20 ns/div, a complete waveform across the scope’s screen would consist of 200 ns
of time (20 ns/div x 10 horizontal divisions). The required memory depth to fill this
time while still sampling at 4 GSa/s is then just 800 points (200ns/250ps = 800).
If you set the scope’s timebase to a much slower time/div setting in order to
capture slower waveforms and longer time, then the scope may need to
automatically reduce its sampling rate in order to fill the required waveform time.
All DSOs and MSOs do this. For example, let’s assume that you want to capture a
relatively slow signal and need to set the scope’s timebase to 10 ms/div (100 ms
across screen). If the scope’s maximum memory depth is 2 M points, then the
scope will need to reduce its sample rate to 20 MSa/s (100 ms/2 M = 50 ns
sample period).
Although in most cases this is not a problem, because capturing slower waveforms
doesn’t require fast sample rates, what if the input signal consisted of a
combination of low-speed and high-speed characteristics? For example, what if
the input signal that you want to capture is a 30 Hz sine wave with very narrow
glitches riding on it? Capturing the 30 Hz sine wave doesn’t require a fast sample
rate, but capturing the narrow glitches may require a very fast sample rate. Let’s
set up a test to capture a signal such as this.
1
Connect the channel-1 probe to the Demo 1 terminal and ground.
2
Press
[Default Setup]
on the scope’s front panel.
3
Press
[Help]
; then press the
Training Signals
softkey.
4
Select the
Sine with Glitch
signal; then press the
Output
softkey to turn it on.
5
Set channel-1’s V/div setting to
500.0 mV/div
.
6
Set the scope’s timebase to
10.00 ms/div
.
7
Press the
[Intensity]
button (under the Entry knob); then set the waveform trace
intensity to
100%
using the Entry knob or on-screen slider.
