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How update rate affects signal visibility
Capturing random and infrequent events on an oscilloscope is all about statistical probabilities. The key to improving the prob-
ability of capturing a signal anomaly is to minimize dead-time and take more pictures of the signal in a given timeframe. Here is
an example with Tek and Agilent scopes both connected to a target with a glitch that occurs 25 times per second.
Seeing subtle signal detail and infrequent events requires a scope with fast waveform update rates. Don’t take a scope vendor’s
banner waveform update rate specifi cation at face value. Test it yourself. It’s actually pretty easy to characterize a scope’s
update rate. Run a moderately fast signal (e.g. 50 Mhz) into a scope channel. Measure the scope’s average trigger output signal
frequency. This is your scope’s update rate for the specifi ed timebase setting. Test the update rate of the scope under various
setup conditions. Setup conditions that Agilent suggests varying include timebase range, memory depth, and number of chan-
nels, including analog, digital, as well as channels assigned for serial decoding.
Tek MSO4104
•
Product data sheet: 35,000 waveforms per second.
•
Update rate = 18 waveforms per second with
10 Mpts and digital channels turned on. Resulting
measurement shown.
•
Probability of capturing the infrequent glitch = 0.09%
after running for 10 seconds.
•
Average time to capture just one glitch = 128 minutes.
Agilent MSO6104A
•
Product data sheet: 100,000 waveforms per second.
•
Update rate = 95,000 waveforms per second with auto
memory and digital channels turned on. Resulting
measurement shown.
•
Probability of capturing the infrequent glitch = 99% after
running for 10 seconds.
•
Average time to capture just one glitch = 1.5 seconds.
Memory* Scope settings Measured update rates
Infrequent
signal activity
Critical
signal jitter
?
?
?
* Agilent and LeCroy memory depth selection was automatically selected. Memory depth = display window times sample rate with up to 8 Mpts for Agilent.
** Tek measurements taken with version 2.13 fi rmware.
Initial setup
Change timebase
Add digital channels
Increase memory setting
Turn on serial decode
Timebase setting
20 ns/div
10 ns/div
20 ns/div
20 ns/div
20 ns/div
TEK MSO4104A**
55,000
2,700
125
35
0.2
Digital Channels
-
-
On
On
On
LeCroy WR 104Xi
27
27
27
27
25
Serial Decode
-
-
-
-
On
Agilent MSO6104A
95,000
95,000
95,000
95,000
95,000
Tek
10 Kpts
10 Kpts
10 Kpts
10 Mpts
10 Mpts
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