This Article is about which high frequent events are captured with the Glitch detect function, even with a slow time base setting.
- About time base, memory depth and sample rate in Fluke ScopeMeters
- Glitch detect
- Displaying Min Max pairs
About time base, memory depth and sample rate in Fluke ScopeMeters
To make signals fully visible in the display of an oscilloscope, the horizontal divisions summed together, should be set to at least one period of the signal.
The lower the frequency of that signal, the longer the period and hence, the longer the time base TB; the time/division.
The relation 12*TB = memory depth /sample rate shows that in order to slow down the time base speed (sec./div.),
the sample frequency must decrease, since the memory depth is fixed. In other words: when slowing down the time base, the frequency of the signal that can be captured would decrease.
The only solution seems to be simply increasing the memory depth, but there are other techniques available to capture the high frequent information at long time base settings.
Glitch detect
To detect high frequent glitches and amplitude changes, also at low time base speed, a second process, parallel to the sample process is going on.
This process samples at 125 MS/s continuously and this rate is fixed; independent from the time base setting, so each 8 ns a sample is taken and digitized.
The result is not immediately displayed but kept in a register. And only when this value is exceeded by a next sample, it is replaced by this higher sample.
This is what we call the Max-register. The same way a Min-register is created but now a sample that has a lower level replaces the previous one.
The two values from both registers together is called a Min-Max-samples-pair. And by the time the display system (based on selected time base speed)
is ready to display yet another sample, only then is the Min-Max-sample-pair handed over to the display. Each pair holds the amplitude-extremes as were collected
over a certain period of time, using a digitizer that samples every 8 ns.
Displaying a Min - Max pairs
Each division of the display consists of 25 positions so the total number of display-positions is 300, per trace.
The interval between the positions depends on the time base. For example; with a time base of 40us/div.
the number of positions is 40us/25 = 1,6us. For each position the Min-Max-pair is taken from the register and
displayed and the register is emptied and the process is started again.
In this example, 200 original samples are included to determine one Min-Max-sample on the display.
See also table above, where glitch detect details are summarized for different time base settings. Because the sample interval remains 8 ns,
the number of samples that are included to determine a Min Max pair increase when the time base setting is lowered.
To capture the waveform of a signal in full detail, the sample frequency should be at least 5 times the signal frequency.
For capturing “only” the Min and Max values of the signal the sample frequency this is not needed and glitches of 8 ns can be captured and the envelope is displayed.
Fluke ScopeMeters® have 300 display positions, and Min Max pairs are displayed on each position with another color
than the wave form. See figure where the Min Max pairs are displayed in shades of red. See image above.
Conclusion
Thanks to high sample rate and the Min-Max (or ‘glitch detect’) system, the envelope of high frequency signal parts can be well detected and displayed, even at very slow time base settings.