Automated waveform measurements are important analysis tools, but the results of these measurements are only as good as the data used to calculate them. The Tektronix 3 Series MDO calculates measurements using the actual acquisition data. And while this can involve a large number of calculations, it provides a large and consistent dataset on which to base measurements. The Keysight X3000T uses a subset of the acquisition data. When display data is adjusted, for example when the display is “zoomed”, measurement quality may be impacted on the Keysight instrument. This post compares the measurement stability of the Tek 3 Series MDO and Keysight X 3000T while zooming in on displayed waveforms.
The same signal but different results
In this example we’ll perform the same measurement on the Tektronix 3 Series MDO and Keysight X3000T.
Both oscilloscopes acquire more data samples than they can display. They both convert the sampled acquisition data to display data before the waveform is written to the display. But there is a difference in the data source each scope uses for measurements. As a result, the 3 Series MDO gives consist measurements, regardless of the zoom factor. The Keysight X3000T can give varying results, depending upon the zoom setting.
To show this, we’ll set up rise-time measurements on both instruments and attach them to the same signal and then acquire it. We’ll gate the measurements with cursors so we can see exactly which edge we’re measuring. This test is easy to duplicate.
Image from the short video on this measurement comparison.
Consistent measurements on the Tektronix 3 Series MDO
As we change the zoom settings on the acquired waveform, we see that the measurements on the Tektronix 3 Series MDO remain the same, within normal expected variation. This is because the acquisition waveform data is being used for the measurement, not the display data, so changing the display zoom setting has no effect.
1. With horizontal zoom scale at 1 ms/div, measured rise time is 18.88 ns
2. With horizontal zoom scale at 400 ns/div, measured rise time is still 18.88 ns
3. With horizontal zoom scale at 200 ns/div, measured rise time is 18.88 ns, no change
4. With horizontal zoom scale at 100 ns/div, measured rise time is 18.88 ns with consistency
Measurements that change with zoom setting
On the Keysight X3000T the rise-time measurements on the acquired waveform change with different zoom settings, because the display data is being used for the measurements.
5. With horizontal zoom scale at 1 ms/div, measured rise time shows “30ns”
6. With horizontal zoom scale at 500 ns/div, measured rise time changed to “<23ns”
7. With horizontal zoom scale at 200 ns/div, measured rise time changed again to 22.75 ns
8. With horizontal zoom scale at 100 ns/div, measured rise time further changes to 22.56 ns
If you read the Keysight User Guide, a note cautions against zooming while making measurements on acquired waveforms. It says,
“Because measurements and math functions are performed on displayed data, you affect the resolution of functions and measurements.” (page 244)
Once you know about this issue on the X3000T you could avoid display functions like zoom and math while making measurements. Or, you could avoid the X3000T altogether, and go with a Tektronix 3 Series MDO. Then you can measure and zoom on your waveforms all you like.
I invite you to watch a short video showing this measure and zoom comparison. You can also download a factsheet comparing the 3 Series MDO to the Keysight DSO/MSO-X 3000T for a comparison of specifications.
Instruments used in this comparison & references:
Keysight Technologies MSO-X 3104T
V 7.30 May 2019
Tektronix 3 Series MDO
V1.4.26, Jun 2019
Keysight InfiniiVision 3000 X-Series Oscilloscopes User’s Guide
Manual Part Number 75037-97061