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Graticule error?

Question asked by ScopeDope on Apr 6, 2020
Latest reply on Apr 8, 2020 by ScopeDope

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I've recently purchased a fast rise time pulse generator for future use on a TDR application. The generator is spec'd to produce a pulse with a rise time of ~40ps, albeit with sufficient scope bandwidth on hand. I'm using a MSOX2024A for my measurements here.

My scope shows a rise time measurement of 1.47ns while the vertical major divisions are set at 2ns/div. The viewed rise time looks more closely to ~1.75ns as compared to the measured value. It is not really a big deal I'm just curious why the two rise time values don't align more closely. Perhaps I'm just nit-picking or I've been cooped up too long with this shelter in place virus thing.

Any comments appreciated.

72011orz Apr 8, 2020 4:24 PM

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algoss

Apr 8, 2020 7:10 AM

What threshold setting is the scope using, 20/80, 10/90, something else?

What threshold setting are you using, or rather, how did you choose the marker levels for your manual measurement?

I suspect that you are using something closer to a 10/90 setup, and the scope is using 20/80. Also remember that the scope is probably using a different algorithm for calculating "top" and "base" (needed for calculating the thresholds) than you are. Based on years of experience, I tend to trust the scope's automatic measurements, but you have to understand how they are being made.

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ScopeDope @ algoss on Apr 8, 2020 11:03 AM

Hello Al.

I checked the threshold settings and they are set at the default of 10% lower and 90% upper. The horizontal and vertical markers are superimposed automatically on screen when you choose the rise time measurement.

Using the cursors feature which then allows the positioning of the X1 AND X2 cursors does not provide any benefit other than I can then measure along the time division scale of my choosing. I'm inclined to think that you are correct about the scope using a different top and base algorithm or something to that effect.

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72011orz Apr 8, 2020 4:24 PM

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ScopeDope @ 72011orz on Apr 8, 2020 5:49 PM

That makes perfect sense as shown in your superb graphic. Kudos! I somehow came up with different values for the minor horizontal ticks and not realizing until just now that these values will change too dependent on the time/div setting.

It falls into place now, the vertical measurement span and the numeric measurement value. Well done, sir.

Thanks again for the assistance.

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algoss Apr 8, 2020 7:10 AM
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What threshold setting is the scope using, 20/80, 10/90, something else?
What threshold setting are you using, or rather, how did you choose the marker levels for your manual measurement?

I suspect that you are using something closer to a 10/90 setup, and the scope is using 20/80. Also remember that the scope is probably using a different algorithm for calculating "top" and "base" (needed for calculating the thresholds) than you are. Based on years of experience, I tend to trust the scope's automatic measurements, but you have to understand how they are being made.

Al
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- ScopeDope @ algoss on Apr 8, 2020 11:03 AM
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  Hello Al.
  
  I checked the threshold settings and they are set at the default of 10% lower and 90% upper. The horizontal and vertical markers are superimposed automatically on screen when you choose the rise time measurement.
  
  Using the cursors feature which then allows the positioning of the X1 AND X2 cursors does not provide any benefit other than I can then measure along the time division scale of my choosing. I'm inclined to think that you are correct about the scope using a different top and base algorithm or something to that effect.
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72011orz Apr 8, 2020 4:24 PM
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1 person found this helpful
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- ScopeDope @ 72011orz on Apr 8, 2020 5:49 PM
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  That makes perfect sense as shown in your superb graphic. Kudos! I somehow came up with different values for the minor horizontal ticks and not realizing until just now that these values will change too dependent on the time/div setting.
  
  It falls into place now, the vertical measurement span and the numeric measurement value. Well done, sir.
  
  Thanks again for the assistance.
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Graticule error?

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