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Oscilloscope Measurement Uncertainties - MU Components for Vertical Deflection


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  Post Number #1  
Old 17th May 2011, 11:59 PM
EMacP

 
 
Total Posts: 14
Please Help! Oscilloscope Measurement Uncertainties - MU Components for Vertical Deflection

Hi All,

Very urgent. Need your response. Currently my MU components for vertical deflection are type a and type b consists of upper and lower cursor, master std. However I think i missed out one more that is the minor division. However, how can i determine the value? Is there a specific formula? Please help me...

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  Post Number #2  
Old 19th May 2011, 06:25 AM
Stijloor's Avatar
Stijloor

 
 
Total Posts: 14,993
Re: Oscilloscope Measurement Uncertainties - MU Components for Vertical Deflection

Quote:
In Reply to Parent Post by EMacP View Post

Hi All,

Very urgent. Need your response. Currently my MU components for vertical deflection are type a and type b consists of upper and lower cursor, master std. However I think i missed out one more that is the minor division. However, how can i determine the value? Is there a specific formula? Please help me...
Any Calibration/MU experts that can help?

Thank you!!

Stijloor.
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  Post Number #3  
Old 20th May 2011, 03:29 AM
Hershal's Avatar
Hershal

 
 
Total Posts: 2,428
Re: Oscilloscope Measurement Uncertainties - MU Components for Vertical Deflection

Oscilloscopes have unique uncertainties. Based on the description, I presume you are discussing a digitizer which has some uncertainties the same and some different than analog scopes.

Digitizer: First and most obvious is the trace width. That will affect the measurement just a little, even if you use the cursors. There is also parallax, which in truth is included with the trace width in a digitizer. Trace width is a rectangular distribution.

Calibration of the cursors is a huge help, but you have to take the uncertainty there as a normal distribution.

Applicable to both digital and analog:

Minor division is not a huge influence, but being between minor divisions is as that cannot really be accurately eyeballed. So take the minor division as the value with rectangular distribution, if you experience that.

Also, different input impedance may have an effect. For example, 50 Ohm, 600 Ohm, 1 MOhm.

If you are near the upper end on your bandwidth (BW) then there could be additional losses. For example, if you use a 50 Ohm coax above 400 MHz.

Watch rise time. The formula is .35 over T rise equals BW, or .35 over BW equals T rise. However, the ratio is not always perfect. Check both BW and T rise, then calculate to make sure you get the same results.

Analog: Hard to get any real reading under 2% due to trace, stability, bandwidth, and so forth, so just use an uncertainty value of 0.5% of reading and you should be OK.

Hope this helps.
  Post Number #4  
Old 21st May 2011, 11:23 PM
EMacP

 
 
Total Posts: 14
Re: Oscilloscope Measurement Uncertainties - MU Components for Vertical Deflection

Hi Hershal,

I understand your reply. for your information the MU components involving Vertical deflection is as follow:

1. Type A
2. Lower and Upper cursor
3. Lower and Upper Parralax error
4. Specs/Uncertainties of standard use.
5. Vertical resolution ---- this is the one I am stuck with.

I will explain a little further, currently the digital scope is an 8-bit scope. This mean that there is 2^8= 256 steps level for the scope to sample. Let assume I set my scope at 1V/div and I am setting my source for a 6div=6V. My problem now is how to convert this to uncertainty? Any formulae???

So whoever has any idea, please assist me.
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