Oscilloscope - AC gain calibration

Benjamin Weber

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#1
Dear community,

I am using an oscilloscope for quite a lot of measurements including AC voltages. The scope specifications cover detailed values for DC gain accuracy. The only thing related to AC measurements is the bandwith (max. -3dB at cut-off frequency). Using this for measurement uncertainty of AC voltage measurements, I can only use the 3dB (appr. 30%, but I am sure the scope is better). The standard calibration by the manufacturer's ISO 17025 accredited lab covers nothing more.

My idea was now to perform additional calibration for the frequency response in the range I am using the scope (bandwith is actually 60 MHz, I use only up to 1 MHz). One way would be to apply discrete frequency values with defined input signal shape (e.g. sine) and RMS or peak values. The other way would be to use the impulse response?

Does anybody have experience with AC gain calibration of oscilliscopes?
 
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dwperron

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#2
A scope is a very versatile tool, but you have seen that it does have its limits.
A couple of questions will provide information that is needed.
Are you using a probe or a direct input via a cable?
What is the input impedance of the scope when you are making these measurements?
 

dwperron

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#4
A high impedance measurement at higher frequencies using a cable can lead to impedance mismatch issues, and your results can vary widely.

You might be better off using a properly compensated X10 scope probe for these measurements, since it effectively moves the input termination of your measurement to the probe tip, reducing cable mismatch errors. Then you could pre-calibrate the probe to the frequencies of interest to your measurements.

I have attached a really good primer on scope probes that might be helpful to you.
 

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Benjamin Weber

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#5
Using a probe is not really option due to the rest of the test setup (I need robust connections for field testing).

Thank you very much for the primer on probes! In the section on scope bandwith affecting the measurement results (p. 35) it says, that if I use the scope at maximum frequencies below the actual bandwith I can improve AC accuracy to be better than 3dB or 30%. My scope has a specified bandwith of 60 MHz. According to the rule of thumb of the primer, the scope can achieve appr. 3% for a maximum frequency of 1/5 of it's bandwith, i.e. 15 MHz. My maximum signal frequency is 1 MHz and I usually work with a bandwith limition at 5 MHz. This is only a third of the 15 MHz.

Do you think it is justifiable to use the 3% accuracy for my uncertainty budget? Or do you see othe major negative influences here?
 

dwperron

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#6
My experience would agree that your numbers sound realistic.

Is it justifiable for your uncertainty budget? No.
You will need to do your own Type A study to come up with accuracy over the frequency range, documenting your method and results. You can then use that number as a contributor in your overall budget. This will be acceptable.
 

Benjamin Weber

Trusted Information Resource
#7
I will use this estimated AC gain accuracy as one component among others contributing to the final expanded measurement uncertainty. To undermine the improved AC gain accuracy I will do some detailled investigation at discrete frequencies in my frequency range.

Thank you very much for your help!
 
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