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Author Topic: How to perform basic quality evaluation in a new Oscilloscope probe  (Read 3109 times)

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Online Kiriakos GR

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The most basic quality evaluation that you may perform is measuring it own resistance at x1 and x10 (probe switch) positions.
Naturally you will need a high quality multimeter for this job, so the measurement to be close to described as unquestionably correct.


The best method to do this measurement is to have one accessory that is a female BNC plug to dual banana.
Dual banana gets plugged directly to multimeter inputs, and the next step is to connect the Oscilloscope probe at the top of it.
In this measurement you have to connect the ground alligator clip with the tip of the probe so this to become a close circuit.

One high quality probe will measure 300 Ohm at x1 and at x10 this will be exactly 9.00 Mohm.

For example one mediocre in quality probe, this can measure 360 Ohm & 8.9 or 8.8Mohm.

In order to measure one x100 Oscilloscope probe this must be measured by a multimeter capable for nanosiemens.
The resistance of such a probe expected to be 99Mohm and therefore regular Ohms range can not help in this measurement because most multimeter can measure up to 50MOhms.

At NS (nanosiemens mode) one good quality probe will measure 10.10 ns.
The conversion of ns to Mohm is simple by dividing by 1000.
Example:   1000 / 10.10ns = 99.009KOhms or 99Mohm.

Nanosiemens range can not offer high accuracy and usually this is between 1% & 2% in good quality multimeter.
I did confirm this measurement with FLUKE 28II and BRYMEN BM869, and both meters measured 10.10 ns
 
By taking in to account this plus/minus 2% accuracy those 99Mohm it can vary between 97.02 and 100.98 Mohm, either way what is important is the probe to measure no less than 99Mohm because by the additional 1Mohm at the Oscilloscope input, this will not be at the target point of 100Mohm.

In summary my measurements’ made known to me that at x1 my probe is 1Mohm + 300 Ohm.
At x10 this is 9+1= 10Mohm which this is the target point.
And the x100 probe very correctly measured to be at 99Mohm + 1Mohm from Oscilloscope input, this becomes a 100Mohm as expected.


Now an advanced quality evaluation in a new Oscilloscope probe, this requiring testing at specified by the vendor probe as bandwidth limit.
This has importance mostly to people working with RF applications.
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