Calibration of oscilloscope doesn't make it accurate?

Calibration of oscilloscope doesnt make it accurate?

Hello,
We are taking scope readings on our SMPS with a Lecroy Wavesurfer scope. The readings dont seem to be accurate. When a scope is calibrated, and found to be inaccurate, is it true that the scope just gets given an inaccuracy report and returned as "calibrated"?
Readings like eg the peak voltage on the source sense resistors dont appear correct.

Re: Calibration of oscilloscope doesnt make it accurate?

This is a case of actual calibration adjustment and a cal check. Some calibration houses will issue a calibration error % figure without actual correcting the cal error. The argument is that it may be worse to disturb settled positions for calibration pots, etc. I feel if electronic calibration via software is available, then it should be done.

Re: Calibration of oscilloscope doesnt make it accurate?

Thanks, i thought that they legally had to declare the device "uncalibrated" if it was out of spec on accuracy?

Re: Calibration of oscilloscope doesnt make it accurate?

One would think that they will mention it in their lab report. Instead of asking for a calibration, maybe ask for calibration and adjustment.

Re: Calibration of oscilloscope doesnt make it accurate?

thanks, i suspect you're right , but surely, if a scope say is inaccurate, and outside of spec, then surely it cant be "calibrated"?

Re: Calibration of oscilloscope doesnt make it accurate?

Discuss it with the calibration house and hear what they say. Maybe they require you to ask for a service and calibration instead of just calibration.

Re: Calibration of oscilloscope doesnt make it accurate?

Some assumptions in this thread are wrong. Modern oscilloscopes have various kinds of internal gain and waveform adjustments, mostly performed in the digital domain. Many of it are already modified in self-calibration procedures, e.g. with a loop-back cable connection from test signal output to signal inputs.

In practice, it's unlikely that a digital oscilloscope fails the relative wide level accuracy specifications (e.g. +/- 2 %)without an actual defect. If this happens, the fault should be normally detected in calibration. There's a certain possibility that not all vertical attenuator settings are calibrated so that a defect isn't noticed.

A calibration protocol worth it's name will always indicate which settings have been checked against which specification.

On the other hand, it's relative easy to perform your own calibration check for the range of interest.

If I would bet, there's a 95% chance of an inappropriate measurement setup and 5% of a faulty calibration.

Re: Calibration of oscilloscope doesnt make it accurate?

I saw the scope in question is a Lecroy Wavesurfer, and I agree that in most cases, automatic internal calibration should take care of it, unless there is a problem that prevents the auto-cal from doing it satisfactorily.

Some instruments I have seen have internal preset adjustments, which are then fine tuned with DAC's. I have seen error messages that indicate that the auto adjustment error is outside of the range of the DAC and needs to be referred to service.

I am not familiar with the internals of the Wavesurfer though.

Re: Calibration of oscilloscope doesnt make it accurate?

We have some of the older model Wavepro's and there is a service menu you can access with certain key combinations, which allow you to alter internal calibration values.

Re: Calibration of oscilloscope doesnt make it accurate?

treez said:

is it true that the scope just gets given an inaccuracy report and returned as "calibrated"?

Click to expand...

Yes there is calibration report, this informing end user of actual measuring error.
Oscilloscopes tightest accuracy this is 1.5% according to most oscilloscope specifications.
What is your own model, accuracy specification ?

calibration report this is task in priority so some one to move in step number two, this called adjustment.
Adjustment this is additional work, depending type of equipment, this requiring a fee.

SMPS and accuracy in measurements this is a major challenge for low-end Oscilloscopes (education grade ones), and user he must be expert so to use appropriate settings and one 100X probe.