Uncertainty of a measurement with a scope - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: May 30, 2019, 09:37:15 am
content: Hi,in the last days I had some discussions about the precision of scope measurements. I tried to find vertical specifications, but the scopes we are using at work (mostly Keysight and Tektronix) are only specified at DC (~2.5%) and the bandwidth is defined at the -3dB point.We have a few scopes which are yearly calibrated, but the calibration is also only checking the bandwidth and DC accuracy (and a few more things, but nothing that gives a hint of the vertical accuracy).Does that mean I can only be sure the signal is within 3dB, if I'm below the bandwidth of the scope? And are there scopes which are specified?I would like to measure peak to peak voltages (a couple of hundred volts, sinusoidal -> RMS is also ok) at 1-3 MHz. In my opinion a scope isn't a voltage measurement device, it is only capable to check not to measure. But for the given signal there are not much alternatives.It seems not only hard to measure such a signal accurate, it seems also hard to define the uncertainty in the first place.Does anyone have some hints?ThanksPhilipp

author: tggzzz
author_rank: Super Contributor
timestamp: May 30, 2019, 10:03:00 am
content: Quote from: e61_phil on May 30, 2019, 09:37:15 amA scope is not an appropriate tool for measuring the RMS or peak value of mains-level voltages. It can be done, but you need very specialised probes (e.g. pay very careful attention theprobe'svoltage derating as a function of frequency), and ideally specialised scopes, plus a knowledge of how tosafelyuse and not use scopes and probes.If you aren't interested in the waveshape, use an appropriately rated voltmeter with appropriate frequency response. You may need to make custom measurement jigs (e.g. a voltage divider), but be careful of safety with sources that can deliver a lot of energy.

author: e61_phil
author_rank: Frequent Contributor
timestamp: May 30, 2019, 10:08:20 am
content: I haven't found a voltmeter yet which is able to measure such frequencies and voltages accurate. Most meters aren't capable of measuring such signals at all.If one builds a divider you have to measure that divider first, which is again a problem.Btw: These voltages aren't very harmful. The oscillator will break down if you touch it. We have a couple of other sources which are REALLY harmful (for example 10kV which is capable of delivering 3A).

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2019, 02:07:25 pm
content: Quote from: e61_phil on May 30, 2019, 09:37:15 amIt means that the accuracy specification only applies at DC and low frequencies. At the -3dB bandwidth limit, signal amplitudes will be down by about 30%. Between that and some lower frequency, the frequency response depends on the shape of the frequency response curve.Quote1 to 3 MHz is pretty low so the accuracy will be close to the DC accuracy specification. But keep in mind that the probe has its own accuracy specification and must be compensated properly.Some probes have a DC attenuation adjustment which can be used with a calibrated source to calibrate the system to better accuracy. +/-0.5% might be feasible this way.Quote from: e61_phil on May 30, 2019, 10:08:20 amSampling voltmeters can easily make accurate RMS and peak voltage measurements from DC to 100s or even 1000s of MHz.For example, the old Racal-Dana 9301 RMS sampling voltmeter was specified to meet its basic DC accuracy specification of +/-1% of FS and +/-1.5% of reading up to 500MHz. Above that, a calibration curve is used at reduced accuracy up to 1.5GHz.

author: e61_phil
author_rank: Frequent Contributor
timestamp: May 30, 2019, 02:23:25 pm
content: At low voltages there are some Millivoltmeters (like the one you showed). But in combination with 1000Vpp there is very few if any.That means one have to use a divider. But how should I calibrate the divider?"accuracy will be close to DC.." isn't enough. I would like to have traceable specs. Some tests at 1MHz with small voltages (Fluke 5730A without amplifier) showed a couple of percent deviation with a (calibrated) Tek scope and linearity of the scope is also in the percentage range.The probes (1:100 probes from Fluke, Philips and Testec) are very accurate at DC, but at 1MHz they are already a couple of percent down (~5%). A quick test with a Bode 100 VNA showed some rolloff between 1MHz and 3MHz (probe bandwidth was specified with 300MHz). Therefore, I doubt one can achieve 0.5% here. Or do you mean 0.5% at DC?Any ideas how to calibrate a probe at 3MHz with low uncertainty?And has anyone experience what will happen with the accuracy at 1000Vpp?

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2019, 03:24:02 pm
content: Quote from: e61_phil on May 30, 2019, 02:23:25 pmThen you need a calibration source to work with the combined probe and oscilloscope.QuoteAlso consider that the probe's specifications assume a 25 ohm source impedance. Probe bandwidth decreases with higher source impedance.QuoteI would start with a good high voltage divider probe like a Tektronix P6013 or P6015.Jim Williams had some things to say about system calibration at high voltages and medium frequencies inLinear Technology application note 65.

author: bdunham7
author_rank: Super Contributor
timestamp: May 31, 2019, 04:40:23 am
content: Quote from: e61_phil on May 30, 2019, 02:23:25 pmWhat you are proposing is not simple. Fluke made thermal RMS converters for old meters like the 8506A that would more or less do what you are wanting, but I'm not sure if they (or anything) would work with your circuit. What makes this extra difficult is your source obviously has a high impedance, but the impedance of most probes (and DMMs and everything else) drops off pretty fast as you go up in frequency. The higher the DC impedance of the probe/test equipment, the more pronounced the effect is. 1000 volts but not enough current to shock you implies an output impedance greater than 1M. At 3MHz, a 1pF capacitor has an impedance of 53K. Perhaps this is not an accurate analysis of your system, but you see the problem.

author: e61_phil
author_rank: Frequent Contributor
timestamp: May 31, 2019, 05:00:00 am
content: The output impedance isn't that much of a problem. At first it is controlled in a feedback loop and second: the detector circuit runs ins parallel. So even if the voltage drops under the load of the probe, the detector will see the same drop.The thermal converters I know are running at low voltages. For higher voltages you have to divide it first.A good divider should help. One gets the benefit that relative measurement is enough to characterize the divider. But it seems still quite hard at low uncertainties.@David: I have a some P6015 and 6015As, but these are also not accurate at my voltages. But I will read the AN65!