Scope Calibration: How is it done? - Page 1
Forum section: electronics / testgear

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 19, 2016, 02:03:32 am
content: I have a LeCroy Xi series scope. I would like to get it calibrated but want to know more about how this is done.Is there specialized test equipment to do this? Is it mostly done through special software?Richard

author: Fungus
author_rank: Super Contributor
timestamp: October 19, 2016, 04:31:53 am
content: Quote from: RCHRDM on October 19, 2016, 02:03:32 amIt's done by paying money.No, really! The only real point of 'calibration' is that you have a certificate afterwards. You can't do that at home.If you don't care about the certificates then the built-in calibration is enough. Those things have a decent internal voltage reference and recalibrate themselves at every power-on. They can even detect changes in ambient temperature during use and do an auto-recalibration if it changes.Any home calibration will really be just to check if it's broken or not. That's easy to test - get a voltage/frequency reference and connect it up. Check the readings. If they're correct then you just 'calibrated' it.

author: tautech
author_rank: Super Contributor
timestamp: October 19, 2016, 06:51:11 am
content: Quote from: RCHRDM on October 19, 2016, 02:03:32 amWhy ?What indications are there that it needs a Cal ?You can do some sanity checks if it's just for hobbyist use. A DC voltage, a good DMM and a known reasonably accurate frequency source should indicate if there's anything out of wack.Service manuals have the requirements for Calibration and the steps involved.

author: JPortici
author_rank: Super Contributor
timestamp: October 19, 2016, 02:52:53 pm
content: Try to dig out the service manual. There you should find all the info, required tools and suggested tools that meet the required spec.The tools are of course an accurate enough DC voltage source, accurate AC voltage source (waveform generator) in both amplitude and frequency, an accurate enough frequency counter, some good quality BNCs and accessories.Unfortunately, you don't always have self calibration and even then it might not "calibrate" properly, like still giving zero volt dc offsets between scales that could and should be zero'd in software if you can't in hardwareAs fungus said if you need it certified that it is calibrated the only thing to do is paying.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 20, 2016, 03:57:20 am
content: I think the scope needs calibration because the measured voltage of a sine wave from a calibrated signal generator is not correct. Frequency is good though.I got it used and it also seems to be missing some calibration files on the hard disk.I understand about paying. I guess I am trying to find out if the hobbyist can calibrate it him/herself since I don't need a certificate. I have not been able to find anything in the manual about making any adjustments like this.By the way, how do I get this forum to notify me if there is an answer to my posts?

author: tautech
author_rank: Super Contributor
timestamp: October 20, 2016, 04:17:36 am
content: Quote from: RCHRDM on October 20, 2016, 03:57:20 amGo to your Profile/ Modify Profile/Notifications.Or at the top of any forum page: Show New Replies to your posts. (If logged in)

author: tautech
author_rank: Super Contributor
timestamp: October 20, 2016, 04:22:48 am
content: Quote from: RCHRDM on October 20, 2016, 03:57:20 amCan you check the sig gen is correct on another scope.Does the sig gen's output impedance match the scope's input impedance?That's an easy mistake to make: Sig gen= HiZ, scope=50

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 21, 2016, 03:52:47 am
content: Thanks.

author: tautech
author_rank: Super Contributor
timestamp: October 21, 2016, 04:05:57 am
content: Quote from: RCHRDM on October 21, 2016, 03:52:47 amWere are you at with this?Mr W, the forum's LeCroy expert is travelling IIRC, he'll give some hints as to what might be wrong. Keep a eye out for his activity when he returns.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 21, 2016, 04:22:33 am
content: Thanks! What is IIRC?Richard

author: tautech
author_rank: Super Contributor
timestamp: October 21, 2016, 04:28:59 am
content: Quote from: RCHRDM on October 21, 2016, 04:22:33 amIf I recall correctly.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 21, 2016, 04:29:14 am
content: Do you have a full user name for Mr. W so that I might send him a personal message?Richard

author: tautech
author_rank: Super Contributor
timestamp: October 21, 2016, 04:32:11 am
content: Quote from: RCHRDM on October 21, 2016, 04:29:14 amhttps://www.eevblog.com/forum/profile/?u=44979

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 21, 2016, 05:07:52 am
content: Thanks!

author: komet
author_rank: Unknown
timestamp: October 21, 2016, 07:01:08 am
content: I just recently saw a Scope Calibrator on our local ebay:http://www.ebay.ch/itm/Fluke-5820A-Oscilloscope-Calibrator-5-Channel-with-2-1-GHz-Bandwith-Extension-/302111064982?hash=item4657390396:g:ZPoAAOSw-CpYBhMWI don't know what the thing does exactly, though.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 21, 2016, 02:31:52 pm
content: Thanks.

author: orin
author_rank: Frequent Contributor
timestamp: October 22, 2016, 01:07:04 am
content: Quote from: tautech on October 20, 2016, 04:22:48 amIt would have to be a pretty good sig gen even if the impedance is right. If your scope is rated at +/- 3% voltage accuracy, your sig gen would have to be better than +/- 0.25dB power amplitude accuracy. A quick look at the 8642A manual and it is spec'd at 1.0dB amplitude accuracy. A 1dB power change is about 12% voltage change... how far out is the "offending" 'scope?

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 22, 2016, 06:43:06 am
content: I have an Agilent E4400B signal generator. The output accuracy is +/- 0.5 dB from +10 to -120 dBm.The scope has +/- 1.0% of full scale DC gain accuracy, if that is the correct specification.I will check the scope again tomorrow.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 12:26:36 am
content: Looks like the frequency measurement is good but the voltage measurement is 946 mV where it should be 1.000 Volt.

author: newbrain
author_rank: Super Contributor
timestamp: October 23, 2016, 12:58:17 am
content: Quote from: RCHRDM on October 23, 2016, 12:26:36 amWhich is still inside the combined tolerances of the scope and SG:Quote from: RCHRDM on October 22, 2016, 06:43:06 am10^(0.5/20) ~= 1.061.06*1.01 ~= 1.07So, in total about 7%, more than the error you see.It's a bit pointless trying to measure an instrument with one of lesser precision.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 01:01:57 am
content: Thanks. Could you please explain the math?Richard

author: orin
author_rank: Frequent Contributor
timestamp: October 23, 2016, 06:07:34 am
content: Quote from: RCHRDM on October 23, 2016, 01:01:57 amThe sig gen spec in dB is a power difference which is defined as 20*log(voltage ratio). To go from 0.5 dB voltage ratio, you must divide by 20 and take the antilog of the result. Hence 10^(0.5/20). This gives the max voltage error of the sig gen which turns out to be 1.06 or 6% (if you used -0.5dB, you'd get -6%).Now the scope has an error of 1%, so assuming the sig gen is on the limit and the scope is on the limit, you could see a 7% error and the scope and sig gen could both still be in spec. Only if you saw a greater error could you conclude that the scope is out of spec.Since you see an error of 5.4%, the test is inconclusive.I'd feed the scope 1V DC and see if it shows between 0.99V and 1.01V on the scope. If it does, I'd guess everything is fine.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 06:47:46 am
content: Thanks! I appreciate the details.Richard

author: Fungus
author_rank: Super Contributor
timestamp: October 23, 2016, 07:39:08 am
content: To measure voltage accuracy just connect a battery/power supply to the input of the 'scope and a good multimeter simultaneously. See if the voltages are the same.Messing about with a signal generator introduces too many unknowns.

author: Performa01
author_rank: Super Contributor
timestamp: October 23, 2016, 09:07:26 am
content: You have to be careful with the DSO specifications.Do not confuse DC and AC accuracies.If a scope is specified to 1% DC accuracy, you can only verify that with a reasonably stable and accurate DC source. Cheap Power supplies aren’t exactly up to the task, but modern DDS function generators in DC output mode would be appropriate, especially if you have a reasonably accurate DMM (at least 0.1% DC accuracy) to monitor the output voltage.AC accuracy of any scope will never be better than 1dB for the lower part of the specified bandwidth. For instance, I have a 300MHz scope that is specified as DC ~ 10%(BW) +/-1dB, i.e. max. 1dB error up to 30MHz. Actually, it’s a lot better than that - I’ve measured the 1dB bandwidth to be >200MHz.Anyway, 1dB error is roughly 12%!For checking the AC accuracy of a scope, a calibrated signal generator is fine, especially if it’s accurate to 0.5dB. As others have already pointed out, 946mV for 1V input looks plausible and is only -0.5dB off, which would be perfectly within the signal generator tolerance alone, not even taking another dB or so for the scope itself into account.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 03:05:54 pm
content: Thanks.I have a DC voltage source that has a 2.5 Volt setting. It was made by Agilent.Is 2.5 Volts OK to put into the scope and not over load it?

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 07:04:44 pm
content: With a calibrated or accurate 2.5 VDC source and using a 10X probe, the scope is reading 2.4420 mV.Richard

author: tautech
author_rank: Super Contributor
timestamp: October 23, 2016, 07:17:45 pm
content: Quote from: RCHRDM on October 23, 2016, 07:04:44 pmDoes your DMM agree with your voltage source?Do ALL channels read low?From this datasheet:http://teledynelecroy.com/japan/pdf/cata/waverunner_xi-a_spec.pdfQuoteCheck again with a 1:1 probe, in fact check with more than just one or set up a direct cable connection.We need to know if all channels have a similar reading or if there is only a problem with 1.EditDoes the graticule measurement roughly agree with the OSD measurements?

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 07:35:38 pm
content: C1 = 2.4411C2 = 2.4407C3 = 2.4413C4 = 2.4430Using same probe. Will check more values later.Richard

author: orin
author_rank: Frequent Contributor
timestamp: October 23, 2016, 07:42:36 pm
content: Quote from: RCHRDM on October 23, 2016, 03:05:54 pmShould be fine for a 1Mohm input.

author: Fungus
author_rank: Super Contributor
timestamp: October 23, 2016, 07:45:59 pm
content: Quote from: Performa01 on October 23, 2016, 09:07:26 amOr use a battery.

author: tautech
author_rank: Super Contributor
timestamp: October 23, 2016, 07:55:41 pm
content: Quote from: RCHRDM on October 23, 2016, 07:35:38 pmThat looks OK for consistency but you need to check other connections, either direct and other probes to be sure.Process of elimination.Before any adjustments are made to the scope (if any are needed), any scope is only as good as the quality of it's PSU. When and IF you go there, all rail voltages need to be spot on and ripple levels very low.Have you checked LeCroy's website for Service manuals for your model, if they're are available they should at least have the PSU specs to ensure it meets specs.With the scope set for 1 Minput you can safely go to many 10's of volts.EditsYou need to check for inaccuracies at various input attenuation settings too.IF you're relying on the OSD measurements it's best to use an input attenuation setting to have the voltage step (0V - test voltage) using nearly all the display vertical axis.Have an experiment with different input attenuation settings to actually see the differences in readout of the same input but with different V/div settings.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 09:49:10 pm
content: There does't seem to be a service manual available for the Xi series.What is "OSD"?With a straight in connection from the 2.5 VDC source and with the input impedance at 1 MOhm, I am seeing a voltage of 1.5723 Volts. Should I be trying this at 50 Ohm impedance?Richard

author: Vgkid
author_rank: Super Contributor
timestamp: October 23, 2016, 10:24:06 pm
content: For dc/ low frequency(most) ac use the scope in 1Meg mode.that 1.5x volts seems off.

author: RCHRDM
author_rank: Regular Contributor
timestamp: October 23, 2016, 11:57:30 pm
content: I changed the input to 1 MOhm DC and got 2.4657 Volts (or close) from the probe or direct input.Unfortunately, I don't know the secret of changing the calibration settings. I assume this is done with some proprietary LeCroy software. In fact, my calibration folder in the scope program directory is missing some important files. I found a few Xi scopes for sale on eBay and asked one of them to send me their calibration folder. It won't calibrate my scope, of course, but at least I can take a look at them.I have been getting quotes for calibration that range from $185 to over $600. LeCroy is $425 and I have to ship across the country

author: newbrain
author_rank: Super Contributor
timestamp: October 24, 2016, 06:49:02 am
content: Quote from: RCHRDM on October 23, 2016, 11:57:30 pmO frabjous day! Callooh! Callay!Rejoice, RICHRDM, as your scope seems to be in spec, though barely (I ascribe the 1.57V to be a wrong measurement, maybe with 50instead od 1Mas you wrote).The value is within the guaranteed 1.5% (the 1% is typical):QuoteAssuming a perfect 2.5V source: 2.5 * 0.985 = 2.4625, so, yes, barely inside.Which range are you using?The spec is given on the full scale, if e.g. you were using 10V full scale (1V/div assuming 10 divisions) the deviation could be up to 150mV.

author: Vgkid
author_rank: Super Contributor
timestamp: October 24, 2016, 07:04:52 am
content: That 2.4Xv is a lot better. The 50Ohm input impedance would load down one of those Voltage reference chips to much. especially if you are using a regular scope probe, then even at 1X, the cable would form a voltage divider.