Auto calibration in modern digital scopes and digital multimeters - Page 1
Forum section: electronics / testgear

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 30, 2015, 06:26:30 pm
content: In traditional analog scopes and older multimeters all the calibration had to be done manually with the proper adjustment of several potentiometers.But how does the auto calibration work in modern digital scopes and modern digital multimeters?While there is an auto calibration feature on most modern digital scopes and digital multimeters, they still have to be sent in for a complete calibration in a specialized calibration center or at the factory, when they are completely off in their readings.Does this mean that there are two levels of calibration: a high-level calibration (done by the end user) and a low-level calibration (done by a specialized calibration center or at the factory)?What is covered by the high-level calibration (which the end user can perform himself)?What is covered by the low-level calibration (which can only be done by a specialized calibration center or at the factory)?Does the high-level calibration perform some kind of further fine tuning, to further improve the base calibration?Does the low-level calibration set the base calibration, based on low-level electrical properties?

author: fivefish
author_rank: Frequent Contributor
timestamp: July 30, 2015, 06:47:30 pm
content: I don't know what's the cutoff date for your definition of "modern" but a Tek2445A is an old 1987-era analog scope... and already have calibration values stored in it's RAM. From what I understand/read, these values are populated at the factory for each specific unit when your scope goes out the door.There's another aspect of calibration and that is NIST traceable calibration.See this for example:http://support.fluke.com/calibration-sales/Download/Asset/3029357_6200_ENG_A_W.PDF

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 30, 2015, 07:12:41 pm
content: Yes, I recognize that there can be older models that already stored the calibration parameters in RAM.Still my question holds about the auto calibration.What can be done by the end user, and what can not be done by the end user?Are there two levels of calibration?

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 30, 2015, 07:33:04 pm
content: Quote from: pascal_sweden on July 30, 2015, 06:26:30 pmIt seems you don't really know what calibration is. FYI: it does *NOT* mean that an instrument is adjusted, that's an additional (optional) step. Calibration merely means comparing an instrument's parameter against a suitable calibration reference and noting down any deviations. If you want it adjusted then this is a separate procedure (which is usually also billed separately).Old instruments were full of trimmers, pots and other stuff that didn't age very well, which means that the instrument had to undergo certain re-adjustment procedures regularly to maintain specification conformance. With modern kit this is less of a problem, mostly due to the fact that modern devices operate digital, and because tightly integrated components of modern days can satisfy much narrower specs than old ones. That means that the adjustment needs for modern instruments are minimal, and most of the adjustments can be done automatically through self-calibration (i.e. no external calibration reference is required). This also means that, for a lot of modern kit, if it's outside spec then it doesn't need "calibration" (or better, adjustment), it simply means it's defective and needs repair.Just check the Service Manuals of some modern DSO, or even something more complex like a Spectrum Analyzer. These days the only remaining manual adjustment are some voltages, if there's any manual adjustment at all. Compare that with the Service Manuals for older kit, which were much thicker because of all the adjustment procedures described in them. Modern manuals usually only contain a performance verification section.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 30, 2015, 07:42:31 pm
content: So the Self-calibration option in the Rigol menu does not change the internal calibration parameters which were stored at the factory? What does it change then?Calibration or adjustment are two words that can be interchanged I believe. I don't like word carping. What I am really looking for, is just an understanding of what calibration/adjustment is done in the factory, versus what calibration/adjustment can be done by the end user (Self-calibration option in the Rigol menu).Note that all certificates that come with a scope are called Calibration certificates, and not Adjustment certificates. Whether these are potentiometer settings or calibration parameters stored in RAM.I still believe that there are two levels of calibration/adjustment in this respect. The parameters stored at the factory (low-level), and the parameters that are affected by the Self-calibration option in the Rigol menu (high-level).And then the big question remains: what parameters consitute the low-level parameters (only set/changeable in the factory), and what parameters consitute the high-level parameters (changeable by the end user through the menu). So we are back to my original question

author: eas
author_rank: Frequent Contributor
timestamp: July 30, 2015, 07:45:00 pm
content: It is probably useful to distinguish between calibration and adjustment. Part of the point of traceable calibration is to establish controlled points of reference for offset, slope and linearity. Any automatic internal consistency checks and compensation done by an instrument will only be as good as the points of reference.Then there is the question of how worthwhile these internal consistency checks and compensation are given that many 20 year old DMMs can remain within their 1year accuracy specs without "self-calibration" or adjustment.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 30, 2015, 08:23:41 pm
content: Quote from: pascal_sweden on July 30, 2015, 07:42:31 pmAsk Rigol, because that depends on the specific design of the instrument. For most simpler DSOs all self-adjustment does is to compensate for drift of the input amps in the front end, which is done by applying a test signal internally. This shouldn't change the factory adjustment, but I wouldn't put it beyond Rigol that this is what happens in their kit.QuoteYou may well believe that to be the case, but that doesn't make it true (it's a common mistake made by hobbyists/beginners, though). Just go ahead and send a non-confirming instrument for calibration only, and you'll see you get it back as failed, not adjusted. Because calibration is not adjustment.QuoteYes, because all these certificates do is to confirm that the scope conforms to its specifications, and the certificate serves as a trace (proof). It doesn't say "has been aligned to specifications", because that's a given (if a new scopes reaches the customer with failed calibration then it's defective).QuoteWhat is done at the factory is an *adjustment* of certain parameters (of which with modern kit many won't change over the device's lifetime), followed by a (usually traceable) calibration to confirm spec conformance.QuoteThat depends on the type of instrument, and even within an instrument class that depends on the specific design of an instrument, so there's no universal answer to that question.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 30, 2015, 08:36:55 pm
content: So the only feedback on my original question is some word carping and nitpicking about the meaning of calibration (which indirectly implies adjustment at the factory when it comes up to a calibration certificate), but no real answer on my actual question.So typical for Germans living in the UK!Maybe you have to write to Rigol then, and make them rename the Self-calibration option in the Rigol menu to the Self-adjustment optionOtherwise users might get the impression that their feature would only verify if the scope is calibrated, but would not change anything.Anyhow, I will use adjustment below, just to follow your guidelines, and hopefully get a real answer:Let's try to formulate why I directed my original question: If the Self-adjustment on the Rigol scope passes, does that mean that your scope is making correct measurements? Based on the above discussion my impression is that it does not guarantee anything! Is that correct?Is it correct that the Self-calibration/adjustment can not correct corrupt factory calibration parameters when something went wrong with for example the firmware upgrade procedure (parameters overwritten in flash)?Why can't the manufacturer implement a Self-adjustment procedure that can discover that there is something wrong with low-level settings/parameters?Note that the digital multimeter from Rigol, needs to be sent in for calibration/adjustment, when the readings are off, while the multimeter has also a Self-adjustment feature. People have reported successful self-calibrations in the menu, and still the readings are off. I guess it's the same for the Rigol scopes.If Self-calibration does not help, your scope is broken? Is that correct?So the factory will not just adjust settings then, but really have to replace parts?Or the company could still just restore the correct factory calibration parameters which got corrupt, and which can not be fixed by the end user?It would be good to know what the Self-calibration/adjustment actually does, and what it doesn't.This way you can better predict how reliable the scope is, when there is a pass in the Self-calibration/adjustment option, and what potential problem there could still be anyhow.

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2015, 09:02:05 pm
content: Quote from: Wuerstchenhund on July 30, 2015, 08:23:41 pmExactly.Download a Tek TDS2000 Service manual.In Adjustment Procedures, (10 pages) there is 106 adjustment steps using precision equipment to allow autocal to compare and tweak to.Now this is in a Service sub menu in the scope UI.WHO other than a Cal center has the gear to perform this accurately and this is a 20 yr old scope.Other than for traceability who'd want to and unless you have the gear why bother.The medium term drift of gear these days is so low that it should not bother the hobbyist.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 30, 2015, 09:39:26 pm
content: Let's try a different approach through some more structure:===================I) First generation scopes:===================Calibration/adjustment had to be done with potentiometers.Calibration settings were not stored in RAM.No Self-calibration/adjustment (potentiometers have to be changed manually).End user can calibrate/adjust scope using service manual and external reference.Alternatively end user can send scope to calibration center or factory who performs the above with professional equipment and reference source.No confusion with 1st generation: only manual calibration/adjustment is possible.======================II) Second generation scopes:======================Calibration settings are stored in RAM. Maybe some adjustments are still done by potentiometers, but this does not matter to the end user, as end user can not initiate self-calibration anyhow.No Self-calibration/adjustment available in end user menu.End user can not initiate Self-calibration/adjustment as menu option is not available.Only option end user has it to send in scope to calibration center or factory who updates the calibration settings in RAM, and possible adjusts also some potentiometers.No confusion with 2nd generation: calibration/adjustment can only be done by calibration center or factory.=====================III) Third generation scopes:=====================Calibration settings are stored in RAM.Self-calibration/adjustment option is available in end user menu.End user can initiate Self-calibration/adjustment as menu option is available.Unknown 1: Are there still some settings done by potentiometers in 3rd generation scopes?The fact that self-adjustment can be done digitally would suggest that there are no adjustments left that are still done by potentiometers. In 2nd generation of scopes, it did not really matter whether end user knows about this or not, as he can not initiate Self-calibration anyhow, but now in 3rd generation, it is important to know, to know how much is covered by Self-calibration.Unknown 2: Does the Self-calibration also repair the calibration settings that are stored in RAM?In 2nd generation of scopes, it did not really matter whether end user knows about this or not, as he can not initiate Self-calibration anyhow, but now in 3rd generation, it is important to know, to know how much is covered by Self-calibration.Unknown 3: If Self-calibration passes, does it mean factory calibration parameters and possibly some left potentiometer settings are correct?Lot's of confusion with 3rd generation:End user has several options. He can perform Self-calibration, or he can send scope to calibration center or factory. When end user sends in scope to calibration center or factory the calibration settings in RAM are updated, and possible also some potentiometers are adjusted.There is no confusion when the scope is sent back, but it is not clear what Self-calibration does or can do, and how reliable it is. Does it cover everything, including the factory calibration parameters? What if there are also some potentiometers in the scope?It should be documented with the Rigol scope, what is covered, and what is not.Right now Self-calibration option generates more confusion than it actually helps.It's not even clear if the Self-calibration passes, that the important calibration parameters which were stored in the factory are still correct, and not corrupted. At least it could detect if they have been altered or overwritten.

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2015, 09:54:24 pm
content: Quote from: pascal_sweden on July 30, 2015, 09:39:26 pmNo

author: mazurov
author_rank: Frequent Contributor
timestamp: July 30, 2015, 10:00:40 pm
content: Quote from: pascal_sweden on July 30, 2015, 08:36:55 pmIf internal reference that is used in self-adjustment of your instrument is still within spec then the answer is yes. To answer with certainty you need access to known (external) reference to somehow compare it with the internal one.

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2015, 10:12:53 pm
content: Another point that needs clarification:Self Cal is not Auto cal.Many scopes can be set to run a Quick cal while in use.

author: Bud
author_rank: Super Contributor
timestamp: July 30, 2015, 10:17:12 pm
content: @pascalYour channel1 seems to be way off, i think the scope needs a repair, not calibration or adjustment.To your question what is calibrated by self calibration i can say based on my digging into a piece of shit Ds2072a, a known reference voltage is injected into the front end and the calibration routine adjusts the gain of the front end stage so the voltage at ADC input is of a specific level. It also adjusts vertical position control offset. This relays on the assumption the ADC internal reference voltage is correct and the calibration DAC reference voltage is correct. If the assumption is valid, it is very much ensures validity of the volt per division levels . If the routine calibrates anything else - do not know.Factory adjustment may compensate for aging of components, perform timing adjustments, power supply ripple check, reference DAC check, things like that. Now, based on what i found inside the piece of shit DS2072a, i have much doubts such thing as factory adjustment/calibration exists at all at rigol. I do not think they ever check anything after the pcb gets assembled.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 30, 2015, 10:22:43 pm
content: I had only generic questions regarding calibration/adjustment here in this posting.Did not report any specific issue regarding my scopeRegarding your comment about DS2072A. Seems like you are not happy with the DS2072AIf I understand correctly, the Self-calibration only affects the vertical resolution (voltage/div), and not the horizontal resolution (time/div)?

author: commie
author_rank: Frequent Contributor
timestamp: July 30, 2015, 10:31:35 pm
content: I can see noone has played with a dual slope integrating adc driven by an AVR or PIC,With any calibration you have zero and span, originally or with analogue stuff, this comprised of two pots to adjust hence zero and span. Now with a micro you can still have analogue sero/span pots whilst the reading, on a lcd, can be the adc counter direct giving maximum potential accuracy. However, if you have a micro to hand then it makes sense to make the the zero/span calibration in software.To do this, you will have to change the reading variable from an integer(maximum potential accuracy) to a float, once this is done calibration of each range takes only 2 cycles inputting zero and then span, finished.Now, auto calibration means there is a drift free source meter built into the product(impossible), given calibration costs of £150 or more many people will rush for the instruments that claim push button self calibration.Mr Gauss will have something to say about this.

author: onlooker
author_rank: Frequent Contributor
timestamp: July 31, 2015, 02:47:50 am
content: According tohttps://en.wikipedia.org/wiki/Calibration"In general use, calibration is often regarded as including the process of adjusting the output or indication on a measurement instrument to agree with value of the applied standard, within a specified accuracy. "As an example, HP3457A's service manual had a section about calibration that included all kinds adjustments as part of the calibration.Another example, the "auto-calibration" on my scope actually fine tunes the internal calibration constants to adapt to temperature or other drift.Then, for the purpose of making money, a calibration lab would certainly like to stress the distinction between calibration and adjustment.

author: MarkF
author_rank: Super Contributor
timestamp: July 31, 2015, 03:06:52 am
content: Pascal,I've wondered the same thing because I've not been able to see any perceptible difference before and after run the Calibration from the menu on my DS1000Z. Just from watching what is displayed, it may be nothing more than adjusting the scaling for the display drawing for each individual vertical scale setting. Not what is normal thought of when you talk about instrument calibration.

author: Fungus
author_rank: Super Contributor
timestamp: July 31, 2015, 03:12:50 am
content: Quote from: commie on July 30, 2015, 10:31:35 pmIt's only an oscilloscope, not a high precision multimeter...Most of them use 8-bit ADCs so they're only about 1% accuracy at best.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 31, 2015, 05:34:34 am
content: Quote from: pascal_sweden on July 30, 2015, 08:36:55 pmAs said before, your question can't be answered universally becausethat depends on the type of instrument, and even within an instrument class that depends on the specific design of an instrument, so there's no universal answer to that question.QuoteXenophobe or wartime complex? Anyways, I'm not sure what my nationality and my place of living has to do with it, but I'm looking forward to your explanation.QuoteWhy should I write to Rigol about another thing they got wrong? Especially since they probably already have a long list of things to get right, which they may or may not work on.QuoteWell, it's a Rigol, but in general, yes, if your scope passes whatever the their self adjustment procedure is called plus any self-tests then yes, you can reasonably expect the scope to work within its specifications.If you want to be sure however you'd need to have the scope calibrated.QuoteAgain, this depends on the instrument. But as some Rigol kit (i.e. one of their bench DVMs) has lost factory calibration after a firmware upgrade because Rigol decided to change the memory layout in a new firmware version, it's entire possible that this could also happen to Rigol scopes.Big brands however should be pretty safe, as they generally put much more care into their firmware than cheap brands like Rigol.QuoteBecause some of the parameters require external signal sources which the user of the instrument is usually unlikely to own.QuoteI guess that was the DVM which had its factory calibration erased after a firmware update? Anyways, the way Rigol does things is often not the way a good manufacturer would handle things.QuoteYes.QuoteAgain, that depends on the instrument. With Rigol kit, they usually replace the one large main PCB that is in the box, which means the whole signal chain gets replaced with another part with (hopefully) correct factory adjustment.More complex kit from the big brands usually use modules which contain the factory adjustment data for the part of the signal chain the module is responsible for in an EEPROM, so if a module is replaced the whole instrument is still within specification.QuoteWith most big brand instruments anything the user does leaves the factory adjustment data alone, even during a firmware update. In older instruments, factory adjustment data can often be lost if some backup battery loses its voltage, and if the data is lost then usually it can't be restored (which is for many instruments people came up with some method to backup the data before the battery dies). Modern instruments use EEPROMs or flash which doesn't require a backup battery to maintain the data.QuoteFor big brand instruments, if you want to know what the self adjustment does you can check the service manual, which will also contain flow diagrams for troubleshooting if something is wrong.For Rigol, well, you could ask them, but in general I guess you'll be out of luck.

author: fivefish
author_rank: Frequent Contributor
timestamp: July 31, 2015, 06:16:07 am
content: QuoteSpeaking from my own experience, specifically of the DS1054, the scope can pass it's own self-calibration (i.e. without any error message) and the scope can still be faulty in it's reading/display. i.e. giving wrong measurements.My CH1 is off by 1.2volts for every 10Volts of DC input. And no amount of multiple self-calibration fixes it. Board needs to be replaced by Rigol. (my hunch is a voltage reference chip on the affected channel is out of spec)

author: coppice
author_rank: Super Contributor
timestamp: July 31, 2015, 06:18:02 am
content: Quote from: MarkF on July 31, 2015, 03:06:52 amWhy would you expect a change after calibration? Assuming the machine was properly adjusted before calibration it SHOULD be pretty much the same after calibration. In a simple modern DSO, adjusting the scaling is about the only thing that needs to be done during calibration. All the timing comes from a crystal that either works or fails.In general, there are far fewer adjustments in modern instruments, and the tweaking of many of them is fairly easy to automate. It might require a little operator cooperation, like "disconnect all test leads" or "loop these connectors to those", but a lot can be done without additional equipment. The biggest reason for sending most instruments away for calibration is to ensure they are not faulty, rather than to get them tweaked properly. Instruments still tend to do a poor job of identifying their own subtle faults, so this can be a very valid reason for a full calibration check in a custom calibration lab.

author: deadlylover
author_rank: Frequent Contributor
timestamp: July 31, 2015, 09:00:51 am
content: I have an Advantest R6581 8.5 digit multimeter, it's capable of "auto calibration".Auto calibration isn't a silver bullet that can magically correct for drift over time or temperature, but it can help mitigate some of the drift. To put it real simply with an analogy, auto cal is kind of like 'nulling' your multimeter probes before taking a measurement, it just ensures the instrument is performing at maximum capability.It has nothing to do with adjusting itself for true accuracy, so it doesn't actually replace a 'calibration' in the strict sense of the word, if that makes any sense...During an auto cal, the meter measures its own internal references and then compares the results with the references' known entered values from an external calibration (from a cal lab for example).This data is stored in memory and is used to apply corrections for subsequent measurements in actual use. Generally, an auto cal is performed 24 hours after the previous auto cal, or after a room temperature change of a couple degrees Celsius.Ideally, you can cancel out any errors/drift from the measurement circuitry, so all you are left with is the drift from the actual internal references.Just as an example, the tempco of the R6581 on the 10V range is 0.5ppm/C, but improves to 0.15ppm/C after auto cal.Someone with much more knowledge will be able to explain it better than I could, I am also awaiting their response. =P

author: MarkF
author_rank: Super Contributor
timestamp: July 31, 2015, 09:32:55 am
content: Quote from: coppice on July 31, 2015, 06:18:02 amOne thing that has always annoyed me some is that the trace for each channel (@ 0V, Gnd, the CH ref point) doesn't line up with the little channel arrow on the left side of the trace. It's always a little higher or lower depending on the trace. Seems like a calibration would take that out.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 31, 2015, 10:15:46 am
content: I am sorry about my comment. I had bad experience with a German support person some time ago, but that should not have given me the right to generalize. Sorry again once more.If modern scopes don't use any potentiometers at all anymore, it should be easy to automate the calibration/adjustment process through SCPI, LXI, etc and provide means to recover the low-level calibration parameters.Instead of sending in your scope, you could lend or hire the calibration/adjustment device from your supplier, connect it to your scope, and initiate a menu option to activate the process. Both the scope and the calibration/adjustment device would communicate with a protocol set that is specifically designed for automating calibration/adjustment. The calibration/adjustment device and scope are connected with a network connection, and the calibration/adjustment device has 4 BNC output connectors that can be connected to the 4 BNC input connectors on your scope.This would be more convenient for some people, as they can keep their scope in their lab.The process would be reversed. Instead of sending in a scope, you just lend or hire the calibration/adjustment device. It should be possible to make this calibration/adjustment device at reasonable cost, and the supplier could have a pool of these devices, so that multiple users at same time have access to these units.Labs that have several scopes, could hire the calibration/adjustment device for a whole week for example, and then can take up the calibration/adjustment at their own pace during that week for their convenience. It would save a lot of shipping costs as well: instead of sending in a dozen of heavy scopes, just ask the supplier to send in one calibration/adjustment device and return it after all scopes have been calibrated/adjusted at your own facilities.With this setup, when the message PASS comes up on the screen the end user really knows that the device meets specs, in contrast to the current Self-calibration option, which does not guarantee anything.Calibration/adjustment should be fully automated in these days, now that the potentiometers are gone, and all calibration parameters are stored in FLASH memory.If the process is fully automated, and can be initiated by a simple menu option in the scope, then even a kid can take up calibration/adjustment. It would make calibration/adjustment available to the people, and it would be much more economical in contrast to the overcharging practices used by calibration centers today!

author: McBryce
author_rank: Super Contributor
timestamp: July 31, 2015, 10:35:50 am
content: That might be fine for a hobbiest, but for business customers self-calibration / certification isn't worth the paper it's written on. You needindependenttraceable verification.McBryce.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 31, 2015, 10:38:09 am
content: This can be solved with digital signatures between the calibration/adjustment device and the scope.It would store a digital calibration certificate in the scope, that proofs that the scope was calibrated last time at date XYZ.

author: Fungus
author_rank: Super Contributor
timestamp: July 31, 2015, 10:41:12 am
content: Quote from: pascal_sweden on July 30, 2015, 08:36:55 pmI'm going to quote this thread next time the "Let's force people to use their country flags" thread appears.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 31, 2015, 10:59:08 am
content: Quote from: pascal_sweden on July 31, 2015, 10:15:46 am

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2015, 01:23:34 pm
content: Quote from: pascal_sweden on July 31, 2015, 10:38:09 amYes, but only with universal manufacturer agreement.......won't happen.

author: McBryce
author_rank: Super Contributor
timestamp: July 31, 2015, 02:23:55 pm
content: Quote from: tautech on July 31, 2015, 01:23:34 pmEspecially as there's no motivation for them to do it.I'm still dreaming of the day that they standardise the interface for active probes!McBryce.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 31, 2015, 06:20:08 pm
content: Quote from: pascal_sweden on July 31, 2015, 10:15:46 amWell, I'm not a support person. On the other side, I have to say my worst support experiences were with support staff in Germany (not all from German companies, though). However, this should be irrelevant to the discussion at hand.QuoteFor simple devices, maybe. For more advanced instruments however this doesn't work, as often modules are aligned individually in a proprietary automated test fixture.QuoteAgain, it could work with a primitive scope like a Rigol, but with devices that consist of more than just a single PCB with all connectors on it it very likely wouldn't.QuoteSounds great, aside from the fact that even the rental of suitable calibration equipment is magnitudes more expensive than sending the unit to the manufacturer for calibration.And it also ignores the main reason people send stuff for calibration, which is that they need proof that the instrument conforms to its specifications, provided by a lab that is accredited to a specific national or international standard (i.e. NIST). Without that a calibration certificate is worthless for most professional users that require calibration.QuoteSavings, you say? For say GBP500 I can send a very expensive high end scope to the manufacturer for calibration. That includes shipping with a 24hrs courier service, and in most cases I'll have the scope back say a week later (if I need it quicker then I can have onsite calibration as well, but just for the sake of the argument let's just leave this option aside). Many 3rd party labs can do that even cheaper.Using your method, I would have to rent various kit, costing me a lot more than that even if its just for a week. But for the sake of the argument, let's just assume we calibrate more instruments and that the pro-rata cost for our scope would be just GBP100. That leaves a saving of GBP400, right? Wrong, because someone needs to spend the time performing the calibration, and this person won't work for free (and also needs a certain amount of knowledge to perform such tasks, so you can't just have the janitor doing it). Most labs I work in deal with pretty advanced kit, which means engineering staff is mostly highly qualified therefore well paid. Having to deal with the calibration means I have to use at least one of my engineers with a charge-out rate of (just to give you an arbitrary number) of say GBP120 an hour to spend say three hours with setting up the calibration equipment, making sure it works, performing the calibration, and then cleaning up behind him. Even worse, the time this engineer spent on working on the calibration is time he didn't spend working on other high value projects where his work would have added value that could well exceed GBP500 per hour.And even after all that I still have no traceable certificate because the whole calibration setup wasn't accredited.Now your idea doesn't sound so great any more, right?QuoteAdjustment *IS* mostly fully automated these days, but that doesn't mean you can do it in your lab, which is what you seem to assume.QuoteI'm sure many of the engineers that spent years for gaining the necessary skills and experience to work in metrology would beg to differ.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2015, 06:05:25 am
content: Quote from: Wuerstchenhund on July 31, 2015, 06:20:08 pmOne of my most rewarding experiences with customer service was with Rohde and Schwarz,where they found a part for a 20+year old Sideband Analyser.Most companies would say:- "Sorry,no longer serviced",especially as it was a specialist bit of gear,but they dug around in the dusty corners of their store & found it.Of course,we than had to wait months.for the next container load of R&S stuff to come from Germany,but that was the "suits",not the customer service guys!Telecom Aust used to have several Roadcases full of Tektronix Calibration stuff which was sent around fromplace to place to check and/or adjust the Tektronix analog CROs.Sending stuff halfway round the world for calibration just "wasn't on".Even sending it to a "Cal" lab in Oz was fairly impracticable.Besides,as one of the leading Electronics organisations in the country,it was felt we could do just as good a job "in-house".Hubris?----perhaps.The CROs very seldom needed significant tweaking,except after repair work

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2015, 07:45:08 am
content: Quote from: vk6zgo on August 01, 2015, 06:05:25 amIMO this also applies to most TE that hasn't been abused and is relatively new (~20Yrs).It's just some think it is their right to be able toequipment for one reason or another.It escapes me as to why.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 01, 2015, 08:53:15 am
content: Quote from: vk6zgo on August 01, 2015, 06:05:25 amYes, R&S seems to have their great moments, even for hobbyists and individuals using old R&S kit.In my experience however, and notwithstanding the occasional great moment for very old kit, their support is very good but not great. Contacting their help desk for example can sometimes take several days for a reply, and registering an instrument with them has on many occasions taken them in excess of two weeks.The other, more annoying thing is that R&S spare parts (don't know for Hameg stuff), with a few exception, are generally *very* expensive, even compared to other big brands like Keysight.QuoteI wouldn't say it's hubris (some of the places I worked had in-house cal labs, fully NIST/MIL accredited), probably just a solution fitting for your employer's requirements.It's also easier with CROs as most analog scopes had pretty poor specs, with quite wide tolerances, and weren't exactly high precision instruments (i.e. mostly used for looking at a waveform and then assessing if its correct or not, plus reading some basic signal parameters from a coarse scale), and if your organization could live without calibration that is accredited to a national or international standard then sending the basic kit that is required for adjusting a CRO around could be the better option over sending a large number of scopes around the country.But that is unlikely to work equally well for more complex instruments or modern-day scopes.QuoteNot surprising due of the general wide tolerances analog scopes had, especially the typical standard low bandwidth scopes (i.e. Tek 455) of these days.

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 01, 2015, 10:20:25 am
content: =================================================No potentiometers for adjustment anymore in modern digital scopes=================================================If there are no longer any potentiometers in modern digital scopes for adjustment, then why would one need a test fixture to align the board? Everything can be done by updating the internal calibration parameters in FLASH memory.=====================================================================Not a universal calibration/adjustment module, but a vendor specific module, which can be a nice addition to their product portfolio=====================================================================If my earlier suggested calibration/adjustment module would work for Rigol scopes, there could be a market for this. I am not saying that this module should be made available for every single test equipment in the world.An auto calibration button in the Rigol menu, and a propietary SCPI/LXI protocol between the Rigol calibration/adjustment module and the Rigol scope, to drive the BNC outputs on the Rigol calibration/adjustment module, and read the BNC inputs accordingly on the Rigol scope in a synchronized fashion. During this process, the Rigol scope will adjust the calibration parameters internally to get the measured signal correspond to the signal fed on the input, and to get the waveform displayed in the raster properly.=================================================================The target market could be universities and it could also be used for educational purposes=================================================================I believe it could be a nice addition to the Rigol portfolio. They can sell this to universities, and could even make educational material around this, so that students can learn about the calibration/adjustment process.The auto calibration process in the Rigol scope, could be semi-automated, where a nice instructional message appears on the screen to inform the student what the calibration/adjustment is doing at that moment, and which things are done to adjust.This way the student can learn about all the different aspects of the scope, the internal signal paths, the internal conversion steps and processing steps, all the way from the signal on the input connectors to the actual displayed waveform.

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2015, 10:30:48 am
content: Quote from: pascal_sweden on August 01, 2015, 10:20:25 amSeriously?Who would check it was accurate?

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 01, 2015, 10:46:58 am
content: An independent calibration lab can test/calibrate the combination of the Rigol adjustment/calibration module and the Rigol scope, and can also individually test/calibrate the Rigol adjustment/calibration module, and individually test/calibrate the Rigol scope after the auto calibration procedure has been completed.When that combination is tested/calibrated, the calibration/adjustment module can be used for 1 year independently.With your reasoning, you have full confidence in an accredited calibration lab as such?Even they have to be accredited back once in a while.

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2015, 11:27:25 am
content: Quote from: pascal_sweden on August 01, 2015, 10:46:58 amI can't see the point/advantage in it.Self cal already compares against internal stored precision references and if it fails then it's.Fix it, get it cal checked or buy another, remember at the price some are now you'd think they're made as throw always at the first sign of trouble.Simple I think, keep it that way.There's no need to reinvent the wheel when it's working perfectly well for the vast majority of us.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2015, 11:37:18 am
content: Quote from: Wuerstchenhund on August 01, 2015, 08:53:15 amThis was for my Employer at the time,the Channel 7 network,so the cost side wasn't a huge problem.They were not common instruments---there would only be one at each City TV station.It was very unusual to get good service from European companies.Lead times of months,& in one case,completely losing our order,were what we became used to from such sources.On the other hand,Sony,who supplied a lot of the more common instruments had turnovers of a few days for parts.QuoteI'm sorry, but isn't the whole function of an Oscilloscope " looking at a waveform and then assessing if its correct or not, plus reading some basic signal parameters from a coarse scale"?I certainly wouldn't rely implicitly on the voltage or frequency readout of any 'scope!QuoteIf the results are not just "in spec",but completely repeatable from test to test it supports my contention that they did not drift to any great extent.No,most of the Oscilloscopes were 100MHz or better units---- 7000 series mainframes were very common.

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 01, 2015, 11:41:36 am
content: Note that the Self-calibration available in the Rigol scope today can pass, even when the scope is completely out in the readings. So the Self-calibration is meaningless today. This has been confirmed by several users with a Rigol scope.So there is a value in the suggested calibration/adjustment module with external BNC outputs to drive the BNC inputs of the scope.The equipment which the accredited calibration lab uses to calibrate other products, also has to be calibrated once in a while. If the equipment which they use for calibration is calibrated, they use it for a certain period to calibrate other equipment, and make the assumption that the equipment which they are using is meeting the specifications. So even in this scenario there is a period where one makes assumptions about the accuracy.The same holds for the calibration/adjustment module that Rigol would send out to the customers.So from that perspective, my suggested scenario is not that different from today's practice.It just makes the whole process more efficient and cheaper.

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2015, 11:42:53 am
content: Quote from: pascal_sweden on August 01, 2015, 11:29:59 amThen you shouldn't buy one.

author: G0HZU
author_rank: Super Contributor
timestamp: August 01, 2015, 12:09:30 pm
content: QuoteBefore I joined this forum I would have dismissed your suggestion above because a generic DSO calibrator unit would end up being a very expensive item with very high (and very regular) maintenance/verification costs.But now I see what motivates a significant number of forum users on here I think you probably could market a cheap 'scope calibrator module' for the 'calibration obsessed hobbyist'A typical design would probably prove to have various uncertainty flaws but people would still buy it if it received a few good blogger reviews. In any hobby there are going to be plenty of people who are more than willing to spend money on cheap and dodgy gear if it is marketed well.I wouldn't buy one even if it turned out to have reasonable performance because I don't think I need one. I've had my Tek TDS2012 for about 10 years now and I have had no reason to worry about its calibration status in all that time. It's good enough to be useful (even today) and if I look across to my old Tek 465 that old analogue scope hasn't been adjusted internally since I first bought it and serviced it about 20 years ago.But it is still very useful even today and I really don't care how precise or linear the timebase is and the vertical inputs still seem to give acceptable accuracy allowing useful work to be done

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 01, 2015, 12:20:13 pm
content: Quote from: pascal_sweden on July 30, 2015, 06:26:30 pmI only can add something about the theory of operation of "Auto Calibration " in general, and about DMMs in special...Well, an analogous instrument, let's say, measuring DC volt, DC current, and resistance, needs basic references, like a voltage reference and a reference resistor. Second, a transfer mechanism to set the DUT in relation to the reference, i.e. in parts of 0.000 .. 1.2000 times the reference, that is in most cases an A/D converter. Third, it also needs a range multiplier / divider for coarse range transfer. In case of a DMM, latter mechanism would work decade- wise, like 100mV, 1V, 10V, 100V, 1kV, in case of a scope, if would have a 1-2-5 range sequence, for example.The calibration first fixes the value of the basic references.Then the transfer mechanism may be calibrated, that is in first instance its linearity (often given by design), but also its offset and gain.The multiplier/ divider is mostly determined for its gain in the different ranges.Depending on the topology of the instrument, these three basic calibrations may be done in one step, but has to be done for each range and for each mode.Auto Calibration in most modern instruments will at first adjust the offsets (Auto Zero).Very few instruments additionally are able to measure and adjust the gain factors internally.For that, they need an internal and precise Ratio Transfer Standard.This device can be the A/D converter itself, if it's linear enoughby design.Or it could be a special ratio bridge, like the Hammon type divider, which is able to make really precise 10: transfers by "auto alignment".The only existing DMM is the HP3458A, as an example for an ultra linear DMM, and the Fluke 5720, 5440 calibrators as examples for the 2nd type.There are also manual devices of that autocal instruments, like the 720A Kelvin Varley divider, and the 752A Reference Divider.( I cannot argue about the Advantest R6581 , if it's really capable of AUTO Calibration in that sense, because I did not find any good user manual or calibration manual about it)Therefore, you can understand the 3458A and the 5720A as being instruments with a built-in calibration lab.And therefore these instruments also cost multiple times the money of ordinary instruments.The only thing, what is NEVER possible to Auto calibrate, is the basic references, "out of thin air".Both instruments therefore need 3 external references, like 10V, 10kOhm, and a standard frequency, but the rest is done completely internally and traceably.(Both also include an AC/DC transfer mechanism)The big advantage is, that you only have to cope with the drifts of the inner references, but not with the gain and offset drifts, which in most cases contribute most to the overall drift specification.There are also some instruments, which also claim AutoCal or SelfCal. (e.g. the Keysight 34465A /34470A and the Fluke 8508A)These may to some extent mitigate some gain drift, but it's not a real Auto Calibration, as the 3458A and the 5720A are doing.If you look into the calibration manual, you recognize, that these still need the whole bunch of external calibrators to calibrate all the different ranges.. so their AutoCal capability is mostly a fake..In case of scopes, that's the same thing.. simply look into the calibration manual, if an external range calibration is required, or not.Frank

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 01, 2015, 12:34:17 pm
content: Quote from: pascal_sweden on August 01, 2015, 11:41:36 amCorrect.QuoteWrong. Equipment which is used for calibration work undergoes regular calibration itself, and the period depends on what the particular instrument was designed for. There is no "assumption" of accuracy, accuracy of the equipment is guaranteed for a certain period of time as long as the equipment is operated in a specific environment and not defective (and in most cal labs calibration equipment is at least checked weekly for correct operation, if not daily).You're making a lot of assumptions here that aren't bound to reality.QuoteNo, it wouldn't. First, calibration equipment prefers a static place with a constant environment (humidity, temperature etc) and not to be sent around on a constant basis as this will impact its accuracy (kit used for mobile calibration usually requires much shorter calibration cycles for itself, or even makes its results untrustworthy).Second, just sending the kit around doesn't make your calibration traceable. Calibrating equipment requires trained personnel.QuoteNot really, especially since you still haven't considered other costs like staffing, training and so on.

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 01, 2015, 01:17:39 pm
content: Let's suppose we go for the calibration/adjustment at a calibration center.Which calibration centers in Europe provide calibration/adjustment for Rigol scopes and Rigol multimeters?How much does it cost approximately to calibrate/adjust a Rigol scope (e.g. DS1054Z) or a Rigol multimeter (e.g. DM3058)?If there is no such thing for Rigol scopes and Rigol multimeters, what about Siglent scopes and Siglent multimeters?Which calibration centers in Europe provide calibration/adjustment for Siglent scopes and Siglent multimeters?How much does it cost approximately to calibrate/adjust a Siglent scope (e.g. SDS1072CML) or a Siglent multimeter (e.g. SDM3055)?When you buy a new scope or multimeter, how old can the calibration certificate be? Is there a limit on the age of a calibration certificate that comes together with a new product that is sold?I can think of older stock in a warehouse that has been sitting there for more than 2 years.Which institute takes up the calibration of Rigol scopes and Rigol multimeters before they leave the factory?Which institute takes up the calibration of Siglent scopes and Siglent multimeters before they leave the factory?Is the calibration for new products done by an independent calibration lab or at the factory itself? What about independent calibration as required by the industry for safeguarding authenticity?Does the manufacturer indicate the time period for when a new calibration is advisable?Or is this time period defined by the independent calibration lab after having calibrated the product? How is this time period determined? Based on the internal design and complexity of the product? Or just based on industry standard or practical experience?Do calibration centers in China have the same standards as calibration centers in Europe?Are they all subject to the same international accreditation standard?Do all calibration tests have the same scope and length for new products? Or is there a long and a short version, where A-brand manufacturers would go for the long version and Chinese manufacturers would go for the short version?How reliable is a calibration center? Has it ever happened in the past, that they just fabricate a fake calibration certificate in order to make easy money?

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 02:15:12 pm
content: Not every instrument brand and/or model can be NIST calibrated (or similar governing standards organization).You'd need to check with a calibration center if your specific equipment brand/model can be NIST calibrated.If you have an instrument NIST calibrated, usually it's valid for 1 year (the certificate).It doesn't mean after 1 year, your equipment is already in error (well, it may or may not be.)That's why you'd need to send it back again for re-calibration and to get a new, valid, current NIST certificate.Here's a sample of an NIST certificate.http://www.simco.com/images/uploads/Sample%20Certificate%20with%20Data.pdfAs you can see, not all values are bang-on exact. But everything falls within limits of the said equipment.Otherwise, the equipment you buy is just factory calibrated... which I presume is calibrated against an NIST-traceable (or similar) equipment.But you don't get an NIST-traceable certificate. (even though your equipment may be within limits)This page may be of interest to you. The example is using thermometers, but the FAQ section is interesting. It answers some of your questions.http://www.nist.gov/pml/mercury_traceability.cfmQuote

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 01, 2015, 04:55:09 pm
content: Quote from: pascal_sweden on August 01, 2015, 01:17:39 pmDifficult to say, as buyers option for Rigol kit are rarely the kind of people that require traceable calibration. But as long as sufficient spec/tolerance data and suitable calibration procedures are available from the manufacturer, any scope can be calibrated.QuoteAgain, hard to say. To give you an example, it costs roughly GBP500 to calibrate a LeCroy WaveRunner 64Xi high-end scope, I guess an entry level scope like a Keysight DSOX2k should be somewhere in the GBP250 - GBP300 mark (all plus VAT of course). But prices can vary dramatically across calibration providers.This aside, many large customers (businesses) use service plans which means they don't pay per calibration.QuoteSee above. It doesn't matter what brand, as long as sufficient spec/tolerance data is available then any scope can be calibrated. However, many cal labs only touch certain brands.QuoteAs with Rigol, I don't know any cal lab that says they do Siglent kit.QuoteSee above.QuoteWell, if the calibration is good for one year, but the scope you bought has been sitting in the reseller's storage for two years then the calibration will have expired. That doesn't mean the scope will perform inferior, it just means the proof that the scope is in full conformance to its specs is no longer valid.QuoteVery likely Rigol and Siglent themselves. Note that none of their kit comes with traceable calibration certificates.QuoteAll that is really only relevant for traced calibration, which none of the cheap Chinese stuff undergoes anyways.QuoteYes, they should.QuoteCalibration cycles depend on the design and specifications of the instrument. For example, most older kit had a 1 year calibration interval, newer kit offers 2 or in some cases even three year cycles.QuoteThe reputable ones, yes.QuoteYes, depending on which standards they follow (i.e. NIST, UKAS, ISO).QuoteQuoteFalsifying traceable test certificates is a serious issue, and any calibration lab that would be found to engage in falsifying certificate would most certainly instantly lose their accreditation, which very likely meant they would lose all their customers and therefore their business. I doubt any cal lab that has invested serious money into equipment and accreditation (which is pretty expensive) would jeopardize it for a few quick bucks.It might be different for non-traceable certificates, but they aren't worth much anyways.

author: G0HZU
author_rank: Super Contributor
timestamp: August 01, 2015, 05:57:32 pm
content: QuoteThe real question is wrt competence rather than honesty. Back in the days when our company was quite small and all of our decent test gear was 'cherished' the annual calibration checks were not looked forward to. It wasn't the cost, it was the fact that the item would be gone for a week or two and there was the real risk that some spotty calibration tech would be learning his/her trade inside our gear if something needed adjusting or replacing.Also, the test gear could easily spend several days boucing and sliding about in the back of a van being driven around the country. So sometimes test gear can get damaged in transit. We would only send our HP gear to HP for calibration regardless of cost but even then we got nervous if something needed adjusting. We would much rather they checked it and gave a limited calibration certificate for that year compared to some spotty tech tweaking around inside.For the vast majority of hobby use there really isn't much added value in terms of sending test gear off for formal calibration. I have never done it in over 30 years of owning some fairly decent RF test gear and I don't think I'm any worse off because of this.

author: HighVoltage
author_rank: Super Contributor
timestamp: August 01, 2015, 09:07:13 pm
content: I am required by ISO and TS standards to have my official test gear calibrated once a year and I too worry about damage and mishandling.So, I got to know the cal labs in my area and selected the one I could trust the most and I bring my test gear to them and pick it up.In addition, they must contact me, if any adjustments are needed. Usually my test gear has a history of passing without adjustment and if something needs to be adjusted, something is wrong and needs an investigation.

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 01, 2015, 10:53:16 pm
content: Yes, that's also one of the main reasons why I would not like to send it in for calibration.Then it can get damaged 3 times. During the shipping to the calibration center, by an engineer at the calibration center who does not care at all about other people's property, or on its way back in the post.Not everybody is concerned about not making any scratches on the housing or on the display. Or even worse, not making any dents in your housing. Or just destroy some stuff on the PCB board, if a summer job student works in the calibration center.Damage and mishandling is actually something which I forgot to add as an argument when I suggested the idea of an automated calibration/adjustment module which can be used at home or in a company for taking up the calibration/adjustment yourself.If it is completely or semi-automated, it would not even take up many resources. You just connect the scope with the calibration/adjustment module through the Ethernet cable, and connect the 4 BNC output connectors from the calibration/adjustment module to the 4 BNC input connectors of the scope. This installation setup takes less than 10 minutes. After you select the "automated calibration over LXI" option in the Rigol menu, and the process initiates. So 15 minutes of your employee resource is taken, but apart from that the whole process can run in the background, and during one of the coffee breaks the employee can check how far the process has come. If the process is ended, the Ethernet cable and the 4 BNC cables can be disconnected. Then the next scope in row can be calibrated/adjusted.Even if this kit is only made available for hobbyists it would actually have value, as it seems from the above response that an official calibration by an accredited calibration center costs more than the actual scope.Given that traced calibration requires accredited calibration equipment, whereby said calibration equipment itself should be frequently calibrated, and should be operated in a designated static environment only, maybe the calibration/adjustment module should actually not be used to proof that the scope is within spec, but it can be used to proof that the scope is not within spec.It is easier to proof that something is not within spec, even if the calibration equipment is not perfect, than to proof that something is in spec, as that would require accredited calibration equipment. It can be compared with negative testing (proof that something is out of spec), instead of positive testing (proof that something is within spec).This not-within-spec proof can then be used as an argument to send the scope back to Rigol. Then it's up to them to proof that the scope is in spec with accredited calibration equipment. At least the end user has something to stand on then. Otherwise the end user always has to take screenshots or pictures, and write unstructured/non-uniform emails to Rigol support indicating that something is wrong, which Rigol support will either not understand, or most of the time they will just respond with the message that they can not reproduce the issue.For low-end scopes there is no economical option available today for getting a traceable calibration. The calibration is too costly in comparison with the price of the scope, and then there is also the risk that your scope gets damaged, either in transportation, or when it is handled by a technician who does not care about damaging other people's goods.The Self-calibration which is available in the low-end scopes at this moment can pass, even when the readings are completely off. This would not be the case with the automated calibration/adjustment module as it really measures signals on the BNC inputs of the scope.Of course you could check manually if the readings are off, but not all people have the knowledge or the time to do it manually, and might still miss some measurements.With the automated calibration/adjustment module, they just have to press a button, or respond to some questions in between when the process is semi-automated.All measurements will be made over the full range, and nothing will be missed out.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2015, 02:20:21 am
content: Quote from: pascal_sweden on August 01, 2015, 01:17:39 pmSiglent performs performance tests 'in house' and for products where high accuracy is expected and a Certificate of Calibration accompanies each new unit.Some products have guaranteed 1 year accuracy specs as outlined in datasheets. (eg SDM3055)Others, the manuals state accuracy specs BUTThe oscilloscope must be within the factory calibration interval of one year.All new Siglent products I have in stock have Factory Cal certs and a range of Cal gear is used to perform equipment calibration for these certificates.In the case of a new SDM3055, the Cal cert lists the equipment used as a Fluke 5522A Multi-Product Calibrator and states the test unit's SN.All fairly traceable IMHO.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 02, 2015, 05:04:58 am
content: In another existence I was involved in testing & issuing calibration certificates for Audiometers & other equipment used for testing people's hearing.To this end,we had two "Artificial Ears" (with associated calibrated microphones ),plus two Selective Millivoltmeters.The former were sent away to the National Acoustic Laboratories for calibration on alternate 6 monthly intervals.The latter were sent away on alternate years for calibration.As the only place in Western Australia who could do this work,we also did calibrations for others,as well as our own units.We repaired hearing testing equipment of all kinds,too,including for others.Any repair to Audiometers,orany otherhearing test instrument required full re-testing & issue of a new calibration certificate.Other hearing aid suppliers did not repair their own Audiometers,etc,so it was us,or back to the Manufacturer.The down side of "back to the Manufacturer" was that some were returned calibrated at US Standard SP levels & required a re-calibration to Australian Standard,which from memory was the same as in the EU,& the majority of other countries.Obviously the US calibration was to a very high standard,but the Australian Department of Health was very strict,& did not allow interpolation or extrapolation of readings.There was at least one other commercial calibration lab in the Eastern States of Oz,which was used,but the volume of work still kept our tiny number of staff very busy.

author: PA4TIM
author_rank: Super Contributor
timestamp: August 02, 2015, 08:07:23 am
content: I think Pascal is searching the holy grale of calibration. You can buy and make scope calibrators. A decent V ref and a time mark generator coupled to a frequency standard will show you a lot. But that is all. You need to calibrate those standards an need the exact specs of the scope to know if it is within specs or broken.You do not understand the whole thing around calibration, why it is done and all the things around it. Shipping cal kit to end users ? That can be done, Agilent has build a truck for that. Dave made a video from that. Look that and then read a Fluke 70's book about calibration. It can be found on the web but I can not remember the titel. And then answer your own questions. There is so much more involved in calibration this topic could be 1000 pages and still cover not everything. For some instruments even geographic location, angle of placement (set a lm399 based reference on its side and them measure it again, it is changed) and even air-pressure can play a role. (I think Dave made a video about a counter reacting on some external influence)I repair calibration and measurement gear. Most of my customers are calibration companies. I have a whole bunch of standards and calibrators but my lab is not climatised and certified. I repair the gear the manufacturer can/will not repair anymore. Most stuff is for calibrations on the industrial marked but I also get electronic measurement related gear. I have repaired and adjusted a Fluke 5101. It was send to Fluke and returned with the cal results. It was out of calibration. They do not adjust or repair that.I repaired it and had to adjust things. After I repaired it I had to be officially calibrated again. So all I needed to do is to be sure it is within the specs and working 100%. The cal report then tell the owner how close it is to te specs. (I was in this case worst case 0.5ppm off ) Often the software calibration then can be used to compensate to get it as close to the specs as possible. Some gear is so bad made and instable towards climate and time it has no use to calibrate is. Cheap Chinese scopes fall under this group ;-)My customers do not like it if I adjust things. Calibration is about knowing your instrument and the behaviour over time and external influence. I do not care if my meter tells me a traceable standard is 10ppm off. As long as it does that every time over a long time. But to get the confidence it does that, I need to collect data over time. If for instance it shows +10ppm, if my lab heats up 1 degree and -2ppm if the humidity rises 5% and it changes +2ppm over the first year but the last 4 years it only changed 0.5ppm I can make very accurate measurements by applying the corrections to my measurements based on the cal history and climate specs. If it would be auto adjusted every time, it would be very hard to measure with confidence and correct for circumstances. Because you have no clue about the real state its in. I never use the cal function of my Hameg scope, it is still almost spot on (plus/minus 1 digit in the auto measurements)About Germans and service, Thumbs up, I get an anwer every time I ask Hamg/R&S a question. Last week they mailed me a manual of a 15 year old 8000 plugin. I can not say the same about Fluke. Most time they not even bother to react at allBut now you say, yes but the auto correction solves all that. And that is the first major screw up ;-) but that you know now why, you assume in that case that the instruments internal references are ever during constant. But these things are already explained several times by others.Then about calibrating a toy-like scope. If accuracy, repeatability, traceability and confidence of measurement are so important to you, you bought the wrong scope. Buy one from a good brand with a nist certificate or nist tracable. Calibrate it according the advise of the brand and record the history. Only adjust it if it is to far off. If your lab is not climate controlled it is even more important to know its behaviour.A very crude explanation of the foodchain:There are several levels in calibration. The national standards labs are on top. They guard and maintain the prime standards. These standards are often only used to transfer its value to a working standard. Below that are local calibration companies who send there prime standards to the national lab. And below that there are labs that only serve endusers and send their own stuff to the local labs. They can also work with transfer, work and prime standards. The primes will be tracable to the national lab through 1 or several levels. They use their working standards to check the rest. Companies that need high traceability and confidence can have there own standards that are calibrated by the local cal labs or higher. And others send just some gear to the local labs and use f.i. their best multimeter to calibrate the rest. Some have to calibrate stuff because of regulations or certifications.A scope manufacturer can have it's own cal lab, it can send its standards to a national lab like nist, it can calibrate and adjust every instrument it makes in that lab, it even can send all scopes to nist first (or do that on request). But if you make a lot of instruments that have price as most important spec it will probably have one or more references on the production floor to test every or some instruments. And how well those standards are maintained is the question. To do it useful the scope must get time to get warm enough (at least 1 hour and build the scope reference with selected aged referenced). I do not think Rigol has time or cares for that.The better and completer the specsheet is, without a lot of "best" "typical", "minimal" etc, the bigger the chance the factory does a decent calibration and adjustment. I think most cheap instruments are very crude calibrated/adjusted if they are calibrated to begin with. Most specs of cheap gear will be theoretical based on the used parts and sometimes an averages based on statistics.Only Rigol knows what they do to maintain the chain of tracability and calibration before shipping. Autocal can screw up things. It can mask a problem. I have my own cal lab, I calibrate my own instruments often, but I never adjust a thing. (only if needed after a repair)This way I can predict and backtrace my instruments.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2015, 08:35:53 am
content: Quote from: PA4TIM on August 02, 2015, 08:07:23 amThanks for your insider info.We certainly now understand your profile signature.There are no strange measurements strange enoughQuestion:Have you experienced hidden Service menus for Calibration purposes in test equipment like those found in the likes of Tek 2000 series?Could you share some stories?

author: HighVoltage
author_rank: Super Contributor
timestamp: August 02, 2015, 09:47:28 am
content: Quote from: PA4TIM on August 02, 2015, 08:07:23 amAfter reading your great response, I looked at your website and the pictures of your lab.Very impressive!And thanks for the great inside explanation from the point of view as a cal lab.Very helpful!

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 02, 2015, 10:29:01 am
content: Quote from: PA4TIM on August 02, 2015, 08:07:23 amOkey, I got your point now about Rigol scopes. Use it as long as the accuracy is acceptable, and if it is no longer accurate enough, simply buy a new one, because a traceable calibration costs the same as a new Rigol scopeEven if I was looking to solve something, which can not be solved in reality, I believe the thread that I started resulted in very valuable feedback, and I believe everybody here in the thread learned a lot more about calibration. Thanks for all the contributions!

author: G0HZU
author_rank: Super Contributor
timestamp: August 02, 2015, 12:25:02 pm
content: IMO you are much, much better off if you aim to be able to check and cross check your instruments at home. That is what I have always done. Obviously, this doesn't apply for business use because there has to be some traceability to any testing etc.It helps if you buy decent test gear from the top brands and I've always tried to buy high end stuff that has been marked as obsolete. So it can be purchased relatively cheaply.A couple of points to bear in mind. It doesn't matter how accurate the calibration standards are at the cal lab when your gear is tested for calibration. The real performance in terms of accuracy and stability is dictated by the people who originally designed your test gear. The calibration lab adds very little extra value to this. Buy good stuff and it will have been designed to perform well across time and temperature and frequency/amplitude etc etc.Over the course of a year the temperature in my workroom here varies a lot even though I'm able to use a spare/converted bedroom in the house.A typical calibration at a cal lab will not allow for this and so you have to rely on the manufacturer of the test gear to produce something stable over temperature. Also, a typical calibration won't adjust anything or include much in the way of test data.Then you can factor real world (and often unavoidable) measurement uncertainty factors which often dwarf any deviation/drift from true that was noted at the calibration lab. This is especially true when making high frequency measurements That's partly why (at my place of work) none of us get excited about calibration and none of us look at any bundled calibration data because the calibration check will take place under very controlled conditions that try to minimise these uncertainties. In the real world of a busy workroom/lab, these conditions are rarely replicated so the various sources of uncertainty can often accumulate and dwarf any added 'value' from the calibration process.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 02, 2015, 12:48:33 pm
content: The "real world" does bad things to one's expectations.Not test gear,but an example was at one job where we had older Siemens TV transmitters as well as newerNECs.It was actually possible to adjust the Siemens units virtually "spot on".The NECs could always be adjusted well "into spec",but not quite as close.Come back in 6 months & the NEC will still give the same results---The Siemens would have drifted out of spec.