How to calibrate an oscilloscope and multimeter

[Jschool]

 I know there's a lot of different models out there but would there be a way to give a general step by step demonstration of how to calibrate, say, an old Tektronix or other oscilloscopes?  Multimeter calibration would be easier I suspect but might as well toss it in.

[Kiriakos-GR]

The calibration process is not possible, if you do not own reference sources ( separate devices or calibrator ) plus an signal generator. 
And you will also need another one oscilloscope ( but healthy ),  for confirmation .

[Jschool]

Assuming you have access to that equipment the process would be fairly straight-forward wouldn't it?  I know different oscilloscopes have different ways to start the calibration procedure.  But the way to do it after that should be mostly universal, right?  

I've just never actually seen it done.

What do most people do if they don't do it for themselves?  Bring it to a local college lab and pay $50 for someone there to do it for them?  

I know there are places you can mail them off too but that's an awful lot for shipping.

[Bored@Work]

Assuming you have access to that equipment the process would be fairly straight-forward wouldn't it?  I know different oscilloscopes have different ways to start the calibration procedure.  But the way to do it after that should be mostly universal, right?  

(lets assume we are talking about calibration and adjustment)

The process is not straight-forward. Except maybe for the most primitive oscilloscopes the process also was never universal. And with the rise of more and more electronics, including microprocessors, the processes have again become much different over the last few decades.

The manufacturers define the procedures, define the equipment you need and if they are nice they make that information available, instead of keeping it in house.

Today, the most modern calibration processes are fully automatic, if you can afford the equipment, with no need to open the lid of the instrument. On the other end of the scale there is still equipment out there where you have to solder or desolder components or where you have to go back and forth between several manual adjustments to find an equilibrium between desired values.
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[Mechatrommer]

Kiri is right. you need a good reference, and thats all to it, as you said, it will be straight forward from there, assuming the device has the feature to re-calibrate (software or hardware). and i dont see there is "general" or "universal" way of calibration, only the definition of "what calibration is".

each device with their own specific calibration method... if you are talking built in or tuning the internal circuit method of calibration, without any reference as mentioned earlier. i was misunderstanding the word "calibration" and "compensation", now i know more about what calibration is. "from agilent app note, cant remember which one, impedance measurement iirc"

[vk6zgo]

Again"BoredAtWork" hits the nail on the head!!

The calibration procedure is,with older analog units,usually included in the Workshop Manual.
It is usually in software with the newer DSOs, etc.

It can be fiddly,I remember one Tek Oscilloscope where you needed to short a connection to earth on a big ganged wafer
switch to do one adjustment.
I got it one wafer out!
It blew up one of those nasty stacked regulator setups that Tek & HP love so much.

About the best most of us with home labs or in small businesses can do is verify performance as best we can,but calibration
needs fairly serious instrumentation & a fair amount of time.

VK6ZGO

[Jschool]

Well fortunately the Tek 2337 I just bought comes with the manual so I'll be embarking on this process soon (if it needs it).  Be prepared for questions!   

[Mechatrommer]

Be prepared for questions!   

and only expect answers from "expert" who own the same stuff

[Jschool]

I think I can follow the instructions in the manual well enough -- I do know it has calibration instructions.  I suspect any questions that do come out of the ordeal will be more about the actual setup of the testing parameters moreso than anything specific to the unit.  At least, that is the hope.

[Kiriakos-GR]

I think I can follow the instructions in the manual well enough -- I do know it has calibration instructions. 

Fluke and Agilent offers also a separate document , called as "Service instructions" ,
and they specify some " Calibration practices " , but they do not say the all story in detail.
And so , if you are totally an-experienced about calibrations , there is no way to do anything just by reading the book.   

[vk6zgo]

I think I can follow the instructions in the manual well enough -- I do know it has calibration instructions.  I suspect any questions that do come out of the ordeal will be more about the actual setup of the testing parameters moreso than anything specific to the unit.  At least, that is the hope.

The big problem may be access to the test gear they specify for the calibration procedure.
From what I remember,the Tek books were fairly specific,making the calibration fairly easy to do-just time consuming.

My Employer had a set of calibration gear which they sent around in a big road case, so we had no problem with equipment.

VK6ZGO

[saturation]

The easiest thing is find the service or calibration manual and follow the procedures.

Oscilloscopes have different methods for calibration.  The Rigol 1052e, has self calibration function, just select it.  However, the internal references may need to be checked against calibrated references at some point, but given these scopes are not as accurate as DMMs or counters, its far less critical.  You can spot check amplitude with a DMM, and a frequency counter for the horizontal accuracy.

For a DMM the metrologic way is to compare the DUT with a calibrated reference source specified by the service manual.  The traditional item is usually a calibrator or stand alone individual traceable references.  

However, if you have access to a calibrated DMM that is at least 10x more accurate than what you are calibrating, and have a stable source, not necessarily metrological quality [be it voltage, frequency, current, resistance], say for minutes to hours, to make a transfer from the calibrated meter to the DUT, you can make adjustments, comparing the DUT to the calibrated meter.  Its tedious but its DIY and you shouldn't use this for professional applications because the methods are not standard and may lack traceability.

I first saw Agilent mention this common practice in research labs in the 1252a manual, acknowledging this can be a good substitute:

https://www.eevblog.com/forum/index.php?topic=3020.msg41370#msg41370

This DIY method depends on whether the DMM will accept as its input, the signal you are using.  

https://www.eevblog.com/forum/index.php?topic=3020.msg41434#msg41434

[tekfan]

Well fortunately the Tek 2337 I just bought comes with the manual so I'll be embarking on this process soon (if it needs it).  Be prepared for questions!   

It can be calibrated quite adequately if no one has messed with the attenuator compenstaion capacitors. Usually all that is needed is to check the power supply. If the voltages are in spec I wouldn't bother with an entire calibation procedure. It's safe to assume that the other circuits are still within spec. A good cleaning with a toothbrush or similat will probably be needed.

[vk6zgo]

Well fortunately the Tek 2337 I just bought comes with the manual so I'll be embarking on this process soon (if it needs it).  Be prepared for questions!   

It can be calibrated quite adequately if no one has messed with the attenuator compenstaion capacitors. Usually all that is needed is to check the power supply. If the voltages are in spec I wouldn't bother with an entire calibation procedure. It's safe to assume that the other circuits are still within spec. A good cleaning with a toothbrush or similat will probably be needed.

True!
Most of the time with the Teks we calibrated,if the voltages were in spec,most of the time all was OK.
(we still had to check them all,though because the Commonwealth of Australia was paying us to!

VK6ZGO

[Jschool]

My suspicion is that most times when people put that an oscilloscope needs calibration on eBay that they are going by the date of the last calibration and not actually testing.  I guess the only thing I can do is test it against a 1025E and a function gen since that's the only other equipment I'll have access to.

[alm]

A variable power supply, DMM and function generator should be enough to verify the attenuation, time base and horizontal/vertical geometry. The only thing you can't verify/adjust is frequency response / transient response, which either needs a leveled sine wave oscillator or a pulse generator with fast, clean edges. In some cases it's only a performance verification anyway, it's either OK or it's not and it needs repair.

Much of the dedicated equipment is about doing the calibration faster and easier, which is a big deal for commercial calibration. If you want to follow the manufacturer's procedure, you're likely to need $1k+ worth of equipment if you buy used, and much more if you pay list price. But you should be able to do a reasonable job, at least for performance verification, with improvised equipment. I would be hesitant to adjust anything based on this. I would try to follow the standard procedure as much as possible.

A multimeter is in some ways harder, since accuracy is much more important. Even a very basic DMM will do 0.3%, much better than standard resistors or something like a 7805. AC voltage and current tend to be quite tricky to get accurate, especially at higher frequencies. Reasonably accurate resistance and DCV is achievable by spending a moderate amount of money (eg. the Geller and Voltagestandard products), and current can be derived from that. To do a full calibration/verification, you'll also need ways to generate the higher ranges, like 1000V and 10Mohm.

[Jschool]

 

I was right.  Messaged the guy I bought the scope from and asked why he thought it needed calibration.  "Oh I just looked at the sticker."

And Alm, I think I'm understanding now.  I didn't have a concept of just how specialized and expensive calibration equipment could be.  It makes sense, but I thought there were cheaper ways to go about it.

In looking for a place locally to get it calibrated if need be, though, I found this:

http://oscilloscopeservices.com/mobile.htm

Houston-based mobile oscilloscope calibration service.  Is that trailer lab cool or what?

[nyo]

Most decent products have a calibration procedure for it that calibration labs have access to. The problem is that an Agilent procedure for example will have Agilent products listed as standards. The way calibrations are done is to use a "standard" that is at least 4 times more accurate than device under test. So even if you don't have the standard the procedure calls for, you can still run the tests if using a device that is 4 times better on the specific function. Keep in mind that standards used to calibrate other devices where themselves calibrated by national traceable standards or devices that where calibrated by national standard. The keyword here is traceability.

[tinhead]

for DMM calibration you can build you own DIY calibrator (for DCV, DCA and Resistance)

Voltage ref like (DK numbers):
296-17005-5-ND (REF102CP)

some 0.01% resistors
RNCF1206TKY100RCT-ND
RNCF1206TKY1K00CT-ND
RNCF1206TKY10K0CT-ND
USF340-100K-T-5PPM-ND
USF340-1.00M-T-5PPM-ND
USF340-10.0M-T-5PPM-ND

with some additional effort (DAC,µC) ACV /ACA

However it is not cheap, so if you don't have to do it often it didn't makes that much sense.

[PixieDust]

After watching Dave's video on his calibration attempt of the 2225 Tek, I realised that the process isn't complicated and anyone can do it, the question is, do you have the x4 accuracy for your reference equipment as nyo stated. I always thought there was some voodoo involved. Not sure why this post is in this section of the forum. Really should be under the calibration section. Maybe that section didn't exist when this was originally posted?

[Brumby]

My first point is to ask what you mean by "calibration".

What it means in the realm of professional usage - with NIST (or equivalent) traceability - involves measurement only.  Nothing gets "adjusted" - and there are very good reasons why no adjustment is done.

However, I presume we are talking about adjusting a device so that it provides measurements with some degree of accuracy.....

After watching Dave's video on his calibration attempt of the 2225 Tek, I realised that the process isn't complicated and anyone can do it

This is not a statement that can be made globally.  Not in any way.  Just because one "looks easy" doesn't mean they all will.  Not by a long shot.

Devices can be set up in any way a manufacturer wants to design them and adjustment processes can be simple, difficult or convoluted beyond description (well, almost).  They just have to be able to define the process, supply the necessary equipment and train the operators to do the job.  Getting the process instructions is one thing - but getting the right equipment that is set up correctly could be quite another story.

[CatalinaWOW]

I am guessing that OP is fairly new to electronics.  As Brumby says calibration has different meanings depending on who you are dealing with.  Those working at the upper end of the scale get quite testy about the details.  Here are some loose definitions of the word as I have seen it used in various environments.

1.  A rough verification that the readings from an instrument are sensible.  That when it says 5.0 volts it is probably not 4 volts or 7 volts.  Or that a waveform whose period shows as 10 microseconds actually has a frequency closer to 100 kHz than to 65 kHz or 120 kHz.  This level of accuracy meets many troubleshooting and fabrication needs.  This kind of "calibration" can be done without adjusting anything and with fairly inexpensive and widely available gear and some generalized instructions can be given that will apply to most instruments.

2.  Adjustment of instruments of a similar type in a lab so that they all give the same reading when attached to the same source.  This is the "I don't deal with uncertainty" and "Don't make me guess which one is right" crowd.  Usually the highest cost instrument in the group is nominated as the reference.  There may or may not be an associated accuracy goal.   Such adjustments are not easily generalized over instruments (though external gain and offset adjustments might work in some cases).  They usually require detailed knowledge of the instrument operation and requires higher precision reference sources which may still not be particularly expensive.

3.  Adjusting an instrument so that the readings meet the users needs for accuracy.  These needs may or may not be related to the original specifications of the instrument.  For example a frequency accuracy might be set tighter than the instrument spec (but only held over a much shorter time interval or temperature range requiring frequent re-calibration).  Again, such adjustments are not easily generalized over instruments (though external gain and offset adjustments might work in some cases).  They usually require detailed knowledge of the instrument operation and requires higher precision reference sources which may still not be particularly expensive.  This type of calibration occurs in some production, research and hobby setups.

4.  Adjusting an instrument so that the readings meet the original manufacturers specifications.  This requires detail knowledge of the instrument and its requirements and possession of reference sources traceble to at least the level of the original manufacturer and all of the test jigs and instruments specified in the manufacturers procedures.  Usually this means quite costly equipment and references.

5.  Documenting the performance of an instrument to very high precision.  This requires references of very high quality, traceable to internationally recognized standards, good equipment and often, much time. 

Unfortunately the OP will have to learn a fair amount about calibration and what he intends to do with his instruments before he can decide if any of these meanings describe his needs.