I have 3 pieces of equipment (Tek 3032B oscope, Agilent 33120A function generator, and HP 6625A power supply). Each device was purchased used from different places. None of it has been calibrated lately (and I don't know if any of it has ever been calibrated). I was wondering how much calibration I might could do myself. Lets take voltage for example:
I came across a voltage reference from Analog devices that provides a
+- 0.01% accurate 10V output
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That's just 1mV in 10V! Would it be wise to calibrate my 3 devices to this reference? Or is this a bad idea (i.e. are my devices likely to be closer than that right now?)? I've never had equipment calibrated before and therefore I do not know what to expect.
The right answer is to send the devices off and having them fully calibrated by a service that specializes in that. I imagine they calibrate over a range of voltages, frequencies, and maybe even temperatures. I hope to do this when funds become available. However, I was wondering if the $20 chip mentioned above might not be a good poor man's calibration scheme. Does this seem reasonable?
Scopes, function generators and power supplies are good for getting a general idea of what's going on.
What measurements do you do that NEED high accuracy? For most people, the answer is NONE.
The primary reason for instrument calibration is because it is REQUIRED by some third party certification. When ISO9000 became the rage, I saw a lot of scopes disappear from engineering and production just because it cost too much to keep 'em calibrated...never mind that they were never used for anything that cared about absolute numbers.
A VERY distant second is that absolute accuracy is needed. Most of us don't need either. We're more likely to want precision, repeatability and stability. Calibration won't fix any of those.
Don't know the resolution of your instruments, but I've seen people get all excited trying to make .01% measurements with an 8-bit digitizer.
If it ain't broke, don't fix it. mike
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"Stephen" wrote in message news: snipped-for-privacy@z14g2000cwz.googlegroups.com...
Looking at the idea of self-calibration objectively, let's consider the possibility that you indeed have a decent DC voltage reference. What can you do with it? (1) You can verify the accuracy of a digital or analog voltmeter, but many of the digital bench meters either meet that spec, or closely approach it. (2) You can verify the reference voltages in other instruments, but in order to do that, you need an instrument called a Reference Divider that can compare your reference standard against the voltages in the instrument being calibrated. And the Reference Divider in turn needs to be calibrated, but your DC reference won't do it. You'll need a set of lab standards in order to calibrate the reference divider. And those lab standards in turn need to be calibrated in a primary calibration facility in order to certify their accuracy.
Your DC reference won't do you much, if any, service in calibrating your scope and function generator. Yes, you'll definitely need an accurate digital voltmeter to measure and adjust certain voltages inside those instruments, but your DC reference usually isn't used in those cases.
The rule of thumb in calibrating instruments is that the standard being used should be at least ten times more accurate than the parameter being measured.
This introduces the issue of "traceability". When an instrument is calibrated, its parameters must be compared against another instrument that has in turn been calibrated against more accurate instruments, until the point at which primary standards are used. Primary standards usually can only be verified by entities such as the National Institute of Standards and Technology (NIST), which is a US Government agency. The traceability of an instrument's calibration is the trail of records of calibration of all the instruments used to calibrate your scope or function generator, all the way up to the primary standards maintained by the NIST. In order to properly calibrate your scope and function generator, you'll need a number of other instruments, each of which performs certain measurements in the instrument being calibrated. Each of those instruments, in turn, must be calibrated. I think by now, you should see what I'm getting at... unless you have all of the necessary equipment to perform a complete calibration on your scope, it's much cheaper to just take your scope to a calibration facility and pay for the calibration. It's up to the cal lab to buy and maintain the standards that they will use to calibrate and certify your equipment. Consider that if you wanted to obtain all the equipment necessary to calibrate a 100 Mhz dual channel scope, you're probably looking at $10,000 worth of additional instruments if purchased new; maybe $1,000 worth of equipment if purchased used (Ebay). But you still have the additional expense of having those additional instruments maintained and calibrated. Otherwise, your calibration is just a guess, which can easily be in error, and put your instruments outside their published specifications.
If the accuracy of your equipment is important to you, bite the bullet and pay for the calibration. You'll save money in the process.
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A DAC makes a good (predictable accuracy) reference divider. If you build a calibrator which has a certain accuracy by design, the calibrator would not need to be calibrated. Otherwise you'll end up in a chicken-and-egg situation.
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I read in sci.electronics.design that mike wrote (in ) about 'Oscope, Func Generator, Power Supply Calibraton using 0.01% 10V ref?', on Fri, 29 Apr 2005:
Quite right. However, what CAN screw you up, even in a hobby context, is
*relative* errors between your own test gear. It's a good idea to check each one against the others and make a note of any significant differences. Say you have a 1 V RMS sine wave, but the peak-peak voltage on the scope is 3 V, yet the waveform really does look like a good sine wave. You could puzzle for days.
On one occasion I found that a sound level meter was reading 3 dB low on one range only. An actual fault, but not necessarily easy to spot when in use. I found an open-circuit resistor in the beast.
If you really want to go into accurate calibration, buy a multi-digit digital meter on E-bay. Have THAT (alone) properly calibrated and use it as a transfer standard for the rest of your gear.
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John Woodgate wrote in news: snipped-for-privacy@jmwa.demon.co.uk:
That still is not going to insure his TDS scope is reading right. (all functions)
I do not know if I'd trust anyone other than TEK to cal a TDS scope.I don't know if anyone else has the automated cal stations TEK has for the TDS line.
I believe you can get a price for the TDS cal from TEKs website;
"DaveM" wrote in news:4uKdnRlnd5Y-S- snipped-for-privacy@comcast.com:
No,it's not;its FOUR times better.Of course,10X is even better,but 4X is the minimum.Below that,one must state the accuracy on the cal cert.
This is all excellent advice.
For hobbyist purposes,I suppose he could look for used TEK TM500 cal instruments (TG501 time-mark gen,PG506 amplitude and fast-rise cal,SG503 for BW test,trigger cals)for checking his gear,or a Wavetek 9xxx cal system(scopes to 500Mhz AND DMMs),or similar products.
Then if he finds discrepancies,he could send the questionable piece out for service/cal.
The entire TM500 line is available for reasonable prices on eBay, set a watch and wait a few weeks or months. Or it's available from my lab's instrument shelves. :>)
The 10V +/- 0.01% reference is great, but it will only serve to give you confidence the DC response of the scope on two or maybe three ranges. Combine it with a bridge/differential voltmeter and you can get some confidence in the generator and power supply instrumentation/knobs.
0.01% is probably overkill (maybe not for the 6625A, I don't know how many digits it claims in accuracy) but if overkill is easy, why not?
Combine it with some precision dividers (0.1% and 0.01% resistors are only a buck or few), some DC and AC bridges, and a good time base (luckily a few tens of ppm in time accuracy is only a few dollars), and you can get some confidence the calibration in many (but not all) functions of your instruments. You won't be able to really follow the calibration procedures from step 0, because that requires some other capabilities you won't have, but you'll get some confidence that the existing calibration is pretty good. If you find they're off in some range it will probably be good enough just to note the deviation.
Fred Abse wrote in news: snipped-for-privacy@cerebrumconfus.it:
Yes,it appears TEGAM got out of the scope cal business,and transferred TM500/5K service to Dynamic Technology Inc.,who has a blank website. A shame,the modular TM500 concept seems like a good one.
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