DIY DMM calibration - Page 1
Forum section: electronics / general

author: rhb
author_rank: Super Contributor
timestamp: November 04, 2018, 08:40:12 pm
content: I want to set up to calibrate my primary DMMs, two 34401As and a 3478A. I've got a DMMCheck Plus, so I know from it that at 5 V I have a little over 100 uV error on one of the 34401As.The manual is not really clear as it presumes a commercial calibrator and doesn't spell out the reference inputs required. It simply says the DMM will prompt for the next input. As I don't have the instruments called out in the service manual I'm reluctant to start a cal until I know what I need. From the manual, it appears that I need full range on all the ranges. Is that correct?I made a 4 wire short from bare 12 AWG copper wire folded as needed to fit. The ends took a little filing to be an easy fit into the banana jacks. Using this I was able by rezeroing to eliminate most of a 1/4 ohm error (I discovered this while checking Chinese 50 ohm terminators), but it still reads slightly off relative to my other 34401A and my 3478A. This particular instrument had been repaired prior to my acquisition and was not functional until I cleaned off the flux residue from the repair which replaced one of the LF377Ms. U312 IIRC.As the manual was quite emphatic about the short being "all bare copper" I used the wire as I could think of no other way to meet that requirement with anything else I had on hand. I've attached a picture. I almost had it finished but ran out of wire and had to start over.I didn't find much using google other than an article at xdevs.com and a thread here from a few years ago. The xdevs.com article was very good, but used equipment I don't have and shall have to make or buy. Making it seems far more interesting and educational than flinging money about.The low ranges are obviously pretty easy, but it gets problematic above 10 V. Does anyone have any suggestions? At higher voltages I'm baffled as to how to calibrate the calibrator. At 100 V, 10 ppm is 1 mV.I want to develop the ability to do a thorough and consistent cal across all my instruments so that the relative errors are small. That way occasional checks should keep me close enough to NIST for hobby work. So now that I have T&M gear I need to sort out maintaining it.

author: lukier
author_rank: Unknown
timestamp: November 04, 2018, 09:33:12 pm
content: 34401A is not an autocal instrument, needs a value for every range / function, see the calibration section (e.g. "Gain adjustment can be accomplished using the following input values").Seriously, don't go there, it is a massive rabbit hole. You'll end up buying Fluke 5440B, Fluke 5200A, then Fluke 540B to calibrate 5200A against 5440B, then making DIY Fluke 752A to recalibrate 5440B from 10V, then buying Wavetek 4600 to generate AC/DC currents and various expensive shunts to test it. Then buying one Agilent 3458A after another to double check everything. Oh and a box of random Vishay resistors. And even then you won't have > 100V AC source. Don't ask me how I know.DIYing some of this stuff will be way more expensive than getting a good score on eBay. Fluke 5440B is still probably the best DCV calibrator out there. Good luck DIYing custom Fluke 5200A transformers or thermal RMS sensors. Even if you DIY everything you will need a working 3458A or two to verify this DIY gear.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 04, 2018, 09:39:28 pm
content: Exactly what lukier describes happend also to me...The 34401A is very linear. If you have known voltage near 10V and stable sources for the other voltages you can transfer the other voltages with the 34401A. (Talking about DCV only)

author: Kosmic
author_rank: Super Contributor
timestamp: November 04, 2018, 10:05:46 pm
content: The best advice would be to do everything you can to be friend with someone working at a cal labBut seriously if you can't live with the offset. You can probably find a deal at a calibration lab. Not requiring the cal certificate can be cheaper sometimes.

author: rhb
author_rank: Super Contributor
timestamp: November 04, 2018, 11:43:50 pm
content: Reminds me of a machinist story.A young salesman is calling on a manufacturing shop. While waiting for the person with whom he has an appointment, he's chatting with the secretary."What do you make here?""We make gears, all types of gears.""Really? I thought you had to buy those."

author: TiN
author_rank: Super Contributor
timestamp: November 04, 2018, 11:54:51 pm
content: Well, you can do like me and build your own Fluke 5720A from scrap. Now you seem to start understanding why so many people here have 3458A. Due to ACAL you just need 10V ref and 10k ref to calibrate all ranges.But realistic, its just 100 or so $ to get cal lab to calibrate your 34401A. you can't even DIY calibrate ONLY DCV with that money anyway, 34401 have good demands on DCV accuracy.I did a cal for friend 34401A recently.Before cal (last cal factory in 2000):https://doc.xdevs.com/docs/HP_Agilent_Keysight/34401A/ecal/XCR_34401A_ERIK_OCT_19_2018_AS_RECEIVED.pdfAfter cal:https://doc.xdevs.com/docs/HP_Agilent_Keysight/34401A/ecal/XCR_34401A_ERIK_OCT_30_2018_AS_RETURNED.pdf

author: rhb
author_rank: Super Contributor
timestamp: November 05, 2018, 12:40:40 am
content: ROFLTiN, I'm doing my level best *not* to follow in your footsteps. I don't think I could afford it. Besides which when would I ever find the time to play with RF?On a serious note, if I have a 5 V source calibrated against a 3458A, can I set the 10 V range using that? While 10 V would clearly be preferable, it's not clear to me from reading the manual if I can use some other known values. If the latter is possible, then I can get ~96 Vdc from a set of 9 V batteries in series, each one measured using the now calibrated 10 V range. If I "sprinkle Gauss water" on the problem I get a sqrt(10) reduction in the uncertainty for the 100 V value. There are, of course, a *lot* of other issues that would need to be addressed in such a scheme.The current cal seems to me the most difficult because of heating effects. However, if I can hook the two DMMs in series I *might* be able to adjust them to read the same. I'd really like to hear from someone who has tackled the problem. I *really* don't have space for a 3458A though I'm sure I'd buy one if I came across a good deal and didn't get beat out by the rest of this crowd.Ultimately I'd like to be able to get a single instrument calibrated every few years and use it as a transfer standard for other instruments.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 05, 2018, 12:51:46 pm
content: I think you need 0.9 to 1.1 of FS to adjust the 34401A. But you can transfer the 5V to another 10V source. I would do the same with your 100V range instead of measuring the batteries with another load.@TiN: How do you calculate the 1.0x ppm uncertainty for 10V? And it seems the DUT limits aren't correct. The DUT uncertainty stays the same, no matter what input. But very nice cal sheet

author: rhb
author_rank: Super Contributor
timestamp: November 05, 2018, 02:27:15 pm
content: After doing the zero adjustment the 34401As read within 56 ppm of each other measuring a fresh 9 V battery. The drift due to temperature is of the same order. I ordered one of Doug Malone's 10 V references as he has them in stock.I did locate a cal lab in Arkansas, or rather a Dallas lab with a facility in Little Rock. They're accredited to cal 4.5 digit DMMs. Not a useful place to send a 34401A or even a 3478A. From their website I'd say most of their work is cals of scales, gas pumps and similar with a legal requirement of annual calibration for use in trade.I'm not going to do any serious cal work until I get good environmental control. The 1/4 ohm error did need to be corrected, but I think I can live with the full scale error until then.I posed a broader question some time back, but never really got a satisfactory answer. Commercial cal is not affordable for all my instruments. The dominant, "trust it until it misbehaves", approach seems to me rather unsatisfactory. My biggest concern is that if I'm measuring voltages and currents in a circuit and RF power that the measurements are to a common reference frame.However, a good bit of this is being a metrology nut. I worry about dimensions, geometry, etc. So I have gauge blocks to check my calipers, mikes, etc. And plan to get an AA grade surface plate when I can find one in stock at a affordable price with shipping. I'd like to get a 24" x 36", but those weigh in at 450 lbs, so I'm likely to settle for an 18" x 24".

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 05, 2018, 02:40:19 pm
content: Quote from: rhb on November 05, 2018, 12:40:40 amSpeaking really seriously, you should have a look into the 34401A and 3478A service / calibration manuals.There's described, what's exactly necessary to calibrate them. In short, a DMM calibration requires a set of over precise 30 artifacts, i.e. a precision source for each of the different modes and their different ranges, at about Full Scale for each.Having a 5V and 1mA source of completely insufficient accuracy really has absolutely nothing to do with 'calibration'.That's always what people are misunderstanding when they buy these devices (fortunately, this one is called DMMCheckonly).Even a 3458A won't help you, seriously, as you'd still need several stable sources for all different modes and ranges.So, very, very seriously, forget about that DMM Check to do any adjustments on your DMM, I won't even use that for verifying your 34401A, as it's simply too inaccurate.PS: The current heating effect can be neglected, or in other words, is already taken into consideration by the specification, which you also should read carefully.Frank

author: TiN
author_rank: Super Contributor
timestamp: November 05, 2018, 03:29:28 pm
content: 3458A can calibrate off lower voltages at cost of uncertainty increase. But yes, DMM Check is useless even for 34401A cal, leave alone 3458A. And even if you given 3458A for free, be ready to spend 1.3K+ for it any day, if ADC go drifty or bad. So far my luck with 3 meters is 100% (meaning need new ADC replacement). Keeping and maintaining 3458A in its best capacity and calibration is high cost ownership. I spent over 10K already for my 10V, 10Kohm and 1 Ohm standards to get them at ppm-level uncertainty, and its not getting cheaper because annual cal costs (I don't send 3458s or other 8.5d meters, only working standards to maintain my stationary standards in cal). Another point why I inverst over 2 years and over 20K in 5720A setup , as I these calibrators also Autocal instruments, unlike way cheaper older Flukes like 5440, 5450(have this one too) and 5200 etc or Datron 470x and 480x series.So I'm with Frank on this one, paying 100-200 may sound useless for calibration, but its not doable in DIY way for cheap, and no math equilinrics can help that.Even if you have calibrated 3458A nearby, it is not enough to cal 34401A.e61_Phil, I didn't verify DUT spec much, rather pick for worst case using 24 spec.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 05, 2018, 03:50:08 pm
content: The specs (including 24h) includes a part of the measurement and a part of the range. Which means the 34401A can't (or let's say isn't specified for) measure 1V in 10V range to 19ppm. The spec is 15ppm of reading + 4ppm of range. Which is 55ppm for 1V in 10V range.How many data points do you use for one cal point? I think it would also be interesting to show the standard deviation of the measured point.I attached an example of an ohms calibration of a Solartron 7150plus against my Fluke 5450A. The Excel Sheet is by far not as beautiful as your report, but I show also some statistics. Otherwise one can be fooled by too good results with cheap meters (like the Solartron). On some points (100R for example) the deviation is only 4 ppm, which sounds great, but the standard deviation is 255 ppm.

author: rhb
author_rank: Super Contributor
timestamp: November 05, 2018, 07:26:42 pm
content: The DMMCheck Plus is just that, a check. BTW Doug has already shipped the 10 V reference I ordered last night.However, I got a PM from Adrian with regard to his products at:www.ab-precision.comwhich look at the lower end of the line to be well suited to my needs and purse. I've not yet searched for reviews, but it's quite attractive kit.In looking at my 34401As, I see that the heavily used unit with the slight calibration issue has an oddball SN. Instead of the xxxxAxxxxx it has a larger sticker in the same spot with USxxxxxxxx. Does anyone know what that means? It was repaired at some point, but I'd be very disappointed if HP left the flux residue I had to clean off which made it non-functional on receipt. So I assumed that repair was by someone else (an LF357M was replaced, U312 IIRC).I've read the 34401A service manual calibration section, but it was not especially clear what you needed. I did find the information eventually, but the first part simply says the meter will prompt the user to set the next cal value on the calibrator.A lot of the people in this section are concerned with the absolute best accuracy they can achieve. That is not my interest. My interest is being able to maintain my T&M gear to a reasonable level of accuracy and consistency. I have a B grade (+- 50 millionths) set of gauge blocks and a B grade surface plate. I *could* buy a set of 1 grade blocks, but I think that for the cost I can build an interferometer based tester. I'd much prefer that. I have the same view of the electronic side of things as well as all other aspects of metrology.My goal is the best self cal facility I can acquire for $1-2K. So my goals are a couple of orders of magnitude more modest than TiN and Andreas. My sole interest in their playground is the aging behavior of precision references.The 34401A may only be an LM399 based instrument, but I was both amazed and pleased that it measures the 3458A calibrated DMMCheck Plus 5 V output as 5.00000 V despite never having been recalibrated since it left the factory in 1991.

author: TiN
author_rank: Super Contributor
timestamp: November 05, 2018, 08:21:34 pm
content: SM for 34401A tells you about all points needed for adjustment + few used by newer fw meters for linearity of the ADC.Calibration for 34401A for it's specifications is what should be considered reasonable. Otherwise what is the point to touch meter at all? Sure you can adjust and test it for single range, let's say 10V, but then you still have unknown accuracy for other ranges/function. In you example you want your B blocks accurate to 50th mil'th , so you would not calibrate them with shop ruler, eh?

author: rhb
author_rank: Super Contributor
timestamp: November 05, 2018, 11:56:51 pm
content: You seem to be insinuating I think that a DMMCheck Plus is adequate to calibrate a DMM. That is *not* the case and nothing I have said can be reasonably interpreted to suggest that. But if I measure 5 V on a calibrated DMMCheck and one DMM reads 5.00000 VDC and the other reads 4.99980 VDC, why should I not adjust the one which reads low? As a practical matter for the things of interest to me, that's the most important range. When I get the ability to do the other ranges I shall. The entire point of this thread is what will permit me to do that without spending the price of a house on the kit.What do you use to calibrate oscilloscopes, RF synthesizers, LCR and RF power meters, spectrum analyzers and similar? Do you have the specific equipment called out in the manual? How do you make repairs?I was looking at an 8648C on eBay for which the seller, a cal lab, wanted around $1100. When I asked about a cal they told me $900 extra. And there was a long thread in a mailing list about the quality of cal labs. Or more properly, the lack thereof.I have no known need to measure anything with the 1000 VDC range. So should I not calibrate the lower voltage ranges because I don't have a reference source for 1000 VDC?You seem to be insisting that someone should not eat unless they have cake and prime rib.Things like the Fluke 5720A were developed to speed up calibrations. The alternative is a lot of other stuff which makes a cal take longer and which require more time and effort to maintain. But the 5720A came *after* the alternatives, not before.In a previous thread I suggested the notion of modeling the aging of a reference to reduce the aging error and got an untold amount of grief for even suggesting such a thing from people who *have* the data to let me to test the method, but who will not let me have *their* data in ASCII form so I can make the experiment.So instead of spending a weekend analyzing existing data, I am forced to get new references, build a sampler for my DMM, an environmental chamber and collect data for several years. I surely do appreciate all the generous help.I retired at 55 to look after my parents. I was able to do that because I performed services on contract to very large oil companies that some of their research staff claimed could not be done. It worked so well I did quite a lot more of that at a very stiff hourly rate. Of course, defining the volt to a few ppm is much more important than keeping wells from blowing out as Macondo did.I was doing regional studies of the GoM while they were drilling Blackbeard. My *regional" predictions of the pressures they would encounter suggested they would not make TD with the original well plan. After spending $225 million dollars, they had to P&A 3000 ft short of the objective. Exxon wanted to try again, but the partners weren't willing and Exxon could not afford to eat the risk alone. While completing the 180,000 sq mile regional study, I was doing a brisk trade on the side doing pore pressure predictions for the 2nd and 3rd wells on prospects that my super major client had tried to drill previously and failed after spending $50-100 million on each attempt. So I was succeeding where others had failed repeatedly. And big oil pays very well for producing good results for the simple reason that the alternative such as the Macondo blowout is mind bogglingly expensive.So how about sticking to what I actually say?

author: tggzzz
author_rank: Super Contributor
timestamp: November 06, 2018, 12:10:49 am
content: Quote from: rhb on November 05, 2018, 11:56:51 pmBecause, according to Murphy's Law, the low one is correct!To get one of your meters to agree with another, all you need to do is open your toolbox, get out a screwdriver, and twiddle one of the meters. (And you don't need a DMMCheck)To get one of your meters to agree with other people's meters, you need to open your wallet, get out some cash, and send the meter to a cal lab.

author: TiN
author_rank: Super Contributor
timestamp: November 06, 2018, 12:16:15 am
content: My intention was not to insult, but to point out the purpose why HPAK recommend what it recommends for calibration. DMM check uncertainty is not good enough to tell you what reading is true, 5.00000 or 4.99980. If you adjust meters to agree, you may get wrong value on both of them as result. Typical requirement for calibration is to maintain TUR at least 4:1 between standard and DUT. Sure you can do so by using 5700 or whole fleet of single references, but later ones cost even more than 5700. If you doing this just foe learning, sure, go for it. I don't do this for business or profits either, that's why my goal is do get best possible results. Obviously 5720 is overkill for meter like 34401, 6.5d calibrator is enough (or 5440 + 5450 + AC source calibrator).I don't attempt any calibrations of LCR, scopes or RF stuff, as I know that calibration standards for all that will cost another 5720. I'm in progress of repairing scope, and once that will be done I'll send it to Tek, since calibration cost is fraction what the project cost and I will get confidence that unit is working and tested properly by calibrated gear at Tek, instead of guessing. But as you pointed out, indeed I'm not interested in DIY scope cal.So excuse me if all that insulted you. English isn't my native. 34401 is good meter that you can adjust only ranges you need. Ones like 2002 must complete full cal or they reject to save cal data.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 06, 2018, 01:04:53 am
content: It can probably be done old-school, if you find a good deal on a KVD, maybe Julie, ESI or Fluke. I ended up finding a good used 752 because nothing else was good for the 10:1 and 100:1 transfers. Then you need some decent references (no, one isn't enough) and a dozen other items. Plus standard resistors and shunts.

author: rhb
author_rank: Super Contributor
timestamp: November 06, 2018, 03:52:39 am
content: First of all, I am more concerned that related measurements with different instruments be consistent mathematically than I am absolute accuracy. I'm more likely to get into trouble because measurements with different instruments don't agree than I am with the absolute error.Secondly, I'm concerned with how I keep things going if sending something to a cal or repair lab is not an option either because of price or there is no possibility to get it done by a 3rd party at all.Although I shudder to use the word, this is more a "survivalist" issue. What *really* matters to survival of a community (Thinking you can do it all is hopelessly naive. It takes tens of thousands of people.) is the ability to repair things. When things collapse, which they inevitably will, maintaining the technology upon which we all depend is far more important than all the guns, food and ammunition you could ever buy even if you were Bill Gates. Silicon Valley is full of millionaires who think their personal jet pilot will fly them and their family to safety and leave the pilot's family behind. That's *not* going to happen. More likely is the pilot shoots the owner and family and takes the plane. In that situation the owner needs the pilot, but the pilot has much less need of the owner.Although my electronics bench is confined to a little under 1/3 of a 7' x 10' room, I have a 1500 sq ft shop in back with *almost* everything you could imagine. Still need a TIG, plasma cutter and a few other bits, but except for a lack of organization, it's a pretty complete complement of tools. Unfortunately, the last 10 years I've had my nose rubbed in the frailty of memory. Things I was once proficient at now take several iterations. But adjacent to my 7' x 10' workspace is a ~500 sq ft library with 5000+ technical books covering everything you can imagine with the one deficit of biological sciences which is only 3-4 ft of books.I'm a big fan of "old school". I hope to build a cupola for casting iron up to 50-100 lbs and a crucible furnace good for 10-20 lbs of aluminum or brass. But smaller is better than nothing. Many years ago I went through the exercise of fixing a Dumont 60 MHz scope and a Tek 465 and then rounding up the gear to calibrate them. So I am all too familiar with the difficulties that poses. Hence, having bought a bunch of gear, I've focused on how to support it.I now have pretty much all the T&M gear I need to make repairs and build things, but I'm still missing the ability to support the gear. i have some of it, but lots of gaps. So my focus has shifted to identifying the gaps and filling them. With time off to just play.This thread got started when I was checking some 50 ohm BNC terminators and discovered an almost 1/4 ohm discrepancy between my two 34401As. Interestingly, the unit that best agrees with the DMMCheck is the instrument that was never calibrated since it left the factory in 1991. Which raises a question about the quality of the calibrations of the other unit. Both came from eBay, so I have no data. After setting zero the resistance discrepancy dropped to 0.007 ohms on the 100 ohm scale. But I've got lots of error from the lack of temperature control, so it's anyone's guess which meter is correct.Long ago, Conrad demonstrated that hobbyist level metrology need not break the bank if you're prepared to put the time and thought into it. I'm trying to do the same thing, but many years later and with a larger budget and more demands.I can see no reason why, if one goes to the trouble Conrad went through for his resistive divider and does not exceed the current limits on an LM399, that one can't do a reasonable (e.g. 100 ppm) job of deriving lower reference voltages from a good 10 V source. Subject to all the usual headaches over tempcos, noise etc. If someone does, I'd be most grateful for an explanation of why, and how we ever developed things to the current state of the art. I'm a scientist, not an engineer. So I revert back to analyzing everything from first principle.Setting up to do things *really* old school is quite a lot of work. But is is how we got to where we are and if we can't still do it, we are in deep trouble. It could become the only option.I have no idea when or how, but the day is coming when the trucks stop delivering food to the grocery store. It could be a Carrington event, a high altitude nuke EMP, influenza or something no one ever heard of. But when that happens, things will get *very* ugly as most of us die. After about 90 days, things will settle down and the survivors will have to try to find a way forward. I only realized it a few years ago, but my whole life has been focused on trying to build a technological ark to get through that storm. Unfortunately, I don't have children and so no one to turn all that I have built over to. I very much hope not, but in the end the effort may fail. I'd hate for that to be the case, but I've had fun learning to run a lathe and mill and repair a 100 MHz analog scope.Most of the population would be horrified to learn that humans will become extinct. But the paleontological statistics indicate that any other outcome is *highly* unlikely. It is not possible to even count the extinct species as many left no trace.And TiN, I never thought you intended to be offensive nor do I think that of Andreas. But communication difficulties and personality have created some friction I'd love to see go away. I miss my job intensely. I had Stanford and similar credential PhDs across the hall. I just want to collaborate with some people of your caliber. All that stuff about working for oil companies was just by way of justifying my optimism that I can develop a correction for aging. But then it may have already been done as the DMMCheck matches to the last digit on an instrument last calibrated by HP in 1991. So it might just be held as a trade secret. I shall be most interested to see what the 10 V reference shows.My apologies for the tl;dr, but I *really* care about how one goes about recovering technology, because my expectation is that 100 years from now, at best, the elite will live as did George Washington (generally considered the richest man in the colonies), Jefferson and others of that era.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 06, 2018, 04:15:55 am
content: Ha! I'm not the only crazy. There's a reason I keep a technical library on pretty much all disciplines of science. A lot of industrial history too. I even picked up some old boy scout handbooks so I'd have at least something on survival in the wild. My metal shop is more than reasonably complete. Though I could rebuild civilization, I suspect it will muddle along well after I'm gone. Anyway, if NIST calibration isn't your goal, just internal lab consistency, there's no reason you can't set up dividers and standards more than sufficient to the purpose. You might start by building a precision source. I've got an Analogic (Data Precision) 8200 and it's the handiest thing on the bench. Sure, it's a voltage and current calibrator, but it's also handy for plotting curves of diodes, LEDs and other things. It's a good source for comparing precision resistors. Sometimes I just want to inject a stable and high resolution voltage or current into a circuit. If I didn't have it, I'd have to build it. OTOH, if I didn't have it, I wouldn't realize how handy it was. FWIW, I've developed a lot of products and instrumentation and worked with some really sharp PhDs. In 40+ years of doing this, and I'm talking really sciencey science, I'veneverhad need of anything accurately calibrated. Lots of resolution, yes, but accurate, not so much. OTOH, it was usually disturbing if different pieces of equipment didn't agree. The two watch problem.I just spent several evenings repairing two scopes and my curve tracer- old boat anchor stuff, but perfect for my fat fingers. Newer equipment might be nice, but can be completely non-serviceable. At work we have several $16k Agilent (Keysight) LCR meters. Great units, but completely non-serviceable. I suspect almost nobody has the standards to calibrate them, much less fix them. Keysight has a flat repair and calibration fee. It's $3000. Not happening anytime soon. I know a lot of the more recent Tek scopes have custom chips that, if bad, make the unit a doorstop. I know my 7000 series mainframe suffers from that, but at least they're common used and don't cost too much.

author: tggzzz
author_rank: Super Contributor
timestamp: November 06, 2018, 08:30:15 am
content: Quote from: rhb on November 06, 2018, 03:52:39 amThe DC resistance of a BNC terminator is only a small part of the story.Consider that many "50ohm" cables are, by design, very definitely not 50ohms. Have a look athttp://www.rfcafe.com/references/electrical/coax-chart.htmand you will see that, for example, RG58 is 53.5ohms, RG58A is 52, RG58B is 53.5, RG58C is 50And there are many other values you can choose from!More importantly, the VSWR is determined by many other factors. The difference between higher/lower quality terminators can easily be seen with a TDR or network analyser.Having said that, I understand your delight in "getting back to basics" and doing things from first principles. I too find that entertaining. But I wouldn't want torelyon my experiments.If all you want is that your N meters show the same value, then simply connect all N to a voltage source and twiddle N-1 of them. It is left as an exercise for the student which one not to twiddle.

author: tggzzz
author_rank: Super Contributor
timestamp: November 06, 2018, 08:35:30 am
content: Quote from: Conrad Hoffman on November 06, 2018, 04:15:55 amI once worked for a company that made an instrument which measured cable impedance with a resolution of 0.001dB an accuracy of 0.1dB. The customer didn't care about the 0.1dB (in)accuracy.The application was to measure how subsea coax cable's impedancechangedwith temperature. Since that required a room-sized drum of cable, each measurement run took a week due to the thermal inertia. They merely needed a high resolution and stable measurement over that period. They had better ways of measuring the absolute impedance.

author: TiN
author_rank: Super Contributor
timestamp: November 06, 2018, 12:01:49 pm
content: QuoteI'd kill for having own workshop, even small one, not 1500sq.ft.QuoteIt's all related to point of perspective. I'm currently in progress of repairing such a recent(ish) Tek scope, with BGA custom chips. Yes, for many it's expensive doorstop, but instead of saying how bad things are and that BGA package invented (for sure by aliens, to make repairs harderbut no it's PhD's and engineers made it so for better performance), I'd love to try fix it first.If I succeed , I'll have 3+GHz scope for 10% of market price. If not - I learned few things already about scope arch and BGA assembly.Very similar how I started 5720A build project, from getting just one PCB from it for "little" toy project...QuotePrecisely reason why I invest hundreds of hours into my public site to preserve and spread metrology and electronics knowledge, and built calibrator to support my tiny homelab instead of spending 12K+ every year to cal my main 4 meters. First basics of resiliency : don't keep all eggs in same basket and share info widely possible. If doomsday event come to north hemisphere, if your data and efforts also preserved on other side of the globe improves chances for reuse by 0.0001% better.. I earned large negatives from all this voltnut stuff, and don't know any smart PhDs to collaborate , so just do my best to share and discuss the topic with those who care too. My approach does not fit everyone , but it was not supposed to in the first place.As my last post on topic (don't have anything else to add to abovementioned things), I've restart my LTZ ref scan for an example slice. Attached graph from last 60 hours data, measurements of 10 different (expensive) LTZ1000 modules by 4 different meters and nanovolt scanner.And because my meters are all calibrated (for a second - my DIY cals but to traceable commercially calibrated references with subppm uncertainty data) and verified for stability (I know from calibrations, they are <2ppm a year), I easily see that:* Meter HP3458 "C" (black line) drifts +1.0 ppm a day (and will void it's annual spec in 4 days!). This means another $$$$ to replace drifty ADC.* Reference on ch8 and ch6 drift bad over -1.0 ppm a day OR those scanner channels are drifty.* Reference measured by K2002 (K4) is bad (jumpy about +/-0.6ppm). This ref came from eBay 3458 A9 board, so that much to tell about buying second hand reference from 3rd party.Could i have same confidence with unknown history meters from ebay? I'd be just guessing if its my ref unstable, or the meter, or both have same drift-rate so resultlooksstable, but is not? Getting more uncharacterized meters that agree only adds more confusion and false confidence. If I'd try some AI math to predict long-term drift of such, it would base of wrong assumptions too. Garbage data in = garbage data out.But ofcourse, none of this matter if you happy with +/-100ppm uncertainty. Then you don't have to spend lot of $$ and just have literally ANY lab to cal your 34401A for cost of few beer boxes.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 06, 2018, 01:11:01 pm
content: Quote from: TiN on November 06, 2018, 12:01:49 pmWouldn't ACAL bring it back to spec, if "only" the ADC is drifty?

author: TiN
author_rank: Super Contributor
timestamp: November 06, 2018, 01:17:38 pm
content: Yes, but ACAL does not fix INL issues this meter have at -0.4 to +0.3ppm (8 times worse then spec).

author: rhb
author_rank: Super Contributor
timestamp: November 06, 2018, 04:01:56 pm
content: Quote from: TiN on November 06, 2018, 12:01:49 pmThe sad part is most of the people who could *really* make good use of such a facility can't afford it until they are old. The kid who made the IC was *only* able to do that because his parents had invested over a million dollars in a shop.QuoteIf the Tek doesn't work out, have a look at a LeCroy DDA-120, *not* DDA-125. The screen is a 640 x 480 VGA monitor. There's an internal VGA out on any unit with the external VGA output option. So one should simply be able to remove the entire CRT section, install a $90 7" LCD and go.That leaves a huge space where the CRT was located right above the main board. The DDA-120 will sample a single channel at 8 GSa/S. The 125 will also, but you need a special adaptor that connects to C2 & C3.I thought the 7% overshoot of an MSOX3104T was bad, but it's nothing compared to the 20% of the DDA-120. Worse yet, the 125 is spec'd as 1.5 GHz while sampling at 2 GSa/S. I can only assume that the application made rise time measurement more important than the quality of the step response. But it really does have a < 250 pS rise time. FYI the disk drive analyzers are modified LCxxx DSOs.The anti-alias filters appear to be a pair of boards that span the gap between the AFE and the ADC. Unfortunately, the LeCroy documentation is not very good. I've yet to identify the filter in the schematics, so I'll be flying by the seat of my pants. But...There is lots of room where the CRT was to install some Radiall SP6T SMA DC to 18 GHz relays. So my general plan is to get a DDA-120, replace the CRT, remove the crappy filters and install replacement boards with SMA connectors to connect to the relays and a set of proper filters, four 750 MHz filters for 4 channel mode, two 1.5 GHz filters for 2 channel mode and one 3 GHz filter for single channel mode. The beasts are incredibly noisy, but a 3 GHz, 16 Mpt DSO with external time reference for under $1000 is a very attractive project. Especially after trying out both the MSOX3104T and the RTM3104.QuoteThat's very kind of you, but the sort of data I need is a suite of several references with a year or two of data from initial startup. The question is, if I have the first 1000 hours of drift, can I predict the next 1000 hours? But I entirely agree with your assessment of trying to predict aging from your data. I should have no confidence in it either. The early behavior is critical.The aging curves have the general form of abs(a*(1-b*exp(-c*t))). Most of the information about a,b and c are in the initial part of the curve. After that it gets very linear. So if you do not have the first part of the curve the best you can do is the current practice.The reason I think I might succeed is that the equation is ill-posed and traditionally considered unsolvable using least squared summed error (L2). And 30 years ago, a least absolute summed error (L1) solution was computationally intractable.Without actually knowing much about it, I started doing L1 solutions to the 1D heat equation which is an infinite sum of exponentials of the same form. It worked much better than I ever imagined possible. Investigation led me on a 3 year binge learning about sparse L1 pursuits about which I've already written too much.But I did a numerical experiment that is very relevant here. I generated a particular example and then tried to recover a,b and c. What I found was that if I had N samples I could predict the value from N+1 to 2*N to within a percent or two. The error could be reduced further, but at the cost of significantly more computation and not really worth while. Suppose you measure a reference for 1000 hours under normal operating conditions and the drift is 5 ppm/1000 hours after the initial 1000 hours. If the aging behaves as in my numerical experiment then for the next 1000 hours I can predict the aging to better than 0.1 ppm. That reduces the uncertainty by almost 5 ppm. It's not all the error, but a large chunk of it.One can, of course, just use a linear approximation from one cal to the next, i.e. a linear fit to the drift from j to j+1 to predict the drift from j+1 to j+2. That's so simple and obvious that I'd be quite surprised if it's not being done in the FW for most high end DMMs.I'd like to get 10 ppm, but like Conrad, I can live with more if I know the magnitude of the uncertainty.As noted, all this requires multiple references, meters, samplers, etc.With regard to the BNC terminators, this is just initial screening before putting them on a VNA. I'm also checking for fit, etc. I don't want to bother measuring the crappy ones with the VNAs.That's really all I have to say on this. I've got one of Doug's 10 V references in the mail. It's not sufficient, but it's better than I have now and that's a start. Right now my biggest source of error is the lack of temperature control.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 06, 2018, 06:12:39 pm
content: Quote from: rhb on November 05, 2018, 11:56:51 pmWell, the problem with your argumentation is, that you compare references of different stability and accuracy, and in the end you indeed imply, that the DMMCheck REALLY could do the correct judgement, which of the two 34401A reads "correct".That's the classical 'man-with-three-clocks' problem..To get a more scientific approach, you have the DMMs, which are accurate to 15ppm in their 10V range, and stable to 35ppm/year. They disagree by about 40ppm on a 5V reading.The DMMCheck on the other side is specified to have 70 ppm accuracy over 1/2 year, that's about 3 time worse than the 34401As.So it's not directly possible to judge a 40ppm difference by a 70ppm accurate source; the service manual calls for a TUR of 5:1, not the other way round.To achieve what you have in mind requires reference(s) of the same, or better stability class than the LM399, so that's the starting point for the LM399 and the LTZ1000 thread here in the forum..If you have a bank of at least 3, let's say LTZ1000 based references, you can really start your drift analysis and really calibrate the 10V range of your 34401A, but you also need one initial calibration baseline of that uncertainty class, so maybe well below 5ppm, or so.Then you need Transfer Standards, which precisely (<1ppm uncertainty) transfer your reference voltage to the needed values, like 7,xxx V => 10V, 10V => 1V, 100V, 1kV.You asked, how this was done in the past, BEFORE the 5720A, or 3458A.Well, these Transfer standards were the 720A KV divider, and the 752A reference divider, plus a stable 332A / 335B, maybe.I recommend the Fluke book 'Calibration: Philosophy in Practice', where all this historic stuff is described.But you really need such instruments, you can't solve that calibration problem just with scientific statistics.Similar to Conrad Hoffman, I also built these devices on my own, in the beginning, because I also wanted to calibrate my 34401A, from 1990.That's described here:https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg239666/#msg239666But I started directly with an appropriate approach, i.e. 2x LTZ1000, a precise 10/7 divider/amplifier (to 10.24...V, <0.2ppm ratio uncertainty by self-alignment) , and a 100:1 / 10:1 Hamon divider, also precise to < 1ppm ratio uncertainty @ 1kV. I also could once import the initial absolute uncertainty of < 10ppm from a 3458A in my company. All these self-built devices cost < 1000 € / $.(Ok, sooner or later, this story escalated, similar to TiNs passion.)I propose that you better chose the appropriate devices, instead of fiddling with DMMCheck, or similar.Concerning your approach to improve the relative uncertainty of references, I fully agree with you, but again under the precondition, that you use lowest possible stability for your initial reference ensemble.That means, if you have 3 or more LTZ1000 references, you can measure the relative instabilities and estimate the absolute stability from that, (i.e. decide the 3-clocks-problem) so to have an uncertainty of that order of magnitude of about 1ppm also, if you once set a baseline.But it's not feasible, to start with, say LM399s, as these will diverge much more, not giving the necessary stability for calibration of your 34401A.If you once have that stable ensemble, then only you can add less stable references to your statistics, and use these additional baseline points to improve your uncertainty also.These metrological techniques have been state-of-the-art, when Weston cells and these resistive dividers were common, and no Josephson Junction arrays were available.I'm maintaining such a history of 4, up to now 9 voltage references, over about 9 years, which you requested.I'd be interested in a statistical model, which analyses these relative drifts properly, and delivers properties of this ensemble, like probable overall stability, uncertainty, identification of 'stinkers' and handling of 'jumpers', after transport or temperature excursions.PS: You obviously had some problems to measure low Ohm with your 34401As, consistently.That probably may be caused by the well known 2W Ohm Offset problem, due to contaminated front/rear switches, but not by an improper calibration.Frank

author: Kosmic
author_rank: Super Contributor
timestamp: November 06, 2018, 07:58:57 pm
content: Quote from: Dr. Frank on November 06, 2018, 06:12:39 pmInteresting book. Thank you for sharing!

author: rhb
author_rank: Super Contributor
timestamp: November 06, 2018, 08:01:58 pm
content: Thanks for taking so much of your time. After finally finding the correct table in the 34401A service manual it was obvious that tweaking the 34401A 10 V range was not possible using a 5 V reference. I raised the question precisely because I thought that might be the case.My reason for not trusting that particular 34401A is:The zero was wrong.It had been sloppily repaired and when I received it was not functioning.It has a history of cals, at least one only partial.The 34401A that agrees with the DMMCheck has never been calibrated since it left the factory in 1991. Also my 3478A agrees with it. I am more concerned that if I'm using both meters they agree than I am the absolute accuracy.I have three of Jason's LM399s, but not yet packaged so I can run and monitor them. I bought a lot of gear the last 12 months and am still struggling to get everything set up properly. At the moment the DMMCheck is my best reference. I can't use what I don't yet have.My question about what was done before was rhetorical and directed at how Kelvin and others did their work. I've had a general interest in metrology for many years, but books and dimensional instruments consumed my available funds. I'm not familiar with commercial electronic instruments, but I am familiar with the physics and mathematics.Do you have data for references from initial power on? LTZ1000s would be nice, but anything will do if it displays the classic aging curve from the beginning or at least within a hundred hours of startup and I have some idea of the lag between startup and data.At this point I don't know anything at all about a, b & c. They might be constants or some might be a polynomial expression. Until I can do fits and look at residuals I have no way to know.I spent several years dealing the properties of hydrocarbon mixtures at work. In the process I became very familiar with the way that multiparameter equations of state are developed. If you think that electrical references are difficult, try dealing with the physical properties of substances at temperatures and pressures that push the limits of physical instruments. Mother nature has no problem developing a pressure of 15,000 psi at 450 F, but it's really hard to do that in the lab. Getting a single data point can consume a lot of time.I knew nothing about the subject when I got involved with it. But by the end of a year it became clear my client was paying $35K/yr supporting a consortium that was doing very poor quality work and making completely ludicrous claims of accuracy. I mention that because in the process of sorting out what was going on I learned a great deal about fitting unknown functions for which only a very rough notion of the mathematical form is available.I am *not* trying to solve the calibration problem with statistics. As you note, it can't be done. What I am trying to do is characterize the physical aging process. What started as checking up on fluid properties eventually led me into the continuum mechanics of solids and porous media. In the process I learned quite a lot about the thermo-elasto-plastic behavior of solids and similarly arcane subjects. I've now forgotten lots of it, but I buy a lot of reference works when I work on a subject and I still have those. So I merely need to refresh my memory.I think I have a pretty damn good idea of exactly what the mechanics of the aging and environmental shock process of a reference is. How that couples to the electrical behavior is still a mystery, but I have not read anything to date with regard to reference aging that gave me any confidence that the EEs involved had taken the time to learn the appropriate mechanics. It's an exotic subject even for a mechanical engineer.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 06, 2018, 09:39:50 pm
content: my 2 cents ....i have 3457a , 34401a and 3478A ; nice thing for HP \ Agilent you may adjust only some single range and do it manually ; another huge plus , you don't need precise 1.000000 V , its ok with some variation, so you just adjust on display an exact voltage that you feeding (that super important on a resistance cal), vs some global procedure with gpib.as I don't have any reference , has had to cal 3457a , then use it as reference , adjust others, same with resistance , same with DC current, then problem began with AC, for a voltage I use sine generator + TDA2030 that feed a couple transformers on secondary , so primary goes +700 . it reasonably stable.and no luck with AC current , so it not calibrated at all ..... (no big deal so far) as well as 1000Vdc3457 and 3478 are 3000000 and 300000 , means ... for me calibrate 34401 in lab == more uncertainties for 3457second note 34401a has 10M 10G resistance both do calibrating during one shot range, but 3457, 3478 doesn't. - means if you feed 34401a from a resistance divider , chance that cal. fail due to diff V input as DMM switch internal resistance therefore affect divider - it doesn't happens with 3457, 3478on side note , it pretty easy to deal with 5.5 , a magnitude harder 6.5 ; impossible for me 7.5. and the same reason why i bought 3 HP3456anow I man with 3 clocksit seem that clocks drifting pretty low, during my occasional use ... around 2ppm \ year ... ( one recent device with 5ppm disagreement comparing to my 3xclock; so I thinking to do a second calibration circle )

author: rhb
author_rank: Super Contributor
timestamp: November 06, 2018, 10:16:22 pm
content: A man after my own heart. Thanks. It may not get you to the level that TiN and Andreas like to play at, but it strikes me as a reasonable solution to a practical problem.I don't have space for 3 x 3465A, so I'll have to settle for 3 or more voltage refs.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 06, 2018, 11:35:16 pm
content: it may work, if you construct 1:10 dividers + buffer follower. I did 2xLM399 with 10 1 0.1 output all opamp+trans folowwer buffered, and cheap melf 25ppm hammon divider where resistors was sorted to very close tempco and then each resistor altered by filer to precise the same nominal; soldered together , then many layers of acrylic coating .now I'm thinking the same melf divider ( as resistors are very small) and acrylic + glass bottle and a mineral oil ... I think it will work wery for a long term even ..... Will do a single reference resistor first ...

author: alm
author_rank: Super Contributor
timestamp: November 06, 2018, 11:55:48 pm
content: Sounds to me like you have three problems:standardssources for all points you care about (e.g. cardinal points from 100 mV to 100 V, or 100 Ohm, 1 kOhm etc resistors)a way of transferring the value of the standards to compare them to the sourcesThe $$$ calibrators contain all in one instrument. For example, they might contain a voltage reference, a variable power supply, and a voltage of the reference to calibrate the variable power supply. If the divider is 1:10, and the reference is 10 V, then if the power supply is adjusted so that its divided output equals the reference, it will be 100 V plus the uncertainty of the reference and the transfer uncertainty (divider, null detector).The old Fluke 7105 system for DCV calibration, discussed in the first edition of the Fluke book described above, used a Weston cell as reference, a Kelvin-Varley divider to divide arbitrary values down to the value of the standard cell, a null meter to minimize the difference between two voltages and a calibrator. For best performance, the calibrator was merely used as a stable power supply, and adjusted until its value divided by the correct division ratio equalled the reference.If you care about absolute values, then you will have to periodically compare your standards to the outside world. If your accuracy requirements are modest, a DMMCheck could serve as a standard. Sources you can improvise, depending on requirements. A precision power supply, a stack of 9V batteries, optionally combined with some sort of voltage divider might serve as one for DCV. For transferring, you can buy / build a Kelvin-Varley divider and null meter (see Conrad's mini metrology lab articles) or use a DMM for which you verified linearity. For resistance, you may be able to do something similar to the ESI SR1010 series.You're not likely to achieve sufficient accuracy to calibrate a 34401. Even the Fluke 7105 system would probably have been borderline. But it sounds to me like that may not be the goal here.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 07, 2018, 12:20:08 am
content: Quote from: alm on November 06, 2018, 11:55:48 pmKinda sorta .... in my childish practice deal with 1mV in the end much easy then 1µV .. so 100.000V not so hard at all, but 1.000000 are disaster, where all diff. effect adding to skew the value ..

author: rhb
author_rank: Super Contributor
timestamp: November 07, 2018, 01:03:03 am
content: Does anyone know of a *complete* DC error analysis of a 1:10:100 or similar divider? By "complete" I mean at the level of what a mid-career PhD experimental physicist would consider complete when designing an instrument. Every known factor affecting the result included in the equations: thermal masses, currents, device dissipation, thermal resistance, non-linear effects, etc.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 07, 2018, 01:22:21 am
content: interesting approach ... in such case , IMOO , design a schematic, then define a most critical elements affected by environment or element parameter fluctuation, then then less, then somehow equalize it , then add statistical uncertain .... and all this approach perfect for a mass production, but not for a really a single device that you going to build .due to all elements you can carefully select, starting from a voltage reference , and ending with resistors that you can select from batch per lowest thermal coeff ( or connect 2 par\seq with opposite sign to compensate it) and so on ..... in the end nothing stop you to tell the temperature: "screw you" and toss all schematic board to thermal box with constant T .Outsmart God it seems a reason volt-nuts still alive; how the hell, i may assume, that on specific trace I lost 3 microvolts, not 5 or 6 and that need to compensate somehow to reach "0000" in the end ....

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 07, 2018, 05:31:13 am
content: Quote from: rhb on November 07, 2018, 01:03:03 amMy guess is no. If there is an analysis, it's proprietary to the company. IMO, they probably cheated and did the analysis after the wily old techs got the thing working properly. There are just too many physical unknowns. No problem putting variables in equations, but where to get all the data?edit- Ha, wrong again. I own a 752a but only read the operating and adjustment sections.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 07, 2018, 06:46:37 am
content: Quote from: rhb on November 07, 2018, 01:03:03 amSure. Just download the 752A manual from the FLUKE site:https://us.flukecal.com/category/literature-type/product-manualsIt contains an elaborate, physics grade error analysis. All relevant effects are taken into consideration, not explicitly calculated, but sufficient to reproduce the calculus, as far as I can tell as an experimental PhD physicist.In summary, the 100V to 10V / 10:1 OUTPUT is uncertain to 0.2 ppm, and the 100:1 OUTPUT to 0.5ppm, including power dissipation of 1kV to 10V.Division of 10V to 1V and to 100mV is the same.Frank

author: rhb
author_rank: Super Contributor
timestamp: November 07, 2018, 02:13:27 pm
content: Thank you. Not quite as detailed as I'd like, but it makes clear what the important factors are and is a marvelous example of good instrument documentation. I shall be spending a good bit of time studying it.