Reference Sources - For Testing Multimeters - Page 1
Forum section: electronics / general

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 11, 2012, 03:38:46 pm
content: Hello Folks ,My First Post here.I Am Interested in calibrating Some old vintage VTVM's and VOMS That i Am Now importing from USA to india . I would like to Know Some Cool DIY Reference Sources .Like1 : Voltage Reference Sources2 : Current Reference Sources3 : Resistance Reference SourcesIf Possible Capacitance Reference Sources ,Though in VOMS and VTVM's Its of No Use .I Will Really Appreciate if they could be variable .I would Be Importing the following To India Soon For My Restoration And Reselling Work .VTVM's :HeathkitKnightRCA VoltohmmystB&K Precision 177 DynascanVOMS :AVometer [ From UK ]SimpsonTriplettGeneral Once Like Sears , Micronta Etc

author: LEECH666
author_rank: Frequent Contributor
timestamp: June 11, 2012, 04:10:44 pm
content: Not really DIY, but here I go anyway:http://www.gellerlabs.com/SVR%20Series.htmhttp://www.voltagestandard.com/I think the main problem with these references is, that you would need an even more accurate (at least one order of magnitude) calibrated meter to calibrate them. So you will already need some pretty expensive / high end gear to "do it yourself".Florian

author: Circuitous
author_rank: Unknown
timestamp: June 11, 2012, 05:18:03 pm
content: I agree with Leech666, the references from Geller Labs and VoltageStandard will probably do what you need.There's some additional discussion here:https://www.eevblog.com/forum/general-chat/incoming-stuff-voltage-reference/msg104528/#msg104528These should be very accurate right out of the box. But, be sure to follow the guidelines on the ambient temperature that they are calibrated for, and also allow for warm-up time.On the Geller unit, you can have the device calibrated to a specific temperature, which may be handy if you don't have an air-conditioned/heated lab.The Geller site might not be as clear about ordering and shipping, but if you have any questions, just send him a note.On VoltageStandard, there's a new unit that puts out AC voltage and AC current (haven't tried this one yet). Again, if you have any questions, Doug Malone is very responsive to inquiries.

author: Unknown
author_rank: Unknown
timestamp: June 11, 2012, 05:32:00 pm
content: Quote from: LEECH666 on June 11, 2012, 04:10:44 pmThe main thing your buying with the SVR/VoltageStandard in my opinion is improvised calibration, so you should not need a more accurate meter. It may not be NIST traceable, but it should be plenty good for most hobbyists. Note that analog meters are not usually 0.05% accurate. If you're talking about building your own standard, then I'd certainly agree. Initial accuracy is usually much worse than stability, even the expensive voltage references may only have 1% initial accuracy, while long term stability might be in the tens of ppms.A potential issue with VOMs is that the ~5 V from a voltage standard may not be sufficient, and that the high test current for the lower resistance ranges might heat up the reference resistors.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 11, 2012, 07:03:06 pm
content: Quote from: LEECH666 on June 11, 2012, 04:10:44 pmSir ,Hope This Is OK For VOMS And VTVM's ->http://www.voltagestandard.com/DMMCheck_Plus.html

author: LEECH666
author_rank: Frequent Contributor
timestamp: June 12, 2012, 03:53:13 am
content: Quote from: alm on June 11, 2012, 05:32:00 pmYes that's what I was talking about.Quote from: Amarbir[Lynx-India] on June 11, 2012, 07:03:06 pmI'd say yes for VOMs (Volt/Ohm Meters). I am however not the most knowledgable person to ask about this.Never used a Vacuum Tube Volt Meter, but I don't see why it shouldn't work. Probably better to ask one of the old guys with more experience than me. Alm already hinted at some limitations of these low voltage low power references.Cheers,Florian

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 12, 2012, 06:32:29 pm
content: Quote from: LEECH666 on June 12, 2012, 03:53:13 am

author: Unknown
author_rank: Unknown
timestamp: June 12, 2012, 06:55:15 pm
content: It says on that page that it's sold out until June 20, maybe that's why you didn't get a response yet?One thing I don't like is the AC signal. They appear to use a micro controller to generate a square wave. A square wave is not something I'd choose to test a multimeter. The value will be off by sqrt(2) for average-responding meters (which includes most analog meters) that measure the average amplitude and multiply it by sqrt(2) to get the RMS value for a sinusoidal input. Even for a true RMS meter, the exact value depends on the meter's bandwidth. A square wave from a micro will have fast edge rates. Some multimeters will only have bandwidth to cover the standard 50/60 Hz signals. I wouldn't attach a micro controller to a sensitive voltage reference circuit, I'd be worried that it might inject noise into its power supply rail. I hope they've decoupled it very well.I realize that chopping a DC signal is by far the easiest way to get an accurate AC voltage. It used to be a common way to calibrate the vertical amplifiers of oscilloscopes, for example. They would generate a DC signal, verify this with an accurate voltmeter, and chop this voltage to produce an AC signal for a scope.

author: bingo600
author_rank: Super Contributor
timestamp: June 12, 2012, 07:23:28 pm
content: I just got a Geller SVR , IMHO it's more accurate than the voltagestandard one.But also because Geller (Joe) will recalibrate it for $10 + postage.My Agilent 34401A is showing 9.99993v , and is still within specs (cal'ed in 2003)I also got 2 PCB's from Geller , and have build & calibrated 2 DIY's against the "Original".Cheap Ref-Chips on *bay here : 250731549641/Bingo

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 12, 2012, 07:37:39 pm
content: Quote from: bingo600 on June 12, 2012, 07:23:28 pmWell,The Solution i am looking out for has AC+Dc Voltage [ though ALM has put question marks there ] , AC + Dc Current + Resistance .This Is What I am Looking For IMHO .@ AlmThis Needs To Be answered By Them ,I Will Email Them again "One thing I don't like is the AC signal. They appear to use a micro controller to generate a square wave. A square wave is not something I'd choose to test a multimeter. The value will be off by sqrt(2) for average-responding meters (which includes most analog meters) that measure the average amplitude and multiply it by sqrt(2) to get the RMS value for a sinusoidal input. Even for a true RMS meter, the exact value depends on the meter's bandwidth. A square wave from a micro will have fast edge rates. Some multimeters will only have bandwidth to cover the standard 50/60 Hz signals. I wouldn't attach a micro controller to a sensitive voltage reference circuit, I'd be worried that it might inject noise into its power supply rail. I hope they've decoupled it very well."Regarding This "I realize that chopping a DC signal is by far the easiest way to get an accurate AC voltage. It used to be a common way to calibrate the vertical amplifiers of oscilloscopes, for example. They would generate a DC signal, verify this with an accurate voltmeter, and chop this voltage to produce an AC signal for a scope. " .Can You Explain Me Better Please

author: Unknown
author_rank: Unknown
timestamp: June 12, 2012, 10:48:50 pm
content: Generating an accurate DC voltage is relatively easy, both the Geller and VoltageStandard ones do a decent job. Chopping the DC voltage with something like a MOSFET produces a relatively accurate low voltage (very close to 0 V because of the high off resistance of the MOSFET) and high voltage (very close to the DC voltage because of the very low RDS(on) and high input resistance of the meter). The duty cycle then determines the RMS voltage (50% duty cycle of 5 V = 2.5 V). A similar technique was discussed in more detail in Richard (amspire's) power supply design thread.Doing the same for a sinusoidal signal usually involves an oscillator and an amplifier with a feedback mechanism which compares the output amplitude to some reference signal and adjusts the gain. This is much more complex and requires more components.

author: Kilroy
author_rank: Regular Contributor
timestamp: June 12, 2012, 11:32:44 pm
content: If you are going to go very far with this I would be more inclined to procure good low ripple, laboratory grade power supplies first, and make sure they are fully calibrated and referenced. That way you will have 99% of your verification plus the main tool required for calibration work.Reason is, you are going to need to input specific, accurate and stable voltages and currents into these meters in order to perform proper setup and calibrations. Just the full scale movement calibration will require a stable 50uA current for typical 20kohms/volt analog VOMs like the Simpson 260 and Triplett 630/60 series. Something like the Simpson 269 or Triplett 630-NS will need even lower current values due to their higher sensitivities.

author: amspire
author_rank: Super Contributor
timestamp: June 13, 2012, 01:18:49 am
content: Quote from: alm on June 12, 2012, 10:48:50 pmI did post a stable circuit for making an AC reference for Marius (king.oslo).https://www.eevblog.com/forum/general-chat/safe-to-repair-hp-agilent-3456a-6-d-dmm-any-ideas/msg92012/#msg92012I didn't get into calibration though. With AC, the most accurate way to calibrate is with a Thermal Transfer standard that basically matches the heat produced by an AC voltage to the heat produced by a DC voltage.It is not hard to find an electronic way to derive an accurate AC level from DC sources or meters to better then 1% accurate, but when you start needing 0.1% or better, it is amazing how many sources of errors there are. Very few DMM's quote better then 0.1% AC accuracy for very good reasons. Squarewaves have a heap of high frequency harmonics that can cause problems, and you are never quite sure how a squarewave reference reading will correlate to a true sinewave source.One of my to-do projects is to experiment with an AC calibrator circuit that I can check against my Fluke 540A Thermal Reference standard. I would love to find a cheap circuit the does a pretty accurate and reliable AC calibration that I could use to set the level on my AC reference circuit.I did also discuss with Marius a precision 100V DC reference circuit that was self calibrating from a 10V reference.https://www.eevblog.com/forum/projects-designs-and-technical-stuff/100vdc-reference-circuit/Richard.

author: Unknown
author_rank: Unknown
timestamp: June 13, 2012, 01:40:09 am
content: Quote from: amspire on June 13, 2012, 01:18:49 amStill a significantly higher parts count and design effort than sticking an 8-pin uC in there, connect it to the reference voltage and program the PWM to generate a 50% duty cycle signal at some fixed frequency. Any idea how much error the distortion introduces? How much difference would there be between an average-responding meter, a true RMS meter with only a few kHz bandwidth and a true RMS meter with 1 MHz bandwidth?Quote from: amspire on June 13, 2012, 01:18:49 amAC specs are usually much worse than DC specs. Many 6.5 digit meters are slightly better than 0.1% at audio frequencies. Datron 1081 quotes 0.02% 1 year 10 Hz-2 kHz and 0.005% AC/DC transfer 90 days.

author: amspire
author_rank: Super Contributor
timestamp: June 13, 2012, 02:20:19 am
content: Quote from: alm on June 13, 2012, 01:40:09 amYou do not know the error and that is the problem. You can never be sure if the error is 0.1% or 1% unless you can compare it to a true sinewave reference signal. Calibration is all about confidence, and a square wave is not a good waveform for a high confidence calibration.QuoteThey are amazing specs for a DMM, and I know seeing that that Datron have done a massive amount of work to achieve those results. I do wonder if Datron specs are as conservative as Fluke.Definitely though, specs like the Datron specs are the exception. If you were trying to calibrate the Datron, I am sure you would have to use a precision multi-frequency sinewave reference source (or another calibrated meter as a transfer standard), as to get those specs, I am sure there is frequency compensation that needs to be calibrated. That would rule out a square wave for calibration.In general, you want a very stable sinewave source for calibration, and some way for accurately determining the true level of the sinewave source.Richard.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 13, 2012, 08:54:38 am
content: @ All,I got the following Message From Doug " I have been noting the discussion oneevblog.comand would like to say that the AC reference voltage output is NOT the DC reference voltage "chopped" by the microcontroller - it is a true AC (bi-polar) waveform whose amplitude is 5V rms - as verified by our calibrated Hewlett Packard 3458A." .PS : Can a Pro disect It

author: Unknown
author_rank: Unknown
timestamp: June 13, 2012, 02:37:11 pm
content: Quote from: amspire on June 13, 2012, 02:20:19 amIf the error is something like 1%, than you might be better off building a less stable sine wave oscillator (eg. Wien bridge oscillator).Quote from: amspire on June 13, 2012, 02:20:19 amI'm sure that takes a lot of effort, I'm not suggesting one could easily achieve this performance level. Fluke bought Datron (through Wavetek) to get their hands on the 7.5/8.5 digit technology, that's where the 8508 came from after years of reselling the HP 3458A, so it was apparently worth something to Fluke.Quote from: amspire on June 13, 2012, 02:20:19 amAgreed. But this is hard to build for a few bucks in parts, as is probably the case for the VoltageStandard.Quote from: Amarbir[Lynx-India] on June 13, 2012, 08:54:38 amDoug is welcome to join the discussion. I would love to be proven wrong, but the only information available are the fairly terse specs on his website. OK, so chopped DC might be a little too simplified, there is also some AC coupling and amplification involved. It is still a square wave as far as I can see. The fact that it can be used to check scope probe compensation suggests it's a square wave with a fairly fast edge rate. This is confirmed by the statement that it has a variable duty cycle, a sine does not have a variable duty cycle (if it has a duty cycle at all).

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 13, 2012, 04:23:29 pm
content: QuoteWell,Might Be Someone Has This Can Can Check the Signal With The scope and post here ,Would Be Very Interesting To Know That

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 17, 2012, 04:47:07 pm
content: Hello guys ,Can anyone Check This Please And Post For Us All With a Scope ? .

author: Circuitous
author_rank: Unknown
timestamp: June 17, 2012, 08:14:27 pm
content: The DMMCheck Plus is currently sold out, looks like they will be restocked in a week.I plan to order one, and I can check it with a 34410A and a scope.I'll post details when I have the unit.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: June 19, 2012, 02:17:09 pm
content: Quote from: Circuitous on June 17, 2012, 08:14:27 pmHii,Nice ,Do post here when you order also i will also order one unit i was also thinking of taking the penta unit so that i could have some variation in dc voltages

author: ModemHead
author_rank: Frequent Contributor
timestamp: July 12, 2012, 01:01:01 am
content: I recently received one of the new DMMCheck Plus units. The reference AC voltage is indeed a 100Hz square wave that swings from -5V to +5V, with 50% duty cycle. All of my RMS-responding meters read at or near 5.00 and the average-responding ones read about 5.55, due to the 1.11 Vrms/Vavg fudge factor for sine waves. I find it curious that the included documentation did not mention this little caveat at all. Nonetheless, a very useful little device.

author: codeboy2k
author_rank: Super Contributor
timestamp: July 12, 2012, 10:38:05 pm
content: Quote from: ModemHead on July 12, 2012, 01:01:01 amI think people see "AC" and think "Sine Wave", when in fact what you have is a 5V rms bipolar AC source.It's not a sine, but it's still AC, and calibrated 5V rms, so it can be used to test a true RMS DMM.It is just that, a caveat, and the documentation would be wise to include this note about the AC source's waveform.

author: Hrvoje-CRO
author_rank: Contributor
timestamp: July 19, 2012, 06:21:45 pm
content: Quote from: Amarbir[Lynx-India] on June 13, 2012, 04:23:29 pmHello there!Well, i will post it.

author: ErikTheNorwegian
author_rank: Frequent Contributor
timestamp: July 20, 2012, 06:33:33 pm
content: Nice, an excellent source of calibrating oscilloscopes with to!

author: Magicmushroom666
author_rank: Contributor
timestamp: July 20, 2012, 09:37:34 pm
content: Surely the cheapest way to start calibrating multimeters would be to just buy a lab multimeter that you get calibrated which is an order more accurate than the calibration you provide to use as a reference? Buying a load of off the shelf sources etc is going to be a bit of a nightmare keeping them all in cal etc? with a precision meter you can setup your own sources, or automate by making a test fixture.

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2012, 09:46:58 pm
content: Quote from: Magicmushroom666 on July 20, 2012, 09:37:34 pmYou can buy a calibrated lab multimeter for $50?This thread is not about calibrating, it is about testing or verifying.