1) frequency
2) voltage
3) resistance
4) current standards?
What devices do you guys use, and for what instruments? And where did you get them?
10MHz is a standard frequency I've seen used in a lot of places so I'm looking for a 10MHz OCXO on eBay...
But what are the magic numbers for the other values? 1V? 10V? 1mA? 1A?
1Kohm?
What are the things to look out for? I think an OCXO will still be usable after decades but will a decades-old resistance standard still be good?
I've got an AD588 vref chip in my junk box but I think modern parts are probably more accurate and I don't feel like building a circuit. Digikey carries accurate parts but eg. a VRE102C is 134$.
(a) How tight do you need to be? Depending on your hobby, you could use anything from the wall socket (for a '110VAC' standard) to a standard cell with yearly calibration tags.
(b) Invest in a good digital meter -- Fluke is good, so is Fluke. And Fluke is not too bad. Then use it for your "standard", and have it calibrated if you're concerned.
I have a large collection of 0.01% resistors, and a Fluke differential voltmeter that has a decent standard and a Kelvin-Varley divider in it, but really, getting a decent DMM that' scalibrated covers most of it easily and cheaply.
There was a guy on eBay who'd sell you a board with one of the Analog Devices references on it, calibrated to his voltage standard, traceable, for $20 or $30 IIRC. Dunno if he's still selling.
As for frequency, beating with WWV is easy and good to maybe 1/2ppm or better if you're really diligent, and GPS receivers with pulse-per- second outputs can get you a couple orders of magnitude better. Personally, I've used a 40-year old OCXO. Aging slows down with time, so frankly Scarlet, it was da^H^Hvery good. All depends on what you want.
For all but frequency, buy a Fluke DVM that's as accurate as you need. It will measure all those things. Every couple of years, buy a new one. It's handy to have a couple of DVMs around.
A wirewound resistor should be very stable for many years. Measure a bunch when your Fluke is new, and keep them around for verification now and then.
GPS is referenced to a primary frequency standard.
Unless you're actually building test equipment, your 'standard' will be selected at the time to serve the specific purpose, likely from the commercial application environment already in existence. Part of this decision will involve availability and sensible budgeting.
A good multimeter, as an indirect reference; a simple oscilloscope, as an indicator; and simple power supply and signal generator, as sources will usually suffice.
I got a couple of 5-1/2-digit Keithley 191s on eBay for really cheap--about $100 for the pair. They agree with each other and with my HP 6112A precision power supply ($75) to better than 50 ppm with no special care.
Ancient precision gizmos are often cheaper than modern, less precise ones.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Yea, I'm anticipating that at some point they'll decide that they're so good they can make crap and it'll be good 'cause it says 'Fluke' on it. Or they'll get bought by a group of investors who feel that way.
Then Keithly will start making hand-held meters, or some Japanese or German company will start marketing in the US. Something.
You are almost as cynical as I am. It's almost guaranteed to happen.
I purchased some DMM916's from Fry's in Wilsonville, some years back, at just under $55 each, new. I see one at ebay offered at about triple that much and one selling (with shipping) for more than what I paid new and the bidding isn't over, by any measure. I'm sad these are no longer being made and that Fluke killed them.
For hobby-grade stuff, I just compare my two DVMs against each other, and the 'scope. The time I needed a 10MHz signal, I just slapped together a pierce oscillator. like
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, but with fewer parts.
For extra credit, [ ;-) ], I made the RFC (L1 in this diagram) out of a chunk of toothpick with a few hundred turns of magnet wire. For leads, I used my pin vise and drilled a ~.040 hole in each end, and glued in a couple of cutoff resistor leads. I save them, because they make dandy little jumpers for the proto-board. :-)
About the AD588, if it hasn't been cooked, it sholuld still be as accurate as the data sheet says.
If you want NIST accuracy, find a local cal. lab and have them do it - you get one of those little stickers, and an official certificate of calibration. It's not very expensive, maybe even less than that $134 part.
Sorry? I'm talking about three gizmos on my bench, not on the Web.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
If your needs aren't too stringent, you can find much of a good standards bench on Ebay. You didn't state what your objectives for your standards bench are, so it's difficult to make recommendations. What levels of accuracy and stability are you looking for? How much money are you willing to spend for the equipment? Are you looking for NIST traceability/certification? Are you capable of repairing equipment should it break (or is defective when you buy it)?
Here are a few suggestions for some surplus equipment that will make a pretty decent standards bench. For a good single voltage reference, visit the web page at
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Don't know if any of these boards are still available, but they should certainly be adequate for a home lab reference. They are surplus 7-volt HP3458A voltage reference boards. John Daly, the gent who is selling the boards, states that they were either production rejects or surplus spares. They are, however, checked out by an overnight test using an Advantest R6581D Digital Multimeter. Be sure to look for a good voltage divider and/or reference divider to complete your DC voltage bench. Unless you're looking for "something in the ballpark" for a frequency standard, your best bet is a GPS frequency standard. The Thunderbolt GPS
10mhz standard is available on Ebay for less than $150. An OXCO has to be calibrated periodically. A GPS standard is directly derived from a set of primary standards. As long as the GPS satellites are flying, then your
10MHz frequency standard is good, with no calibration required. General Radio resistance standards are also available on the 'bay, sometimes really cheap. Unless they have been seriously mistreated, they are still quite stable and usable as a lab standard. The GR resistance decade boxes are good standards for home use, and they are variable; not fixed values. They are available in values from 0.01 ohm to 1 Meg, with 0.02% accuracy and
10 to 50 PPM stabilities. You might have to get more than one decade box to give you the range of values that you need. A Fluke 382A or equivalent makes a good DC current standard. I have one of these on my bench; it's been operating quite well for several years with no problems. You can find them on Ebay occasionally.
Hamfests are good places to look for decent equipment, many times at much lower prices than you'll pay on Ebay.
Sensible advice. The world is only good to three significant figures, except when it isn't. NIST- or NRC-traceable, professionally calibrated equipment has its place but does it *really* matter that a signal line is 4.852 and not 4.85? Well, sometimes. Even if there's 10's (or 100's) of mV of noise on it?
Keithly's low-end bench DVM is the rebranded ARRAY M3500 (Chinese) thing, which is pretty bad. They probably pay about $500 for it. Everybody sells this one:
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The Fluke 8845A is excellent, and made in the USA.
I used an AD588 to calibrate my Fluke 8060. They're laser trimmed in the factory. You used to could use analog TV stations as a frequency reference. The color subcarrier was extremely tightly controlled, at least during network shows. Alas, they're all gone now. Of course WWVB and WWV are still available.
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