[time-nuts] Voltage standards

WarrenS warrensjmail-one at yahoo.com
Sat Nov 29 04:51:12 UTC 2008


Just to add my experiences and throw in another two cents worth.
I live in California,  and a few years back I did some experiments 
with Geller on the East coast shipping back and forth some of his 10V 
References in the regular mail that I had added a simple 1 transistor 
1st order Temperature compensation circuit to some of his selected aged 
units and proved to my satisfaction that better than 2PPM transfer 
accuracy was obtained across the country. In house I can consistently  
get better than 1 PPM transfer with the same units. Of course there 
are more accurate ways, but for us cheap guys It sure beats the $500 
I heard Fluke charges to do their underlying Fluke 732B calibration.

WarrenS
*********************

RE  Message 8 and a couple of earlier posts on standards

> Date: Sat, 29 Nov 2008 10:38:53 +1300
> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> Subject: Re: [time-nuts] any way to bootstrap a frequency standard
>  into  a, voltage or resistance standard?
> To: Discussion of precise time and frequency measurement
>  <time-nuts at febo.com>
> Message-ID: <493064ED.1030501 at xtra.co.nz>
> Content-Type: text/plain; charset=ISO-8859-1
>
> WB6BNQ wrote:
>> Ed,
>>
>> No way in hell does that thing qualify as a voltage standard.  It  
>> barely qualifies as a VERY short term transfer device assuming the  
>> temperature does not change.
>>
>> If you want a real voltage reference then buy, from eBay, a Fluke  
>> 731B voltage standard.  You will still need to get it calibrated,  
>> but then it will hold under 10ppm for well over a year or
>> more.  Actually, if they are adjusted correctly, you can get less  
>> than 5ppm and it will hold it at a given temperature.  These items  
>> are serious devices and their latest versions (very expensive)
>> are better than a properly maintained and operated group of  
>> standard cells.
>>
> The latest Fluke voltage standards use a selected LTZ1000 plus a
> precision resistor array.
> The earlier versions used selected Motorola zener diode references.
> The latest versions allow the LTZ1000 chip temperature to be cycled to
> largely eliminate hysteresis which may occur when they lose power  
> during
> shipment.
>> Bill....WB6BNQ
>
> Bruce

Since we've kinda strayed from the original topic, I'll throw in a  
couple of cents. The V references in these 6-1/2 and 7-1/2 digit DMMs  
(that I currently own at least one each of):  Keithley 196, HP 3456,  
3457, and 34401, use selected -- some more than others -- National  
LM199/299/399 series references, which have their own heaters. The  
Datron 1080 series (that I own two of), as well as the 1070 series  
and 1060 series, use selected pairs of zeners connected in parallel,  
but not housed in ovens. The Fluke 8502/05/06 (and my Fluke 510A ACV  
std) use the aforementioned Motorola transistor+buried zener  
reference, also not in ovens.

The HP 3458, as mentioned before, uses the Linear LTZ1000/1000A,  
which has its own heater. As to others, the old Fluke 895A, 887A, and  
885A 6-digit analog Differential Meters (with K-V bridges for  
comparing the reference to the unknown in various ways), all use a  
pair of zeners in series, in an oven. A Fluke engineer told me years  
ago that those zeners were selected to have slightly different TC  
peaks so when one was falling in V the other was rising, so that they  
would spread the peak over a larger temperature range. They had a  
very elaborate automated system for testing the temp, voltage, and  
current values to do the pairing.

My Fluke 732A DC reference standard, which uses the Motorola  
transistor+buried zener reference, housed in an oven together with  
all of the various fixed and variable resistors and the other active  
devices, has an extremely low drift rate -- unmeasurable over a month  
in any definitive way -- as measured with a rented HP 3458A. I have  
disabled the batteries because I discovered, thanks to the 3458A,  
that when turned off, cooled off, and turned back on, within a day,  
the output returned within 0.05ppm to it's cal'd value -- I know  
there can be hysteresis, I just couldn't see it.

I have one of the little Geller 10V refs built around the AD587  
reference chips, and if you keep it enclosed and away from air  
currents, it's pretty stable, but would need to be in an oven to  
approach the performance of the LM199 series.

All this is to say that if you can get a working HP 3458A or a Datron  
1280 series (which is also the Fluke 8805), or a Datron 1270 series  
meter, all of which are self-calibrating in various ingenious ways,  
it will serve you well as a secondary standard for measurement on  
everything that  it can measure. I can't, or more properly, won't at  
the moment, spend the current freight of around $4,000 USD for a  
known good working 3458 or 1281.

What to do? Seems like a possible stable, yet low-cost source would  
be to buy 10 or so LM399s and hook them up to a power supply to age  
for 6 to 12 months, then use two or four in parallel (with suitable  
resistor buffering) driving a chopper amp like the LT1050 to give  
gain. Powered by a good, stiff regulated supply, and kept away from  
stray air currents, this arrangement wouldn't cost as much as a  
single LTZ1000A (which in singles is over $100 USD) and -- dang it,  
there's always a catch -- after lab calibration, could be a source  
that might need adjustment only once every few years to stay within 5  
or 10ppm or so. Paralleling more LM399s would be better, and with an  
initial purchase of 10, you could throw away the stinkers and  
parallel all the rest. So, short of having a Fluke 732A or those  
8-1/2 digit meters, that's my recommendation for a low-cost V  
standard. Unless you know someone, it'll cost less to build than to  
calibrate...

Dick Moore




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