Short History of DC Reference Standards & Calibrators

This is not an exhaustive history by any means, just what I know of or could find on the web. Note that links (or comments) in this document are not endorsements, just there to provide a picture or other information. Standard cells were the choice for many years. See my previous thread here. DC references are used in many electronic instruments like calibrators and digital voltmeters. They are also sold as separate equipment for calibration purposes.

I remember a vacuum tube DC calibrator made by Cohu (can't remember the model #). It had a mechanical vibrator used as a modulator and demodulator (chopper). The idea was to chop up a DC voltage to make it AC. Then you could amplify the signal with ac coupled stages. This eliminated the problem of a change in bias in an early stage causing an output stage to be shifted to the rails. Then the AC signal was demodulated back to DC. This was how high gain DC amplification was done in the early 1960s.

I can only assume that regulator tubes were used as references in early equipment, but have no specific knowledge of it. They were certainly used in regulated power supplies in electronic instruments. One example is the Tektronix model 130 L-C meter. These tubes were obsoleted by Zener diodes in later years.

Hewlett-Packard company came out with the 735A DC Transfer Standard in 1967. It was a stand alone reference standard with a zener in a temperature controlled oven. It was a very small box (about 3.5" high, 5" wide, and 11" deep). It had a 1V output, a 1.018V + delta, a 1.019V + delta, and a 0 to 1000uV output. The 1.018 and 1.019V outputs were there to use to compare with Weston standard cells. It's drift spec of 10ppm/month after 30 minutes warmup must have been considered good at the time.

The HP 740B DC Standard / Differential Voltmeter was also introduced in 1967 and had the same reference oven. The 740B had two neon lamp-photocell choppers, one for the DC standard, and one for the differential voltmeter. The neon lamps were lit alternately to effect the chopping action. The photocells for the modulator were on one side of metal block, and the demodulator ones were on the other side, with the neon bulbs in the middle. These choppers were somewhat expensive, and the aging of the photocells caused the units to become sluggish from lower gain after a few years. They lost favor with the advent of op-amps.

Many would think these belong in a museum. A few years ago I purchased a box that contained the guts of a 735A among other things. I put them in a smaller box with 1V output terminals. Later I modified the oven with a divider to have a 5V output (the Zener is about 7V). The 5V output has 1/5^th the error from thermal offset voltages. I had assumed that the unit wouldn't drift if left powered off. After about 1-2 years I powered it up and compared it to a currently calibrated standard. It was 6ppm low. I left it powered on for 1 week, and took measurements each day. It recovered a little each day, and at the end of the week it measured within 0.2ppm. HP got out of this business, so there aren't any new models.

Another early unit was the Fluke 731A. It had the same outputs as the HP 735A, and a 10V output. I wasn't able to find its introduction year or a drift spec. The Fluke 3330B DCV and DCI calibrator was near the same vintage and was introduced approximately 1971. Some Zener references have been known to change voltage when cooled off and warmed up again, up to 2ppm in extreme cases.

Later Fluke introduced the 732A DC Ref. Std. It is a much larger unit than the 731A, with batteries to keep it powered up for shipping. It has outputs of 10V, 1.018V and 1V. Its drift spec is 3ppm/year for the 10V output. Many of these are still in use. I know of some that were modified to bring the 7V zener voltage to the front in place of the 1.018V output. The Datron 4910 was about the same size as the 732A, but had four 10V outputs, and batteries too. The stability was 1ppm/year. With 4 outputs in an average, the noise is lower, so this unit is good for doing short term stability graphs on DMMs. I think you can see the effect of competition in the features and specs of these units.

The Datron 4000 and 4000A calibrators were introduced approximately 30 years ago. They included current and resistance. Many are still in use today. Fluke bought out the Datron products. Their best DC ref. std. and calibrator are the 732B and the 5720A respectively. The calibration of DC reference standards with a quantum standard may be my next thread.