Monitoring SetupIf you want to find out how well your LTZ reference performs, you have to compare it to either one much more stable standard, or to several equally good ones.
This much better standard would be a JJ primary standard only.
Equally good ones would be supervised Zener standard banks as Fluke 734B, 7010 or Datron 4901, or an ensemble of at least 3 DYI LTZ1000 references, or similar.
Also, a precise transfer standard is needed, to be able to compare the raw 7.xxx volt precisely against the 10V references.
I use 2 different kind of references, the first one is the internal LTZ1000A of a 3458A, running on 65°C (instead of 95°C), and a Fluke 5442A which is based on the SZA263.
The HP3458A also serves as a very precise transfer standard, due to its linearity.
Both devices are well aged, 13 years for the HP3458A, 23 years for the Fluke 5442A, and both are running intermittently only. During power down times, their drift should be close to zero. Their combined drift over 3 ½ years is obviously 1ppm maximum, see figure 1.
Both of the DIY LTZ1000 references, including the 7,147V => 7,0000V => 10,0000V transfer derived from Ref_2, have been running continuously for 3 ½ years, and were compared also against the HP3458A. Therefore, my reference ensemble consists of 4 equivalent sources.
Long Term stability measurementsDuring the first 2 years, I have checked only, that all references were stable to within 1ppm of their initial values. Therefore, intermediate data points were missing.
In June 2011, I have moved my complete analogue equipment to the basement, where a stable room temperature of 20.0 … 22.5 °C is available during all seasons.
The room temperature may be constant to +/- 0.2K during several hours and to +/- 1.0K during successive measurement days.
This measure improved the short term stability of all measurements significantly.
In other words, without a stable environment, sub-ppm stability measurements are not possible at all.
Ref_2 and its 10V output drifted about 1ppm in 3 ½ years. (See figure 2)
The 7,000V output has been calibrated initially only, but the 10/7 transfer was calibrated several times, about 2 times a year.
On Ref_1 I performed some experiments, so it was “mistreated” several times, i.e. the temperature control got out of regulation. Thereby it encountered several temperature trips to 100°C (estimated).
Afterwards, its output voltage then restarted at a lower value, and drifted much more than Ref_2, (see figure 3). As the output voltage of the LTZ1000 is lower at 100°C, the reference @ 45°C obviously memorized its short trip to 100°C, and drifted towards that direction.
ConditioningThe last such accident happened in June 2013, which left a hefty additional shift of -3ppm.
I remembered the patent of Pickering, to remove temperature induced hysteresis effects in the LTZ1000.
So I temperature-cycled the complete box to remove the hysteresis (see figure 4).
To do that cycling right, it is important to have big enough, but decreasing temperature differences, related to the stabilization temperature, in this case compared to +45°C.
This means, +100°C gives +55K, storage at -23°C gives a dT of -68K, +80°C equals dT = +35K and so forth.
I found out (see fig. 4 also), that below a difference of about +/- 20K, there is no big hysteresis effect.
So I really doubt, that the Fluke 7001 really did operate efficiently in removing hysteresis, because at a stabilization temperature of +45°C, the lowest available negative temperature difference at around 22°C room temperature (i.e. -23K) is too small to deliver a sufficient 'reset' effect. (The 7001 has been terminated by Fluke in the meantime.)
During that procedure, Ref_2 also showed a hysteresis effect, but in the end returned to its initial value of 3½ years before. This indicates that the cycling really removed the hysteresis in both references.
Ref_1 ended at about +1ppm, and its future drift behavior will show, if it’s now more stable. If it resides at about +1ppm, possibly the LTZ1000 had been heated too much during soldering, although I used a thermal transfer pincer and soldered very quickly.
Further effectsIt is important to have a proper shielding (e.g. case connected to ground) and a quiet power supply.
An external wall plug power supply was used, to avoid magnetic disturbance from the transformer. In first instance, its output of 18V AC induced disturbances on the LTZ temperature regulation.
The case had to be connected to ground of the LTZ output, and that induced a shift of 0.5ppm of each output. Exactly this same effect could also be demonstrated on another LTZ based reference, designed by 'babysitter'.
When I redesigned the wall plug in for an output of 25V DC, the LTZ references became more short term stable, and the 0.5ppm shift vanished.
Short Term stability measurementsFigures 5 and 6 show short term stabilities of Ref_1 versus HP3458A during 10 minutes and 35h, using an aperture time of 2 seconds, i.e. NPLC100.
The internal temperature of the HP3458A increased continuously from 33.7 to 34.8°C during those 35h. ACAL was performed only once, before the start of the measurement.
The 10min measurement shows fluctuations of +/- 0.05pm, which is very well consistent with the transfer specification of the HP3458A, +/- 0.1ppm, and especially with the noise specification of the LTZ1000, i.e. 2µVpp (equivalent to +/- 0.15ppm).
The average 35h drift of ca. 0.3ppm is well below the HP3458A 24h specification, i.e. +/- 0.55ppm, or its T.C. of 0.5ppm/K without ACAL.
Conclusion:The basic LTZ1000 circuitry, output of around 7,2V, running on 45°C and built with wire wound resistors is capable of drifts well below 1ppm/year. Noise and short term stability are below +/- 0.1ppm.
Sophisticated shielding and a low noise DC PSU are very important for obtaining that degree of stability.
The influence of additional ‘gimmicks’ as summarized in part #1, have not been demonstrated yet, but they have to compete in value with the effects demonstrated here.
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