New secondary voltage standard - Page 1
Forum section: electronics / general

author: MiDi
author_rank: Frequent Contributor
timestamp: April 01, 2020, 10:24:21 am
content: Had a very interesting talk with PTB about a project for a new secondary 10V standard they are currently working on.As there is the permission to talk about it, I can give some details - even I do not really understand this sophisticated stuff in detail.This project is aimed to fill the gap between todays secondary voltage standards and JJA.The key specs are <0.14ppm/year drift, TC <0.014 ppm/K and LF noise <0.041 ppmpp (0.014-14Hz).It is based on a standard voltage reference in conjunction with a sophisticated PLL.This allows to calibrate/adjust the voltage with a frequency standard e.g. GPS disciplined oscillator - so this can be made on site without need for cal-lab.The PLL consists of a OCXO as frequency reference and a voltage controlled oscillator.The voltage is controlled by a very precision combined DAC consisting of a very precision DAC for the MSBs and an overlapping precision DAC for LSBs which is adjusted to 10V (+-0.14ppm) and maintains this value due to the PLL and the OCXO.The standard voltage reference is boosted with the DACs to 10V (DAC output is connected to voltage reference) - they act as the voltage divider made of precision resistors in classic 10V standards.Edit 2.4.: corrected all specs

author: branadic
author_rank: Super Contributor
timestamp: April 01, 2020, 10:40:30 am
content: I wonder how they made frequency to voltage conversion, as this is the tricky part, right? The best f-V-converter I'm aware of is AN14 "Ultra-Linear V→F Converter" that achieves "This V→F has a resolution of 1ppm, with linearity inside 7ppm (0.0007%)."If this is simply a 1st of april fool's joke, I call that infantile and a comment that is not in the proper section!-branadic-

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2020, 05:43:07 pm
content: It seems dubious to me. Voltage-to-frequency and frequency-to-voltage converters rely on charge metering in one form or another which ultimately depends on a voltage or current reference. This might be useful if capacitor matching, including charge injection, is better than resistor matching but I am not convinced that is the case. The exception might be precision dividers and multipliers where the capacitors can be identical and the frequency is fixed.The DAC output should be divided as much as possible consistent with the reference accuracy for the minimum trim range to prevent DAC drift from contributing to reference drift.Quote from: branadic on April 01, 2020, 10:40:30 amThe best frequency-to-voltage converters I am aware of enclose a voltage-to-frequency converter inside of a feedback loop.What I would like to know is, where are the precision delta-sigma DACs?

author: guenthert
author_rank: Frequent Contributor
timestamp: April 01, 2020, 06:00:37 pm
content: Quote from: branadic on April 01, 2020, 10:40:30 amI had a laugh neverthelessWell played.

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 01, 2020, 07:10:25 pm
content: That's no Aprils fool, or maybe one which went in the wrong direction.The PTB obviously has a Reasearch Project with WEKOMM, for an improved voltage reference.This is official, and branadic had linked this recently.Can be found on the PTB site.Frank

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2020, 07:42:36 pm
content: I took it seriously because I have seen this proposal before.

author: guenthert
author_rank: Frequent Contributor
timestamp: April 01, 2020, 10:11:35 pm
content: Works for me either way.

author: TiN
author_rank: Super Contributor
timestamp: April 01, 2020, 11:35:33 pm
content: Traceability of this setup would be worst metrologist nightmare, like we didn't had enough of that with SSRs and ageing prediction/etc.Beauty of JVS is that it is "simple" and rely on fundamental physics. It's the details and implementation that are expensive to do

author: Andreas
author_rank: Super Contributor
timestamp: April 02, 2020, 04:23:47 am
content: Quote from: MiDi on April 01, 2020, 10:24:21 amand why not a 1.4. V Standard?

author: branadic
author_rank: Super Contributor
timestamp: June 25, 2020, 11:05:50 am
content: Information in first post is somewhat limited.However, I came across this recent short paper, with PTB obviously being involved:https://www.ama-science.org/proceedings/details/3798So the mentioned approach of the PTB with GPS has nothing to do with the work PTB is doing together with Wekomm.-branadic-

author: ramon
author_rank: Regular Contributor
timestamp: June 25, 2020, 02:54:01 pm
content: After reading, does that means that we can experiment with those old transistor arrays in DIP package and some good resistors (or resistor arrays) and get better performance than a LM399A?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 25, 2020, 03:14:35 pm
content: There are already some low noise bandgap references available (e.g. LTC6655). Noise wise they can compete with buried zener types at a comparable power level - though I don't see a much better performance. The lower voltage may however be more suitable for single chip SD ADCs.The weak point is however the long time drift, that is highly dependent on the case.Separate transistor array would have a similar case problem.

author: niner_007
author_rank: Frequent Contributor
timestamp: June 26, 2020, 07:16:52 am
content: Is it LTZ1000 based? Is all the voodoo to get 10V? Why not just buffer the reference (7V out), you can't really beat that if you want ultimate stability. Wondering what the 1 year stability of a well aged LTZ1000 looks like, someone on the forum said <0.5ppm stability. Fluke can't get it under 1ppm, and they got all the fancy ovens.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 26, 2020, 12:37:53 pm
content: I'm patiently waiting for a Josephson junction array in a TO-18 case!Maybe April 1, 2021?

author: razvan784
author_rank: Regular Contributor
timestamp: June 26, 2020, 01:27:52 pm
content: The paper is a little short on details - construction, thermal matching, environment control measures...Note how, eventhough the DYI bandgap achieves much lower uncertainty than the Zener on a ~200 s time scale, it begins to increase thereafter, indicating poor stability.Maybe one can use power transistors (having huge junctions) to make a low-noise bandgap. Maybe those "perforated emitter" types that Noopy has been tearing down around here. They could be mounted on a heatsink, maybe surrounded with thermal insulation and then placed in an aluminium box - that should help with temperature equalization.

author: niner_007
author_rank: Frequent Contributor
timestamp: June 26, 2020, 09:53:41 pm
content: One other thing, mentioned by other, the key spec of 0.14ppm/year, if it is stability or uncertainty, it doesn't add up, given the DAC and clocks, the error budget seems optimistic.

author: bdunham7
author_rank: Super Contributor
timestamp: June 27, 2020, 04:40:08 am
content: Quote from: niner_007 on June 26, 2020, 07:16:52 amI'm not sure that's actually true. You're sort of making an apples-to-oranges comparison.

author: niner_007
author_rank: Frequent Contributor
timestamp: June 27, 2020, 10:02:39 pm
content: You’re right, they are not using LTZ1000, but rationally, surely they’d deliver better than 1ppm if they could, they are not in a business shy of charging customers $

author: bdunham7
author_rank: Super Contributor
timestamp: June 27, 2020, 10:16:07 pm
content: What I (also) meant was that comparing the results of a hobbyist who, perhaps accurately, determines the uncertainty or drift of his LTZ1000 reference to be 0.5ppm to Fluke conservatively specifying an uncertainty with guardbands and a 99% confidence interval is not an even comparison. If you assume 20% guardbands and 3-sigma confidence (OK, that's 99.7%, but close enough) and a random-ish distribution, then half of the 732Cs will actually be within 0.2ppm. Perhaps my math or something else is off, but I'm not seeing a 0.14ppm standard as such a huge advance unless it is economical or has some other advantage.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: June 30, 2020, 07:46:06 am
content: My Fluke 732B/AF is already at -0.133ppm/a...

author: notfaded1
author_rank: Frequent Contributor
timestamp: June 30, 2020, 03:21:21 pm
content: Quote from: Magnificent Bastard on June 30, 2020, 07:46:06 amI wish I had some B's... only A right now and homemade so far. Some new competition for the LTZ would be neat but time has shown it's not so easy minus Fluke's custom buried zener. It seemed to me at first that there must be new technology that can rival it but people here and Dr. Frank have convinced me it's a lot harder than it seems like it should be. With nano technology, quantum, and you'd think right? Well at least there's some new tech on the timenut horizon vs. voltnut horizon. Optical with microwave and ytterbium and strontium instead of rubidium and cesium:https://spectrum.ieee.org/tech-talk/semiconductors/optoelectronics/optical-atomic-clock-advantage-expands-electronicsSo we may not have a new voltage standard but we're defining the second a lot better these days.

author: branadic
author_rank: Super Contributor
timestamp: June 30, 2020, 06:03:45 pm
content: Be patient, people are working on it, but it will still take some time to come up with the next generation of voltage references.-branadic-

author: The Soulman
author_rank: Super Contributor
timestamp: June 30, 2020, 06:17:08 pm
content: It kinda reminds me of:https://www.eevblog.com/forum/metrology/building-your-own-voltage-reference-the-jvr/

author: dietert1
author_rank: Super Contributor
timestamp: June 30, 2020, 07:38:35 pm
content: Quote from: Magnificent Bastard on June 30, 2020, 07:46:06 amCould you write a sentence or two to explain what you compare to and how you do the comparison? Can you show some kind of log. maybe of several years?I am currently running a comparison of two well built references and although i see a standard deviation of only 0,029 ppm for the daily comparisons, even after 40 days the statistical uncertainty at 99 % c.l. of the drift is about +/- 0.4 ppm/a, let alone systematic uncertainties.Regards, Dieter

author: guenthert
author_rank: Frequent Contributor
timestamp: June 30, 2020, 07:43:16 pm
content: Quote from: The Soulman on June 30, 2020, 06:17:08 pmHow so? The JFET based voltage reference goes back to an observation originally published by L E MacHattie in 1972 (May 26th, not April 1st), has been cited in patents (US4068134, US4158804, US3975648A), exhaustively discussed and can readily be reproduced.

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 30, 2020, 07:51:29 pm
content: Quote from: Magnificent Bastard on June 30, 2020, 07:46:06 amOn my last visit at the PTB I had the chance to compare my LTZ1000 against a 732A which was used there. The 732A was periodically compared to a JJA (I saw the calibration certificate) and they said that the 732A isn't moving by more than ~0.1ppm/year

author: retroware
author_rank: Contributor
timestamp: June 30, 2020, 10:37:47 pm
content: There is a patent from someone at Advantest documenting this technique.https://patents.google.com/patent/US6091281A/en

author: guenthert
author_rank: Frequent Contributor
timestamp: June 30, 2020, 11:34:23 pm
content: The patent is interesting, I would like to see an implementation though.In the patent it is written that "Therefore, in the present invention, because of the fixed relationship between the control voltage supplied to the TCXO and the oscillation frequency of the TCXO, the control voltage shows a high precision DC voltage." This hinges on the relationship between the control voltage and the oscillation frequency of the TCXO being fixed, doesn't it?And if the TCXO is controlled via a variable capacitance diode, then how 'fixed' is the relationship between control voltage and capacitance really? I'd think the geometry of the diode changes with temperature and with that the capacitance. In a GPDSO that doesn't matter, as the error is calculated and corrected periodically using the GPS time, but how is this supposed to be handled here?

author: guenthert
author_rank: Frequent Contributor
timestamp: June 30, 2020, 11:51:56 pm
content: Just noticed, that his has been discussed here before:https://www.eevblog.com/forum/metrology/another-method-to-get-a-precision-voltage-reference-does-it-make-sense/

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2020, 01:47:07 am
content: Temperature compensated varactor diodes can be very stable. Do a search for Bob Pease's comments about varactor diode based operational amplifiers.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 01, 2020, 03:35:59 am
content: Hi,I have looked at the patent. The circuit described in the patent is essential a GPS disciplined (VC)TCXO. The reference voltage output is the tuning voltage of the VCTXO.OCXOs are generally considered to be an upgrade from TCXOs.So if were to monitor the tuning voltage in my Z3801A (for example) I should have a stable reference voltage?If this correct, then the converse should be true, if I feed the OCXO tuning voltage with a fixed voltage I should have a very stable frequency, no need for the GPS disciplining.But if I put the Z3801A in the holdover mode, by disconnecting the antenna the frequency will drift.As a voltage reference it seems very complicated to me. There are lots of opportunities for drift and aging in this circuit:Image is from the patenthttps://patents.google.com/patent/US6091281A/enDoes anybody know if Advantest ever used this reference in a product?Regards,Jay_Diddy_B

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2020, 04:27:31 am
content: Quote from: Jay_Diddy_B on July 01, 2020, 03:35:59 amI had the same feeling, although I don't want to dismiss out-of-hand an idea that I may not fully understand. Just because something is patented doesn't mean it is a good idea. Perhaps someones pointy-haired boss wanted to market a frequency-related voltage standard thinking people would associate it with the Josephson effect. The problem is that there isn't any fundamental principle of physics being exploited here and I don't see anything that would give this system any more stability than current methods. I guess we'll see.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2020, 05:45:33 am
content: I see no real advantage use a crystal and GPS clock as a reference for the varactor capacity. One would probably better just use a brigde with varactor and capacitor (possibly twice to make it a full bridge). The idea has the positive effect that the reference part itself (cap + varactor) would not produce significant heat and could thus be easily temperature stabilized. However there is parasitic capacitance and this tends to be effected by humidity. So if at all it would need to be in a small sealed container. My first estimates do no look that optimistic, as there is quite some TC to the varactors (some 2 mV/K).

author: guenthert
author_rank: Frequent Contributor
timestamp: July 01, 2020, 03:47:14 pm
content: Quote from: Jay_Diddy_B on July 01, 2020, 03:35:59 amI believe in principle that to be true, however (as discussed in the older thread) the relationship between the tuning voltage and the frequency is not in favor for a voltage reference: the tuning voltage swings a few volts for a few ppm of frequency.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 01, 2020, 06:18:14 pm
content: Quote from: David Hess on July 01, 2020, 01:47:07 amThose are interesting little buggers with input bias current of less than 10pA and CM voltage of up to 200V.Inhttps://books.google.com/books?id=Kcv8BAAAQBAJ&pg=PA72&lpg=PA67&dq=Philbrick+P-2&source=bl&ots=spTAKv58_J&sig=UkIDXapzfihaGuwCDrMTuKLhUhk&hl=en&sa=X&ved=0ahUKEwi1-8LQ-YXOAhXCFR4KHXY1CEwQ6AEIKTAC#v=onepage&q=Philbrick%20P-2&f=falsehowever I found an estimation of the capacity to voltage dependency of varactor diodes (delta of 0.03pF for a delta of 1mV) and some anecdotes of stability of the P2A over time which doesn't look too promising (better than 1uVppover an hour or two).

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: July 02, 2020, 04:12:28 am
content: Mine was a special "cherry picked" unit that was sold to the US Air Force (for one of their PMEL labs). It has lower than normal noise and lower than normal annual drift compared to the average 732B. My unit gets calibrated by Fluke-- and only a JJA has enough stability to get the uncertainty down to these low levels. It (unfortunately) was made with one of the early LTFLU's (the non-A version), and so is more sensitive to barometric pressure changes. This is calculated into the calibration and compensated for at my local altitude. I was only able to get the one unit, and it is unlikely that I will ever find another one with these ultra-low drift specs. My interest is in building a number of LTZ based references to build out a sustainable array of references that can be statistically monitored. It's been a long road, but I'm getting closer.

author: dietert1
author_rank: Super Contributor
timestamp: July 02, 2020, 08:24:05 am
content: Quote from: Magnificent Bastard on July 02, 2020, 04:12:28 amThe references i am observing are LTFLU 'A's and their voltage difference exhibits a barometric pressure correlation of about -0.003 ppm/hPa. Do you have some numbers for LTFLU non-A and LTFLU A?So barometric pressure variations may contribute +/- 0.03 ppm typical - a significant fraction of the observed residual (stdev = 0.029 ppm). I am just installing some BMP388 sensors for continuous pressure monitoring, trying to convince myself that a standard deviation of the daily average below 0.01 ppm is realistic. As far as i understand that would be required to predict yearly drift to within 0.1 ppm.Regards, Dieter

author: SilverSolder
author_rank: Super Contributor
timestamp: July 02, 2020, 12:30:20 pm
content: Quote from: dietert1 on July 02, 2020, 08:24:05 amThat's impressive resolution - is that something that can be reliably achieved outside an underground shielded bunker?

author: dietert1
author_rank: Super Contributor
timestamp: July 02, 2020, 01:47:02 pm
content: Until now that isn't precision but resolution. It's a small setup in our lab with the two references and a HP 3456A to measure the voltage difference, with low thermal EMF wiring. Then +/- 0.03 ppm of 10 V are +/- 300 nV. The difference voltage is in fact about 190 uV and in it's lowest range the HP 3456A has a resolution of 100 nV and that is also its observed offset voltage stability. If i just leave it running, apparently it is that stable. That HP 3456A came to us through ebay about 15 years ago. The in-guard power supply was recapped and got some mods.Later i will play with the wiring and observe changes caused by EMI and tune things to make those as small as possible, but that's another story: to make the voltage reference something useful. Build the references into a case with binding posts, with battery operation etc. Currently it is just a little research by a physicist.Regards, Dieter

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: July 12, 2020, 02:32:01 am
content: Quote from: Kleinstein on July 01, 2020, 05:45:33 amWould it not be better to derive the voltage reference from the OCXO's Vref pin rather than the variable VFC pin voltage?I wanted to improve on the 1mV resolution of my cheap Mestek "9999 counts" DMM when monitoring the VFC (currently 2,281 mV +/-500uV) in my mark 2 homebrewed basic GPSDO (hardware PLL with a 5000s TC) and hit upon the bright idea of using the OCXO's 5.127v Vref to drive a 1mA current through a 220 ohm 'ballast resistor' and a set of 5 hand matched 1K resistors to realise a series of DC offset voltages in 1 volt increments from zero to 4 volt in order to subtract in this case, the excess 2 volts off the meter reading allowing me to use the 999.9mV range, to show VFC changes in 100uV increments.I used a 50k trimpot with a 22k resistor in series wired across the 5 k resistor string to trim the volt drop across each 1k resistor as close as was possible with this cheap meter to exactly 1.000 volt each. Exactitude would be nice but in this case, temperature stability was the most vital parameter for my purpose.I'd already experimented with the mark 1 GPSDO using a 3.16v CR2032 cell (a surprisingly stable voltage reference for my much more modest voltage stability requirement) before using a TL431 from my wallwart salvaged parts collection to create a 3.000 volt reference (the OCXO in this case needed circa 3.311v to tune it dead on 13MHz - don't ask!) but would drift by just over a mV for modest 2 to 3 deg changes in room temperature.Short term, this was not really much of a problem in observing the minute to minute changes in the ionosphere's electron density on the phase of the 100KHz pulses out of my fake NEO M8N gps receiver modules that I was phase locking the OCXO's to but I wanted a better voltage reference for my mark 2 GPSDO attempt and making use of a temperature controlled voltage reference built into most OCXOs (including the seven 12v 10MHz CQE units I'd managed to get hold of dirt cheap), just seemed the most obvious solution to saving a few hundred quid on an overpriced 4 1/2 digit bench voltmeter just to be able observe the VFC change in 100uV increments.Previously only being able to observe in 1mV increments meant I could see the effect on the DSO display before I could see any change in VFC voltage whereas now, it's the other way round.Why anyone would think the VFC voltage would be a better choice than the thermally stabilised Vref voltage of your typical OCXO, I just can't imagine, especially when you consider that there's no need to complicate the arrangement with the addition of a GPS timing receiver module - just use the OCXO, almost any OCXO (even a broken one provided the Vref was still intact), on its own in a suitable enclosure.Anyhow, that's my two dollar's worth on a subject I'm only just starting to get to grips with.JBG

author: Kleinstein
author_rank: Super Contributor
timestamp: July 12, 2020, 06:40:30 pm
content: The Vref. pin on an VCXO is usually just an internal classical voltage reference (e.g. bandgap or zener). The main purpose is likely to have a stable for the control and it may be needed internally for the temperature regulation anyway.I don't think it would make much sense to used the VCF pin as a reference, but the general idea behind it may work: The VCF pin usually control a varactor diode, and the voltage versus capacitance curve of a varactor would be used as a reference. Just don't measure / regulate the capacitance with an oscillator, but directly with a bridge. The advantage over the classical zener would be that the varactor bridge has little intrinsic noise and also low heat production. The problem are parasitic capacitors and changes of it. So it would kind of requite a compact, possibly monolytic design to really become very stable.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: July 13, 2020, 01:48:09 pm
content: @KleinsteinThank you for the clarification on the thinking that had prompted the suggestion of using the VFC as a voltage reference. Obviously you can't use the VFC for this simply on account of the ageing effect of the crystal itself.My thinking, as far as solving my own little problem of using a reasonably temperature stable DC offset to utilise a cheap "9999 Counts" DMM to monitor the VFC was that the Vref, even if derived from a cheap commodity 1117-5 ldo used to power the XO and temperature controller would be a far better option than the TL431 I'd tried in my mark 1 gpsdo simply because it was being temperature stabilised and providing a constant (in this case 14mA) current to the XO and thermal regulator circuitry plus one additional fixed 1mA external load (my series string of five 1K matched resistors).I actually ran a voltage comparison test between two of my CQE 12v 10MHz OCXOs just to verify how well these temperature stabilised sources were likely to be. I did see some slight variation of around 100uV between them (5.128 versus 5.172 volts - a 44mV difference) over a two or three day test run. Since this was a significant improvement over the TL431 I'd used in the mark 1, I picked the one out of the two I'd been using for this voltage stability test which seemed to be making the least contribution to the 100uV variation for use in my mark 2 gpsdo.Although it's unlikely I'll be needing anything other than a 2v offset using my chosen OCXO, I designed it as a multi tapped potentiometer of five 1K resistors partly to cater for different OCXO voltage offsets but mainly because I'd still be able to use my cheap "9999 counts" Mestek DMM to trim the volt drops across each individual 1K resistor to within 100uV.I know this level of measurement accuracy falls far short of that normally dealt with by members in this topic thread but it meets my immediate requirements (for now) and might help other cash strapped hobbyists work out a cheaper alternative method to get that extra magnitude of accuracy they crave before being forced to dig ever deeper into their pockets for better test gear.JBG

author: branadic
author_rank: Super Contributor
timestamp: December 26, 2020, 10:09:03 am
content: Thinking about it, the only approach that I know of and that could fit the description by MiDi in the first post is based on NV centers.Robust and accurate electric field sensing with solid state spin ensemblesA solid-state quantum voltage referenceI know that there is a group at PTB working on NV centres as many people do.-branadic-