DIY IVD RBC project - Page 1
Forum section: electronics / general

author: Sengcid
author_rank: Regular Contributor
timestamp: April 25, 2023, 06:48:13 pm
content: Herewith a proposal for the construction of a two-stage inductive voltage divider for calibrating ASL F type resistance bridges plus some practical tips on construction.It could also be used for checking the linearity of DCV meters to better than 1ppm with a sufficiently stable AC source and synchronous rectifier. Any interest?

author: bobAk
author_rank: Contributor
timestamp: April 25, 2023, 07:10:17 pm
content: For a level of 1 ppm and below, inductive voltage divider, it is required to introduce compensation for the magnetizing current, shielding the magnetizing winding, introducing a feedback winding to correct the non-linearity of the core, the core must have low magnetostriction, the "secondary windings" must have a low leakage inductance between themselves and shielding from neighboring "secondary windings" . To correct the linearity, you can introduce additional windings or even additional transformers to reduce the transformer ratio, or you can also introduce a correction DAC.Do not forget that an inductive divider has capacitive dividers connected in parallel and in series, in the form of parasitic inter-turn and inter-winding capacitances. Therefore, electrostatic shields are required. Especially between the magnetizing winding and the "secondary" windings. To DC, you can apply a similar chopper amplifier + sample and hold circuit, you can perform it as a kind of amplifier modulator.Additionally. Forgot. It is desirable to use a low modulation frequency to reduce current through parasitic capacitances, since inductances are impedance. If a variable modulation frequency ivd is used, one should be very careful about capacitive parasites.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 25, 2023, 07:17:22 pm
content: Quote from: Sengcid on April 25, 2023, 06:48:13 pmDefinitely !

author: Sengcid
author_rank: Regular Contributor
timestamp: April 28, 2023, 06:35:30 pm
content: Quote from: bobAk on April 25, 2023, 07:10:17 pm“compensation for the magnetizing current” and “feedback winding to correct the non-linearity of the core” and “variable modulation frequency ivd”: Interesting. Please provide more.“low magnetostriction”: I have not come across this problem with either ferrite or “clock spring” metal-glass type toroidal cores (e.g. Mumetal, Permalloy etc are trade names). ASL used to make six decade capacitance bridges running at high(ish) levels of flux density (100V RMS at 1.6kHz) and they were silent.“parasitic inter-turn and inter-winding capacitances” and “shielding from neighboring secondary windings”:According to Hill and Deacon (of NPL - see attached) the admittances (mainly capacitance) between strands of a rope are far more significant than admittances to other windings, screen or metallic core. The in-phase ratio error is negligible at frequencies lower than 400Hz. The measured ratio errors for a ten-strand single-stage IVD closely match the predicted values based on a lumped impedance model (see attached graph). A two-stage IVD is basically the same.For the very highest accuracy (<0.1ppm) it is usual to use PTFE insulated wire for the first decade.IMHO the use of screens is only justified when “bootsrapping” to replicate operational conditions (e.g. H & D’s test configuration)

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 29, 2023, 10:41:13 am
content: once i tried to recreate Datron 1281 reference DC divider based on IVD:https://www.eevblog.com/forum/metrology/datron-calibrator-7v-to-10v-conversion-with-a-transformer/msg1019807/#msg1019807However it failed at 24h stability test. The linearity also wasn't satisfying. I'm not sure where the problem was.

author: bobAk
author_rank: Contributor
timestamp: April 29, 2023, 11:53:50 am
content: You can remove the feedback demodulator. Introduce an additional winding with low leakage inductance together with the primary winding. Modulate the primary winding with a conventional op amp follower. Before the follower, put an error amplifier that compares the signal from the additional winding and the bipolar modulated reference voltage signal. If simplified.

author: Sengcid
author_rank: Regular Contributor
timestamp: April 30, 2023, 06:36:21 pm
content: Quote from: doktor pyta on April 29, 2023, 10:41:13 amGreetings doktor pyta,I will have another look at your 7V to 10V converter thread and reply there.Given the size of your toroid and low number of turns I am surprised how good the linearity is (< ±10ppm). The Datron xformer is quite a bit larger. See attached.Relevant to this thread are: -1. The Datron single-stage transformer is probably OK for stability (an alternative to precision resistors) rather than absolute accuracy – with CAL data stored in memory. For linearity checks use a two-stage IVD plus conventional (double-balanced) synchronous rectifiers. See attached for practical circuits (from the thread ASL F17 and F700 circuits).2. Life is much easier with a sinewave – none of those nasty transients near your sensitive circuits.3. To achieve better flux matching in the secondary windings one should use a separate primary, wound uniformly around the core (BNNL: balanced no-net loop) in a single layer on the outer circumference and top/bottom but two layers on the inner circumference - a uniform (toroidal) sheet of current. Virtually all the flux is contained inside. For the first design iteration it is more important that the core be completely covered. Choose a different gauge of wire later for the second/third design iteration to achieve the desired number of turns. To minimise capacitive coupling from primary to secondary the drive could also be balanced. See the monograph “IVDs and RTs – the basics” for more detail.4. A rope winding should also be at least no-net-loop (better still is BNNL). Use thinner wire and more turns to achieve a double layer on the inner circumference. You should be operating at well below saturation.5. From the shape of the non-linearity curve the most likely cause is load current. IVDs and RTs do not like any sort of load, resistive or capacitive. Try connecting both synchronous rectifiers to the same tap. Does adding the second (double the load) affect the DC output from the first?P.S. your xformer appears to have 13 turns of rope.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2023, 07:21:51 pm
content: Quote from: Sengcid on April 28, 2023, 06:35:30 pmMumetal and Permalloy are low magnetostriction. For the thin ribon metallic glasses the magnetostriction can vary quite a lot. From near zero for cobalt based to rather high for the common Metgals 2605 for power transformers. Ferrites usually also have high magnetostriction. A good solultion could be nanocrystalline materials (near zero magnetostriction and higher saturation than Permalloy).The problem is likely not so much the noise, but the inversese magnetostriction so that mechanical stress can effect the permeability quite a bit and inhomogenous stress could cause stray fields.Some ferrite cores are quite sensitive to stress (e.g. winding stress).

author: bobAk
author_rank: Contributor
timestamp: April 30, 2023, 11:31:00 pm
content: Let's agree - I'm talking about idv only in the context of dc. In your bridge, most of the problems do not manifest themselves due to the balancing of the system and the constancy of parasitic parameters. At low frequencies, the problem of magnetizing current is more pronounced, rather than capacitance. It degenerates into the problem of linearity and reaction to the load on the secondary winding. Capacitance also has an effect, but its effect is at levels below 1 ppm. Kleinstein answered almost completely about magnetostiction. Sinus is more convenient, yes, but harder to apply for dc. The square current ringing problem can be circumvented by "sampling" between the transients in the inductor and the ringing.

author: Sengcid
author_rank: Regular Contributor
timestamp: May 03, 2023, 06:55:19 pm
content: Herewith an interesting document from Wika. The ratio test unit referred to (RTU: probably a more appropriate name) is almost certainly a two-stage IVD. All 14ppb uncertainty is due to PTB's reference IVD.It is very likely to be constructed from size 11c SM100 cores and PTFE insulated wire - the same as used in the F18.The CTR9000 is an F18 with an extra digit. The three-stage transformer was so good that ASL (John Yewen and Paul Bramley) were able to hunt down the (sub nV) final errors.See the thread "ASL F18 circuits" .

author: MegaVolt
author_rank: Frequent Contributor
timestamp: May 04, 2023, 09:59:08 am
content: Inductive dividers were used in Soviet devices Р3003, Р3017 and others. They have documentation and photographs. Caution in the name Cyrillic. Use ctrl-C, ctrl-V to search

author: bobAk
author_rank: Contributor
timestamp: May 04, 2023, 10:28:34 am
content: There is also П327. I have everything. Here is a general photo of the Р3017 transformers.https://ibb.co/z47sy6x

author: MegaVolt
author_rank: Frequent Contributor
timestamp: May 04, 2023, 10:39:36 am
content: More pictureshttps://ampnuts.ru/3017-performance-verification-module/

author: bobAk
author_rank: Contributor
timestamp: May 04, 2023, 12:12:12 pm
content: A little explanation. P3017 consists of three blocks - a verification block, a measurement block and a microvoltmeter. In my photo there is a measurement block, in the photo from the ampnut website there is a verification block. The whole complex is used for voltage transfer and comparison.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: May 04, 2023, 03:26:22 pm
content: There are all the blockshttps://ampnuts.ru/tag/r3017/

author: dietert1
author_rank: Super Contributor
timestamp: May 04, 2023, 06:30:41 pm
content: My observation about the transformers: Windings are multifilar and the wires leave the tranformer in bundles, to come separate far from the transformer where stray fields are small. One can see that each bundle separates into 4 to 10 separate windings when the wires are connected to the board far from the transformer. I'd guess that's one of the secrets to achieve linearity.Of course there will by quite some capacitive coupling. Maybe such a transformer works better with sine waves and the demodulator won't be switching either.Regards, Dieter

author: bobAk
author_rank: Contributor
timestamp: May 05, 2023, 12:20:47 am
content: All Soviet idv for dc work on a square wave. I have not seen solutions for dc, neither Soviet or Western, on the sine and not turning into a monster with losses in accuracy. Perhaps missed something? It would be very interesting to take a look. All I saw on the sine is ac bridges for capacitance, resistance, inductance, ac dividers and the ac ratio standard.Dieter1 , yes you are right. Such winding taps are grouped into sections with small voltage differences to reduce the effect of capacitance. In the case of a 1 decade dc source, 2 transformers are used, for a 7 decade source there are already 3. Grouping of windings on transformers is performed to reduce the effect of capacitance, magnetizing current, etc. The set of windings is used to build a circuit for correcting and adjusting the linearity and issuing some "standard" voltages, for example, 1.018. The output voltage is switched by the winding switching method.

author: alm
author_rank: Super Contributor
timestamp: May 05, 2023, 11:22:19 am
content: Quote from: MegaVolt on May 04, 2023, 03:26:22 pmThanks for the link, some great information there. Although somehow for me only this link works:https://ampnuts.ru/tag/%d1%803017/. Probably because of the forum screwing up Cyrillic multi-byte UTF-8 characters.Quote from: bobAk on May 05, 2023, 12:20:47 amIs this a higher end version of the Fluke 7105A/Datron 4900 systems that would allow comparing an unknown voltage to a 10V or Weston cell reference? Where the verification block is used to adjust the measurement block, which in turn is used to divide the unknown voltage until it equals the reference voltage?Where does the square wave come from? Is it chopped DC that's AC-coupled to form a square wave with amplitude equal to the DC voltage?

author: bobAk
author_rank: Contributor
timestamp: May 05, 2023, 12:28:17 pm
content: QuoteThe comparison is wrong(similar to fluke 7105a). P3017 is not automated and is not a voltage reference. It transfer or scales voltage from an external voltage reference. For example, it is used for transfer 1.018 and verification(I hope the term is correct verification, not calibration) hp 3458. Its own voltage reference provides metrological performance for approximately 8-12 hours, depending on the operating time. Yes, it is designed to measure an unknown voltage using its microvoltmeter as a null meter or a differential voltmeter.QuoteIt uses 2 zener diodes that create a negative and positive voltage, which is then converted by the modulator into a square wave, bipolar. The modulator is made on two fet p types.If you are interested, here is a modern automated system with a highly stable reference voltage source, a direct relative of the P3017.znp.ru/prod/km300.htmThe principles of operation and transformers are the same.

author: alm
author_rank: Super Contributor
timestamp: May 05, 2023, 01:27:41 pm
content: Quote from: bobAk on May 05, 2023, 12:28:17 pmThe Fluke 7105A is not automated and does not contain a voltage reference either. It will let you either compare an unknown voltage to a voltage reference using a resistive KVD, or can source a voltage referenced to the Weston cell voltage reference. See pictures of both systems on page 1 ofMigrating from dc voltage dividers to modern reference multimeters. From the KM300 theory of operations it sounds like this system is quite similar. It's a very high resolution, stable, variable voltage source from a stable square wave generator, inductive voltage divider and precision rectifier which can then be compared to a reference or unknown voltage.Quote from: bobAk on May 05, 2023, 12:28:17 pmThanks for the reference! The theory of operations in section 5 of the manual gives a decent idea for us relying on Google Translate.

author: bobAk
author_rank: Contributor
timestamp: May 05, 2023, 02:34:24 pm
content: Above, in brackets, I said that 7105 is similar. Km300 differs from p3017 in terms of use. P3017 is essentially its own autonomous complex requiring an external voltage standard and requiring manual adjustment. Km300 itself can be a kind of standard, but for verification of linearity it still requires an external linearity standard and it is automated.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 05, 2023, 05:04:02 pm
content: Possible useful work here, done by Henry Hall-https://www.ietlabs.com/pdf/GR_Experimenters/1968/GenRad_Experimenter_May_1968.pdf

author: Sengcid
author_rank: Regular Contributor
timestamp: May 08, 2023, 06:35:59 pm
content: Quote from: MegaVolt on May 04, 2023, 09:59:08 amThe P3017 transformer is interesting. I note, however, that the ropes (“bunches”) could have been twisted more tightly – necessary for accurate (average) sharing of the stray flux outside the toroidal core.More important is the gap between the “start” and “finish” ropes before the strands are terminated and connected in series. A better way is to keep the ropes together before being separated into twisted pairs (of starts and finishes) and then terminated on pins. See, for example, the xformer PCB of an ASL F26. The point is to keep magnetic flux from passing through any gaps – inducing interference or errors.To put things into perspective, however, the transformer core attracts magnetic flux and significant amounts of interference (power supply harmonics) can be expected. Also, the interwinding capacitance of the main part of the rope (on the transformer) is much greater than that for those short lengths as well as between the primary and secondary. The effect on in- phase accuracy at <400Hz is negligible.Similarly, current carrying pairs should also be twisted – the red and purple pairs (F26) are the energising windings for the (larger) three-stage transformer. The smaller transformer is a two-stage for stepping down and adding in the lowest three decades (from a multiplying DAC).BTW. The F26 three-stage transformer is good for <1ppm at 75Hz. The spec of the instrument was deliberately reduced (no quadrature servo) in order to avoid competing with the much more expensive F17 (later the F700).The P3017 transformer appears to be single-stage with a bit of positive feedback (borderline stable) to correct for the (inherent) in-phase error. It has circa 100 turns of the main rope (a total of 1000 turns). A two-stage IVD requires an extra core but fewer turns and is inherently accurate at a much lower frequency. When hit with a square wave the amount of droop is also much less, compared to a single-stage transformer. The main disadvantage is the larger number of turns, larger core and higher operating frequency required to tolerate higher voltages and maintain a low level of flux density – the same for both types.

author: bobAk
author_rank: Contributor
timestamp: May 08, 2023, 08:55:28 pm
content: The p3017 transformer is not single-stage (if I understand correctly what is meant). The frequency of operation of the idn is selected for a specific version of the transformers, since it is not possible to perform absolutely exactly with a given quality factor, capacitance, inductance, resistance, etc. (including parasitic ones). Since the work on a square wave with all its harmonics. On average, the frequency is 700-800 Hz for p3017 (p3003, p327). Also, the balance between the time of the positive and negative half-wave is adjusted to reduce the dc component of the magnetization signal. No matter how hard you try to create even 50% modulator keys, they will not be ideal.Wiring racks are an obvious and convenient solution, but the developers did what they did))

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 08, 2023, 09:05:19 pm
content: Some details regarding p327 from the available manual.It uses square wave driven transformer.

author: bobAk
author_rank: Contributor
timestamp: May 08, 2023, 09:37:42 pm
content: According to p327. Here the principle is the same, only the approach to demodulation and the operation of the DC amplifier is different. The picture shows only one transformer, in reality there are two. The other one is on the back of the board. Under the aluminum "cans", from right to left, there are an idn modulator and an idn demodulator (+ amplifier modulator). On the block diagram, the section of the demodulator idn and the modulator of the dc amplifier are depicted in a simplified way, there are three transistors.

author: Sengcid
author_rank: Regular Contributor
timestamp: May 09, 2023, 06:45:57 pm
content: Quote from: bobAk on May 08, 2023, 08:55:28 pmIn this context a "single-stage" transformer has one high permeability toroidal core.A "two-stage" IVD has two. See attached for an example.A three-stage transformer has three, not including smaller "equalising" cores.

author: bobAk
author_rank: Contributor
timestamp: May 09, 2023, 08:55:07 pm
content: Then I understood correctly. Two-stage transformer, wherein the second and third transformers are made in almost the same way, depending on which verification or measurement unit, so they differ quite significantly from each other in the structure of the transformers. In the photo in your post is a homemade transformer?

author: Sengcid
author_rank: Regular Contributor
timestamp: May 10, 2023, 06:51:45 pm
content: Two-stage means two cores per transformer. The two-stage IVD pictured was a prototype but proved to be be < 0.1ppm (in-phase) at 375Hz. I used it as a simple variable AC source when testing (@ 75Hz) the linearity of the ASL F17 prototype synchronous rectifiers and carrier generator circuits (sorry, original evidence lost - it was in 1978). The result was as good as our best DVM (Datron) at a few ppm. Herewith some reading for colleagues new to this area of metrology.The full set of monographs can be found via the link.https://drive.google.com/folderview?id=1TbomXeoBbIe-IVADaOOmyyLH9le-3wAtMeanwhile there is a potential customer for a two-stage DIY IVD RBC over in the thread ASL F17 and F700 circuits.

author: bobAk
author_rank: Contributor
timestamp: May 10, 2023, 08:00:01 pm
content: I understand that and that's what I meant. I didn't explain the design very clearly.. Thanks for the documents, let's compare with our approaches and calculations.

author: Sengcid
author_rank: Regular Contributor
timestamp: May 11, 2023, 06:44:47 pm
content: Herewith an updated document to be more relevant to the Datron “Selfcal” type VREF multiplier application. The main conclusion is that the resistance on the input side (analogue switch) reduces the transformer time constant and, therefore, the amount of voltage droop. The resistance on the output side determines the voltage (average and ripple) on the capacitor. The technique is not intended to be accurate but stable - an alternative to precision resistors, probably requiring temperature control and periodic calibration. It must have required a lot of R&D to achieve the target spec.Also of interest here is the Datron (single-stage) transformer. The primary winding (centre tapped with balanced drive) is almost certainly wound uniformly NNL (no-net-loop), not only to avoid transmitting flux axially but also to minimise capacitive coupling to the secondary. Each strand of wire is right next to the strand with equal but opposite voltage on it. The same is recommended with a two-stage transformer though each primary (energising and ratio) would need its own (low resistance) analogue switch.Herewith, also, is another example of RT construction: the ASL F25.

author: bobAk
author_rank: Contributor
timestamp: May 11, 2023, 08:01:29 pm
content: A two-winding(two winding primary) transformer is inconvenient in this case and does not solve the capacitance problem, better results with electrostatic shielding. It is impossible to obtain a uniform remagnetization of the core, and the greater the resistance of the key, the more problems, and also for a small resistance of the key, a certain balance is needed. The circuit at the output of the transformer pulse loads idv since the sample-hold occurs on one clock cycle. This can be partially compensated.

author: Sengcid
author_rank: Regular Contributor
timestamp: May 12, 2023, 06:54:06 pm
content: Herewith another (quite nice) example of RT construction. The CMOS switches used to select the variable turns are on the underside. They are the 74HC version of the CD4051, hence the +/- 5V supplies.The connections labelled EW_S and EW_F (start and finish) could be misleading - they are the primary winding (single stage) and not the energising winding of a two-stage transformer. The extra toroidal core is for the equalisation of the second rope to the main one.The F252 is a later version of the F25.Also, a small correction to the square wave response. Does anyone know the frequency of the Datron selfcal system?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2023, 08:04:30 pm
content: Looking at the circuit the clock for the Datron circuit should be a little under 1 kHz ( 4.9152 MHz / 5000).

author: Sengcid
author_rank: Regular Contributor
timestamp: May 13, 2023, 06:53:40 pm
content: Quote from: Kleinstein on May 12, 2023, 08:04:30 pmVery interesting. Capacitance (between strands of the secondary rope), dielectric loss factor and leakage inductance are an issue - the rope must be very tightly wound (capacitance and leakage inductance combine to create in-phase error - similarly in the time domain analysis). Capacitance between primary and secondary is much less important.Edit: I found the service manual and circuit diagrams via xdevs.https://xdevs.com/fix/d1281/Meanwhile I found this: the (expensive) competitor to a DIY IVD RBC

author: Sengcid
author_rank: Regular Contributor
timestamp: May 17, 2023, 06:45:24 pm
content: From the xdevs 1281 repair thread is a nice pic of the selfcal xformer. The taps are brought out and terminated on the PCB as single wires rather than twisted pairs - not recommended. One could easily pick up the stray flux from the primary winding connections with a simple search coil (10 turns wrapped with cotton thread and coated with nail varnish - an essential bit of kit for nanovolt hunters) plus a x100 amplifier.