AC Voltage Standard - Page 1
Forum section: electronics / general

author: Kennylxix
author_rank: Regular Contributor
timestamp: January 31, 2017, 03:07:21 pm
content: So I'm fairly new to metrology but I find it very fascinating (I just sent my first bench dmm off to calibration.. I've found tons of information on methods to create very precise and cool standards for pretty much every measurement under the sun except one. AC RMS. I've tried and failed to find a single source of a DIY reference or standard (The closest I've come is a dmmcheck but it's pretty limited on amplitude). I also failed trying to find an IC that does any meaningful inversion.Am I missing something or is AC voltage just not really a concern for anyone in the diy community? I want to get to a point to where can calibrate a lower resolution DMM using only home made equipment and this is the one thing I just can't find anything onThanks for any info!

author: capt bullshot
author_rank: Super Contributor
timestamp: January 31, 2017, 03:13:04 pm
content: Don't know a actual project, but from my knowledge a thermal converter would be your way. Start checking some of Jim Willams' application notes, LT built once a nicely integrated solution. But there's no secret stuff inside a thermal RMS converter, so you should be able to build one from scratch (more mechanical challenging than electronically).

author: Kennylxix
author_rank: Regular Contributor
timestamp: January 31, 2017, 04:21:17 pm
content: I've seen thermal converters and they are really cool but what i really want is an actual source. Something that outputs a precise AC RMS voltage . Even better would be a programmable one. Oscillators pretty much already do this but they are designed for precise frequency not amplitude. I know it's possible because multi thousand dollar calibrators have the capability.I'm learning that pretty much anything that can be bought for thousands can be made for hundreds.I just don't know how it's done.

author: CalMachine
author_rank: Frequent Contributor
timestamp: January 31, 2017, 04:46:20 pm
content: I'm in the works of a PWM version of a 10V reference. Once I get that design finished and working as well as I would like it to, I plan on trying an DC->RMS comparator circuit to give out a DC/AC Transfer value. I'm shooting for the stars, here!I contacted Guildline in regards to their 7410... but they told me it is no longer available and they have nothing even remotely similar to offer. I wonder who paid them to stop producing that unit?http://www.weiku.com/products-image/13742223/Model-7410-AC-Voltage-Reference.htmlGuildline took down all information regarding it, so it was a little more difficult finding something about it.

author: Kennylxix
author_rank: Regular Contributor
timestamp: January 31, 2017, 07:52:15 pm
content: Hmm so maybe a precision sine wave generstor that is programmable for amplitude and frequency going into a step up transformer with something like an lt1966 RMS to DC converter monitoring the generator output before the transformer. Then an nice ADC on the RMS to DC converter with an Arduino to both monitor the ADC and control the generator. It would needs some serious calibrating but once it was it should be pretty stable. The lt1966 has a .02% linearity.(Please forgive anything stupid I just said, I'm new to electronics;)

author: Kleinstein
author_rank: Super Contributor
timestamp: January 31, 2017, 08:53:20 pm
content: Classical AC sources with precision amplitude use a thermal converter to do feedback on the amplitude. This is limited in amplitude though and might need stable dividers - precision transformers (e.g. with double core) can be pretty good in this. A lot of work, but in principle DIY possible.For the low frequency range (e.g. 50 / 60 Hz) there is a chance to go sampling a sine and have a precision DAC that can also output DC. One can reasonably check the speed by sending out steps. As the usual DAC chips are reasonable fast, this might not be so hard, if you don't need super high accuracy. The individual DAC steps (or a reasonable part of it) could be measured in DC.The complementary way would be a fast ADC to measure AC voltage by sampling and do RMS conversion in software. So more or less do what HP called "true volt" or the 3458 does for AC measurements, only with an integrated ADC. The problem here could be the speed and compensation of the input stage - so make sure amplitude is frequency independent. Directly at the output of a generator one might not need amplification at all - just some protection. However the ADC might have internal delays / frequency dependence. To a limited extend one could check it with a step response or a square wave signal. So even if you want only 50/60 Hz - have a MHz BW amplifier and adjust like a scope probe to have it straight in the low frequency range.

author: Kennylxix
author_rank: Regular Contributor
timestamp: January 31, 2017, 09:55:14 pm
content: Hmm yeah I understood about half of that:). I would definitely want it pretty accurate as it would be used to check calibration of dmms. I definitely want to look into this more. I bet if someone made one and could produce it even somewhat reasonably priced there would be a market for it.

author: CalMachine
author_rank: Frequent Contributor
timestamp: January 31, 2017, 10:05:39 pm
content: Quote from: Kennylxix on January 31, 2017, 09:55:14 pmI'm in the same boat with you, lol. I'm very new to the design side of things and reading what a lot of people post on hear skims right on my hairline. I get the general theory behind most things, but then implementing, properly, is a whole different story. I love visiting these forums though, as I learn SOO much and it really helps me with my job!

author: pelule
author_rank: Frequent Contributor
timestamp: January 31, 2017, 10:41:42 pm
content: Have a look to folowing thread. Offers some ideas...https://www.eevblog.com/forum/metrology/diy-precision-ac-rms-to-dc-transfer-standard/?allI personally would put my money ito the DAC based solution.

author: Kennylxix
author_rank: Regular Contributor
timestamp: January 31, 2017, 10:55:49 pm
content: Generating the sine wave with a DAC seems like I really good idea. I would imagine that a you wouldn't even have to measure the voltage coming out after the initial setup because the DAC should output the same thing (within the limit of the reference) every time. Is that correct?

author: Kennylxix
author_rank: Regular Contributor
timestamp: January 31, 2017, 11:17:32 pm
content: So the AD5504 can output up to 60v which is perfect. It has a built in reference as well. I tried to look up how you go about actually telling one what to do and came up empty though.

author: Kennylxix
author_rank: Regular Contributor
timestamp: February 01, 2017, 12:14:22 am
content: The biggest issue my ignorant viewpoint can see is translating the desired RMS value to a p-p value for the DAC. This could be somewhat ignored by using a RMS to DC converter to show what the actual output is and something like a pot to control p-p. That would be a kind of "you'll be happy with what we give you" kind of situation rather than being able to program exact wanted values but as long as the RMS sensing is accurate outputting somewhat arbitrary values isn't really an issue for dmm calibration.This is fun. Once again I apologize if what I'm saying doesn't actually make sense.

author: pelule
author_rank: Frequent Contributor
timestamp: February 01, 2017, 12:24:33 am
content: As a group of setvalues build the output sinus (lets assume 10 setpoints for a half cycle of the sinus wave for easy calculation), the full sinus wave requires 20 values, thus an output frequency of 50 Hz requires 20*50 = 1000 values per second.So settling time of the DAC defines the maximum output frequency. Thus I would use a DAC with a parallel input.In the old days we used a ROM containing the set values and clocked the adress with different clock speed to control the ouput frequency. Nowadays I would use a SRAM (or FRAM) to have the output signal form programmable (getting an abritary signal generator)Design critical are:- the switching speed of the DAC / defines the max possible frequency- the switching failure of the DAC generates dissortion- the drifit (tempco, longterm stability) of the DAC the and reference and the impact of load all influence the stability of the output amplitide.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 01, 2017, 01:03:03 am
content: I've got a Fluke 540B thermal transfer standard but even those had a calibration constant. I think Thaler made an AC chip that's no longer available. Here's some info from a N&V article-http://www.nutsvolts.com/magazine/article/the-ac-volt

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2017, 01:39:51 am
content: The way I have done it in the past is to generate a precise 50% duty cycle square wave with accurate peak voltages but this only works if a square wave is suitable.What I have considered is the sampling method which can return both the peak and calculated RMS values over a wide frequency range while the sampler itself can be calibrated with a clean square wave. The problem then becomes how to build a high accuracy sampler.

author: Kennylxix
author_rank: Regular Contributor
timestamp: February 01, 2017, 02:01:06 am
content: I found a paper where they created the waveform with a DAC which they then used a calibrated 8.5digit dmm to measure each step of a single cycle then calculated the RMS value from those results. This gave them a calibrated value for each use of the standard which eliminated issues with long term stability. I think this could be done with an ADC instead of a dmm. I'm pretty sure a 24bit ADC would have enough resolution to get an accurate measurement and would be a lot quicker than the 40mins it took them with a dmm.

author: Kennylxix
author_rank: Regular Contributor
timestamp: February 01, 2017, 02:09:05 am
content: As long as the DAC and reference were accurate enough I don't think the DAC would be needed at all but it would give an extra layer of confidence comparing what the ADC thinks its outputting and what the DAC calculates from the output.

author: timb
author_rank: Super Contributor
timestamp: February 01, 2017, 05:05:22 am
content: Quote from: Kennylxix on February 01, 2017, 02:09:05 amYou really can't easily get a DAC with the required accuracy, INL, DNL etc. to do calibration work without some sort of digital feedback.Besides, even a 18-bit precision DAC can have quite a large temperature variation even at DC bandwidths, which is why modern calibrators still use ADC feedback.There's Jim Williams app note where he uses two precision 16-bit DACs (summed with a precision op-amp) and a 24-bit ADC for feedback. There's even a circuit in the appendix for creating a class-AB amplifier out of discrete transistors capable of a +-120V PP output.I'd start there.

author: Moon Winx
author_rank: Regular Contributor
timestamp: May 06, 2017, 01:26:31 am
content: It's kind of funny that after decades of advancements in electronics, we still are left with a thermal converter as the most accurate way to measure an AC signal. NIST has developed a rather mature AC Josephson Standard or JAWS (Josephson Arbitrary Waveform Standard) that is fundamentally accurate and produces waveforms up to 1 V at 100 kHz or so. They don't use it as the official AC standard as of now but that is the goal. It is a quantum-accurate DAC and works by pulsing Josephson junctions with bitstreams.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 02:18:37 am
content: I wanted to do a project to learn FPGA's and the thing I picked was a PWM sinewave generator with the first harmonic of significant amplitude at something like the 54th harmonic.Currently I am just working to a 4ns pulse accuracy, but I should get to 1ns or lower accuracy once I sort out a few tricks with the FPGA - I am still a VHDL beginner. The thing about this approach is that as long as you can minimize switching issues, the AC accuracy comes down to the DC accuracy, plus errors that are calculable. I only need something like 40 PWM pulses a cycle, so at 1KHz, there is only a 40kHz pulse rate. Using CMOS switches with a 5ns rise time connected to an accurate DC reference, the errors can be very low. It is only the differences between the rising and falling edges that matter, and they can be observed and compensated for. When you only have to make a low pass filter to eliminate the 54th harmonic and above, you can make filters with very accurate gain at the output frequency. You are not dependent with the stability of resistors in a DAC at all.The plan is to also have PWM voltage dividers in the FPGA, that I use as the DC reference for the AC PWM, so I can have a super stable and accurate AC voltage that has an accurately programmable amplitude.The point of this approach is the AC amplitude is exactly calculable from the DC reference voltage and all the error sources are measurable and can if needed be compensated for.

author: chris_11
author_rank: Contributor
timestamp: May 06, 2017, 06:47:23 am
content: I am a bit on a loss why you are interested in a super precise AC calibration of an DMM. 100% - a few ppm of the time it is used to measure mains voltages or 50/60Hz AC, derived with a transformer from the mains.Mains has several % of harmonics due to all sorts of rectifier and triac style circuits. Look it up with a scope (over a transformer) if interested.To make a calibration source for that, any stable 50/60 Hz sine generator will do. Simple DAC type with a decent reference or a Wien bridge with decent amplitude control.If you are interested in the AC meter bandwidth hook it to an signal generator and dial the frequency through. Will be short term stable enough to give you the 3dB point and other surprises.The RF guys measure in dB. 0.1 dB is in the range of 1% amplitude error.If you are in the electrical meter calibration business that is a different deal, but for the troubleshooting DMMs anything better than 1% is overkill.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 07:20:30 am
content: When I started electronics, most people were using 5% accurate analog meters, and they managed pretty well.Most devices do not need precision of any sort.But so what? What if, for example, someone wants to measure the stability of an Analog Devices RMS converter, just because they want to find out how stable the reading of a device they made is. The reason might be to work out how many digits they need on the display for their device. To do so, they need something more accurate. If the reference is 10 times better then the AD chip, then you get accurate numbers. If you try and use a 0.1% accurate meter to test an AD chip and see a 0.1% drift, you have absolutely no idea what was drifting.There is no one explanation that answers your question. 100 different engineers can have 100 different reasons for needing an AC reference."Because it is fun" is a really excellent reason.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2017, 07:55:03 am
content: When using PWM to make an accurate DAC, the logic part made in a FPGA is only the small part to start with. The more tricky part is getting the switches and filter part. TiN had some information an the Fluke 57xx calibrator that uses PWM for setting DC values. It take quite a lot of effort to do that and this is not in the logic part - that is easy today and possible with most µCs.Today ADCs tend to be more accurate than DACs and thus the more promising way is using an ADC to measure a clean sine. The signal could still be with samples from an DAC and a filter, but amplitude reading would be from the ADC. One trouble with DACs is that they tend to also give higher frequency noise that normally can be filtered, but that filter would also add uncertainty. It is easier with the ADC: here, if the signal is pure, there is no need for an extra anti-aliasing filter. It is also possible to directly read the output - thus no more critical amplifier/buffer behind the DAC.

author: chris_11
author_rank: Contributor
timestamp: May 06, 2017, 08:20:54 am
content: I assume that if you pimp the reference a bit in a standard audio USB Stick you get a decent good low harmonic source. They usually have between 16 and 24 bits. The high resolution A/D comes free too. And that with two channels in each direction. The isolation between the channels is limited, because for stereo audio not of much concern. Run both channels in parallel. Would be interesting how good the amplitude accuracy can become, if you improve the reference.At least I found a gain drift spec from those guys of 20ppm/Khttp://www.es.co.th/Schemetic/PDF/AK4395VFP-E2.PDFThat's a starter. This is a typical audio D/A used in the better sound cards.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 08:43:31 am
content: I have no idea of the design choices of the Fluje 5700A, but all you need for an accurate DC PWM integrator is an resistor and a capacitor followed by a very high impedance unity gain zero offset opamp. You do not need an active integrator circuit. The active integrator introduces a whole swamp of extra possible sources of error. The amplifier/filter for an AC PWM output and a DAC are not much different. It is all manageable of you are not running anywhere near the cutoff point of the filter.There are parts available today that were not available in the 70s or 80's - whenever the 5700A was designed.Switches can be as simple as a cmos logic chip. The resistance of the cmos outputs do not matter - only the difference between the high and low mosfet switches. It all can be measured and the errors calculated.It is easy think that the PWM is somehow too simple, but when you build and test them, they are brilliant.There are many high end references that rely on DAC and ADC components. I am sure the companies using those parts have spent a huge R&D budget on verifying the performance of those parts, and they probably have a tight QC process to ensure the parts used in manufacture meet then needs.However, the PWM is simple and does not rely on anything hidden. As I said, all the key parameters are visible and testable. 0.01% accuracy is achievable with very little effort. To get down towards 1ppm, there are always many subtle effects that probably take more effort.Some of the things I have now are FPGA's which can give me a 1ns pulse accuracy as opposed to microprocessor PWM circuits that may only give a 100ns resolution. I also have many great choices for auto-zero offset, low input current opamps. The PWM frequency is irrelevent - only the short term PWM clock jitter matters.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: May 06, 2017, 08:54:22 am
content: I know you're talking about making one, I recently purchased a EDC AC source. I can't find much information on them, but happy to take photos of the internals when I get it and see what makes it tick. 6 decades of switched, 10mv, 100mv, 1v and 1000v at 50, 60, 400, 1000 or 5000hz. I can't wait to get it on the bench and run some performance tests on it

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 09:04:07 am
content: I will be interested to see the EDC AC source. I would love to have full accuracy at 1000V AC, but making an amplifier like that is definitely really hard. My first goal would be to get a very accurate AC output up to about 1.5V RMS. I will worry about the voltage boosting later.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2017, 10:06:37 am
content: When using PWM as an accurate DAC, there are uncertainties in switching times and charge injection effects. Thus for a really accurate value the frequency should not be that high. To avoid too much residual ripple, one essentially needs an higher order filter. The simple RC filter is not that practical.Even if it is only the difference in switch resistance that enters, this can be a problem for really accurate outputs. Large area and thus low resistance FETs add more charge injection and capacitance and are this only a limited option. In addition the FETs resistance is highly temperature dependent (e.g. 6000 ppm/K). For 0,01% this might still be OK, but it gets increasingly more difficult. Some of the errors in an PWM based DAC are just offset type and might thus be less of a problem with an AC signal. However the speed is very limited.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 10:46:55 am
content: Quote from: Kleinstein on May 06, 2017, 10:06:37 amUncertaincies? It is easy having jitter effects much less then 0.1ppm, and rise and fall times of the switch can easily be measured and characterised. A 3 or 4 stage RC filter can easily reduce the noise to less then 1ppm for a 10Khz PWM frequency with a 1 second settling time.With a 1ns timing resolution and a 10KHz PWM clock, it is not hard with edge errors to still achieve 10ppm with a simple cmos logic IC as the switch. The 74VCT family can get down to the sub 2ns switching speeds. I will probably end up with 0.66ns timing resolution (that is what I can get from 1 PLL on the Cyclone II board I will use) and it is also not hard to compensate for unequal edges.QuoteDon't use large area low resitance FETS. You need them for DACs, but not for PWMs. If you use 100K resistors for the PWM and the cmos switch resistance is 10 ohms, you already have an worst case error of 100ppm. It does not matter what the cmos resistance is - only the difference between the high and low channels matter. If you can find a device where the resistances match within 10%, you have 10ppm error. So you are only looking at tweaking the design to get the extra factor of 10 improvement for 1ppm.100K sounds high, but with opamps available now with a 1pA input current maximum at 25 degC, using resistances of 100K from the switch output to the first capacitor is practical. You can use guarding to handle leakage currents. You also have to test the capacitors for leakage.As always, to get 1ppm, you have to work through everything in detail, but as I said, 100ppm is dead easy to achieve.If you want a reference, DACs and A/D IC's can be more difficult. It wouldn't be a very clever metrological strategy to just chuck a device in a reference circuit and just assume it will meet the manufacturers specs. How are you going to verify the accuracies of these devices?Precision PWM sounds hard until you actually try it.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2017, 12:11:51 pm
content: With a high frequency PWM, on relies on fast and accurate transitions. However the turn on / turn off times are not that stable and are not necessarily the same.The bigger problem is loading the reference input: fast switching causes ringing on the supply and the reference voltage - if you need to rely on ns speed switching this needs a layout good for GHz frequencies - not that easy with the layout, and may not be stable with changes in humidity or similar. In this high frequency range a lot of strange, little predictable effects might happen.There is a good reason the 5700A used a rather low PWM frequency. Typical logic outputs are more like 50-100 Ohms switch resistance, 10 Ohms FETs are usually already slower and create significant more switching spikes on the supply. A 100 K resistance in the filter sound reasonable. However this would be just a 1st order filter and there is no way of easy cascading to higher order. Even with buffers in between a chain of 1st order filters don't make a good higher order filter. So the filter circuit in the 5700A has it's reason. Today one might get away with monolithic OPs instead of the discrete JFET pairs. So it can get a little easier there.PWM, if used in the right way might offer a high degree of linearity, but the filter will cause some frequency dependence. So it might be OK for DC and very low frequencies (e.g. < 1 Hz), but nothing really useful in the AC range. Even a filter at 1 KHz will have quite some residual effect in the 50 Hz region. Some of it is predictable, but it depends on the capacitors (not just capacitance but also loss).With ready made ADCs, it is possible to measure the dynamic effects as well. A simple step signal or low frequency square wave usually provides enough information. With modern, fast chips that work up to the MHz range, there is no much dynamics going on with 100 Hz sine signal. It is essentially DC in this range. Linearity of the ADC in the AC range is usually well characterized by THD and can be measured if needed.

author: pelule
author_rank: Frequent Contributor
timestamp: May 06, 2017, 12:18:30 pm
content: For those who interested, here the data sheet of the SWR300 and a AppNote.I worked with in the past.Thaler was aquired by Cirrus Logic in December 2008. The products are sold as Apex Precision Power:http://www.cirrus.com/en/products/apex/thaler.htmlThe SRW300 is obsolet, but the SRW200 is still available:https://www.apexanalog.com/products/vre_selector.htmllhttps://www.apexanalog.com/products/swr200.html/PeLuLe

author: chris_11
author_rank: Contributor
timestamp: May 06, 2017, 12:22:37 pm
content: PWM modulators show clock feed through and unsymmetrical transitions. The same problems you have in any internal sigma delta modulator of AD and D/A. There is a reason that it is extremely difficult to get them to ppm range or sub ppm range linearity. Resolution is not the problem, you can get 32 bits out where the most part is noise, but the linearity is the problem.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 01:21:38 pm
content: Quote from: Kleinstein on May 06, 2017, 12:11:51 pmA device like the 74LVC1GU04 has a typical transition of 1.3ns - maximum of 3ns. The resistance is about 20 ohms. I can work with that. I can easily slow down the PWM frequency if I have to for both AC and DC.There might be ringing. Looking at the effect of this is part of the fun of design.QuoteI gather the 5700A uses 200Hz using technology from many decades ago. You can have the same issues with 200Hz switching when you have older technology components.For DC, there are methods to test for issues like uneven transition time. If you set the output voltage for 5mV, then every 1ns difference in transition will cause a 1% error in this voltage. Once you know the error, it can be easily compensated for both in the case of AC and DC waveforms. If I was chasing the best accuracy, I could have a test done on this at startup. The only number I need is the difference.QuoteA very odd comment. Each RC stage reduces the ripply by over 1000. I can have as many RC stages as I like, but 3 would be more than enough, even with a much lower PWM rate (if I choose to do that).Quotethe 5700A is filtering 200Hz and trying to get a quick settling time. That needs a much better filter.QuoteYou have to have the R/C combination initially for the AC PWM, but then it has to be followed by a real filter - either a 3rd order filter such as a Butterworth, or a 2nd (perhaps 1st) order eliptical filter. The beauty of the Elliptical filter is you can have a notch at the frequency of the greatest harmonics. The filter a can be active or passive.When you pick the right type of filter, the gain of an active filter at 1/50th of the lowpass corner frequency can be known with the same accuracy as the gain error of the amplifier just as an ordinary unity gain buffer. All AC amplifiers (whether for PWMs or DAC buffering) will not have perfect unity gains and this is all part of the design process. There are ways to handle this, but I now have the option of some opamps with very high bandwidth and so the gain error can be pretty low. I have a lot more choices then the 5700A designers had.And I am talking about better then 0.01% accuracy. Probably a 0.01% reference would be a satisfactory result if that was the best I could achieve. A calibrated 5700A on its best range only manages 0.006% on its best range at 1kHz. If I can manage 0.01% before AC calibration (in other words, 0.01% accuracy in comparison to the DC reference), that would be a fabulous result. I have a Fluke AC/DC thermal transfer to check.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 01:39:02 pm
content: Quote from: chris_11 on May 06, 2017, 12:22:37 pmMy PWM clock will be 1GHz or better. When I said the pulse rate was at 40KHz for the PWM, there is no lower frequency clock that the pulse edges align to. As I said in another post, it is not that hard - if I need to - to measure the transition error (probably 2ns maximum for the kind of chip I will use) and compensate. The final error is calculable and is pretty satisfactory. If I need it, I can basically reduce the transition errors to 0.66ns.The reason why I went to do this on a FPGA is initially tried to do a lower quality version on an Atmel microprocessor's PWM and it gets very ugly. The FPGA just makes it easy to get a PWM that works the way I need.I have tested the PWM with a 250MHz clock, but I want to boost it to an effective 1.5GHz clock.A PWM with harmonic cancellation is nothing like sigma delta or any other PWM that has no harmonic cancellation. There genuinely is negligible harmonics in the PWM waveform up to over the 50th harmonic. Without the cancellation, PWM for AC is not at all good. I can match the 5700A distortion figures without huge difficulty.I may only have spot frequencies. I can have fully programmable frequencies but I cannot get best performance for all the frequencies, and I will probably need at least 3 different filters to get down to frequencies as low as 10Hz.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2017, 02:18:53 pm
content: Using PWM as an DAC and using a "PWM" pattern with harmonic cancellation are different techniques. With the harmonic cancellation there would be an increasing amount of higher frequency content. The high frequency part above something like 10 MHz usually does not behave like DC anymore - so damping of a low pass filter will go down again at some point. If not absolutely needed one should avoid such RF magic with impedance matched lines and caring about capacitors ESL - this can be a problem for larger film caps. Things may not be so bad if switching events are far enough apart to that ringing has subsided before the next transition. But it gets nasty if the next transition happens when there is still some ringing : it is no more linear from that point.In real life I very much doubt the harmonic canceling will work well to very high harmonics. So maybe up to the 10 th harmonics.Usually the gain of active filters is much less predictable than that of a simple buffer, it is only the DC gain that is OK. One problem is that capacitors are nor that ideal and there are also parasitic capacitors, that can even be nonlinear like the input capacitance of some OPs. Even with a theoretic suppression to the 50th harmonics, the filter would be still only a little higher than the frequency used. With a variable frequency this gets a really hard as it would need a tunable filter.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 03:14:31 pm
content: Quote from: Kleinstein on May 06, 2017, 02:18:53 pmit works beautifully. I have attached the numbers for a non-optimised 13 pulse/quadrant 1KHz PWM sinewave. These harmonics are the actual numbers with the clock I will use. This includes the inaccuracies in the edges to ideal.With everything above the 53rd harmonic removed, the distortion is 0.0032%. That would be 10 times lower then the 5700A. That 0.0032% is absolutely real figure - that is what can be done with my PWM running at an effective 1.5GHz rate and with the fairly straightforward switch edge transition error correction. There are some other issues with the sinewave generation and I have tested some workable solutions, but I hope you are not asking to try and do the full design right here in this thread.There are always nasty harmonics at the first non cancelled harmonics which is why I suggested an elliptical filer with a notch at those frequencies. Up around the 105th harmonic, it gets nasty again - that is 105KHz. It also has a much lower peak around 155KHz.But so what? It is not as if there is a problem with RF filters. The whole thing can sit in a RF shielded box with RF filters/ferrites on the outputs if necessaryQuoteVariable frequency is possible, but as I said with less than optimal figures. The easiest will be to go for some spot frequencies instead of fully variable. But I will try variable anyway, just for fun.Now if you go back to my original post on this, I never said this was a preferred method over DACs. I said this is the way I was trying - it was the project I picked to learn programming FPGAs, and I pointed out some advantages it does have. The big one was with this approaches the sources of error can be accessed easily. DAC's and A/Ds do a great job, but you do put a lot of trust into AD, LT, etc. You cannot see what is happing inside these ICs. Even if a DAC can be tested and it is found it is perfectly accurate at DC, it is not a guaranty it will be as accurate at the sinewave digitizing frequency. They can have their own quirky problems and filter issues too.I can easily do a 1ppm DC PWM with a great perfect monotonic linearity and stability (compared to the reference) - it is a 20bit DAC and previously I have used some tricks to go to 24 bits. This does work as I have tested it. Costs me under $20 for the parts in Australian dollars excluding the DC reference. Similar parts - costing perhaps another $15 - can exceed the 5700A's sinewave's performance for distortion and accuracy - that is if I push for a really high accuracy.That is alright. It is worth a go, I think.

author: 2N3055
author_rank: Super Contributor
timestamp: May 06, 2017, 07:22:32 pm
content: Are these Don Lancaster style Magic Sinewave PWM sequences.. ?I was thinking to experiment with it..What is your experience with it? Does it really work that well?Regards,

author: chris_11
author_rank: Contributor
timestamp: May 06, 2017, 09:13:40 pm
content: If you are in PWM generation get an actual class D audio amplifier. They are relative low harmonic and have solved those problems already. With a precise stabilised supply rail the AC voltage accuracy should be decent.

author: branadic
author_rank: Super Contributor
timestamp: May 06, 2017, 09:58:34 pm
content: Thermal converter LT1088 is currently available by a german seller on ebay:http://www.ebay.de/itm/TRMS-Wandler-DC-100-MHz-LT1088-/132160053754?hash=item1ec55a61fa:g:sQYAAOSwMVdYEP-6But it is only 1% accurate from DC-50MHz and 2% to 100MHz. I was always wondering how a 50 ohms rf resistor and a platinum element would behave?-branadic-

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 10:28:24 pm
content: Quote from: 2N3055 on May 06, 2017, 07:22:32 pmOnly on breadboard up to now. The problem is it makes one quadrant. Just reversing the pattern makes the second. The third and fourth are the more difficult. The solution I tried that works well is to have two outputs with two resistors to a single cap. Use 0.01% matched resistors plus a trimpot to trim. For the first two quadrants, the second output is high. For the third and fourth,, the first output just inverts the pattern and the second output goes low. It works well.Trimming for resistor matching is very easy. Just output an antiphase. 1kHz square wave from the ports and with an amplifier on the capacitor, adjust for zero AC output.Nicer to have a full wave solution, but I don't.

author: amspire
author_rank: Super Contributor
timestamp: May 06, 2017, 10:43:46 pm
content: Quote from: branadic on May 06, 2017, 09:58:34 pmI did a post on this using a small smd resistor and a small smd transistor glued together with Kapton layer as insulation. I used a diode junction as the temperature sensor. A miniature smd thermistor would have been preferable. I had to put it in a glass jar to remove air currents, but it did match the Fluke AC DC transfer standard to 0 01%. It is much slower then Fluke's sensor. It looked like it would work measuring the relative rate of change, rather then getting perfect static thermal matches.https://www.eevblog.com/forum/metrology/diy-precision-ac-rms-to-dc-transfer-standard/Richard.

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2021, 06:12:25 pm
content: I know it's quite an old thread, anyhow it fit's the topic.Did anyone ever tried to build an AC voltage standard himself?DC voltage references based on LTZ or LTFLU and the like have been demonstrated here over the last years. We also know there areFluke 510Aout there, with the related datasheetshereandhere, but also Fluke 5700 series calibators, quite expensive though.On the other hand audio hobbyist have demonstrated ultra low distortion oscillators e.g. designs based on AN67 or such asVictor's Ultra low distortion (<0.00001%) 1kHz sine generatoralso described in more detail inAn ultra low distortion oscillator with THD below -140 dBorLow distortion oscillator tests measurement circuits.So it comes to mind to combine such an oscillator with a voltage reference such as LTZ for amplitude regulation (AGC), thus similar to what is done in Fluke 510A.Is there any experience on that topic available here?-branadic-

author: 1audio
author_rank: Frequent Contributor
timestamp: July 06, 2021, 07:29:14 pm
content: Interesting- I have both Victor's oscillators and a 510. (And an Optimation AC calibrator). I'll try to set up the Victor oscillator to see how stable they are. They use TL431's for a reference and a less sophisticated AC sense for regulation. The distortion is extremely low. Not so sure on the level stability.I think it may be possible to use a current generation DAC chip with a good reference but it might need temperature stabilization to get decent output stability. I now have two calibrated Fluke 931A's so I can see what stability is possible with some generic DAC's. I would hope its similar than the extensive complications in the all analog approach in the Optimation. Maybe use a dac + multiplying dac + stored correction table would address the stability/accuracy issues.

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2021, 08:17:47 pm
content: Fluke 510A uses LTFLU set to 14.14214 V for a 10 Vrms signal, so replacing the amplitude regulation of the formentioned Wien bridge oscillator and it's pot by a proper DC voltage reference could do the trick?-branadic-

author: TimFox
author_rank: Super Contributor
timestamp: July 06, 2021, 08:49:55 pm
content: I looked at the Apex datasheet for the SWR200DS precision sine generator. Two external capacitors set the frequency, the harmonic distortion of the sine wave is very low, and a precision chopper measures what I interpret as the average absolute value of the sine wave to compare against a Zener DC reference to drive the AGC circuit in the oscillator. Since the harmonic distortion is low (typ 0.1% above 2 kHz without loading, the spec sheet does not specify this very well), the ratio between the mean absolute value ("average responding" on your old -hp- voltmeters) and the root-mean-square voltage is well-defined. Initial accuracy is specified as 0.5%, but the output is more stable than that. The manufacturer claims that the chopper circuit tends to compensate for the temperature variation of the reference zener.I didn't bother to look up the price.One could try using a carefully-designed "precision absolute value" circuit, offset-adjusted and calibrated against a reversible DC source for use at mid-audio frequencies to drive the AGC in a low-distortion analog oscillator.

author: 1audio
author_rank: Frequent Contributor
timestamp: July 07, 2021, 03:45:37 am
content: The apex looks quite interesting however the datasheet circuit doesn't make sense on the chopper/dc reference connections. I think additional filtering on the low pass filter on the output of the chopper will lower the distortion. Any idea how much for the part?I don't have the circuit for Victor's oscillator handy but I'll check it soon to see how its reference is done.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2021, 03:13:51 pm
content: The Wien bridge (or similar) oscillator and then amplitude feedback is a common technique. For the amplitude there are a few methods to choose. Sync rectifier, precision full / half wave rectifier, peak detection and maybe a thermal converter. With low distorion one is free to choose a method different from RMS and still get a stable amplitude. Just for stability the distortion would not even be that critical for stability. It would be however to also get a good DC/AC transfer and also the use with variable frequency.Filtering the ouput of the amplitude detector is tricky, as it is inside a control loop and thus needs to take care about loop stability, not just ripple suppression. The control side of the amplitde regulation is also nonlinear and frequency dependent. So the control loop can be a bit tricky.

author: TimFox
author_rank: Super Contributor
timestamp: July 07, 2021, 03:34:19 pm
content: I own two Krohn-Hite audio generators that directly generate in-phase and quadrature outputs. The AGC feedback uses a sample-hold on one output triggered by the zero-crossing of the other to get a DC output with low ripple.

author: branadic
author_rank: Super Contributor
timestamp: August 09, 2021, 07:55:51 am
content: QuoteDid you already manage to perform such measurement?I was thinking about a similar measurement on Victor's 1 kHz oscillator with Datron 4000A as source for the amplitude regulation.-branadic-

author: Jester
author_rank: Frequent Contributor
timestamp: August 09, 2021, 10:58:36 am
content: RFL and then Clarke-Hess made an ac/dc voltage standard targeted for calibration of low digit DMMs. 0-1000V. I picked up a non working one up on eBay for dirt cheap and fixed it. Really handy addition to the bench. The repair was a bit tricky.https://testequipment.center/Products/Clarke-Hess-828

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2021, 05:01:48 am
content: Quote from: Kleinstein on July 07, 2021, 03:13:51 pmAnd since AC voltmeters are using the same methods, it isn't like something better or unavailable is needed.On the subject of precision rectifiers, the open loop gain element can be AC coupled at the input *and* output to remove offset voltage.Quote from: TimFox on July 07, 2021, 03:34:19 pmI have thought about doing the same thing for calibrating the output of a pulse generator in place of precision resistor networks.