DIY Precision AC-RMS to DC Transfer Standard - Page 1
Forum section: electronics / general

author: amspire
author_rank: Super Contributor
timestamp: April 16, 2016, 01:55:51 pm
content: I have been working on a simple precision DC to Sinewave converter - so if you enter 5V DC in, you get a very precise and very stable 5V AC out. Enter 10mV DC in and you get 10mV AC out.But then I started to think - How do you verify it is accurate?I have a Fluke 540B that can thermally compare the heat produced in a 100 ohm resistor by AC to the heat produces by DC and it has a 0.01% accuracy. The great thing about this solution is you only need to have an accurate DC standard, and you can use it for AC calibration as well.http://bama.edebris.com/manuals/fluke/540b/Can common parts get similar results?Thinking about it, I went for a 100 ohm 402 SMD load resistor glued to diode (a C-B junction of a SMD NPN transistor). The use of a diode as a temperature sensor allows two options - forward biased where you get about -2mV/C coefficient. This is good as we need to know the resistor temperature. My initial target is 100 degC so that I have a safe margin. The diode could also be used in reverse mode using the diode leakage current.The wires are 0.1mm enamelled copper - soldering wasn't much fun. Finding very fine wire is easy - old phone earpieces, small speakers, etc. Mine came from a dud 40mm speaker from a transistor radio.The initial results surprised me. Got the 100 Deg C target temperature at 3V - that is 12 times worse then the Fluke, but the settling time looked useable. I was getting the diode voltage going up and down by 50mV every few seconds. Then I realised that this was just air movement in a room. Stuck it in a jar and it became very stable - no fluctuations at all.Based on my very quick first tests, 0.01% should be achievable. The sensitivity means that 0.01% difference between the AC and DC corresponds to a 20uV DC voltage change. I am not concerned with the high power requirements of my sensor as we have something that Fluke did not have in the 1960's - we can easily get 100MHz+ opamps that should have a flat enough frequency response to get DC to 1kHz accuracies of 0.01% easily.

author: alanambrose
author_rank: Frequent Contributor
timestamp: April 16, 2016, 03:07:34 pm
content: Wow very neat

author: Mickle T.
author_rank: Frequent Contributor
timestamp: April 16, 2016, 03:29:52 pm
content: Nice work!There is another way to make a stable and predictable AC voltage standard via the calculable AC voltage reference.The Solutions : Calculable AC Voltage Referencehttps://www.ncsli.org/c/f/p13/REG_2013.PRE.1147.1876.pptDigitally Generated AC Reference Sourcehttp://www.transmille.net/Presentations/AC%20Reference/Digital%20AC%20Source.pdfGuildline Model 7410 AC Voltage Referencehttp://www.guildline.com/Datasheet/Guildline7410Datasheet.pdfSine Wave Generation Techniqueshttp://my.ece.ucsb.edu/York/Bobsclass/2C/Tutorials/App%20notes/an-263.pdfThis is my first try: simplified version of the 50 Hz - 1 MHz fixed voltages AC calibrator.

author: quarks
author_rank: Frequent Contributor
timestamp: April 16, 2016, 03:57:16 pm
content: Looks very interestingthanks for sharing

author: Vgkid
author_rank: Super Contributor
timestamp: April 16, 2016, 05:27:14 pm
content: Very interesting, I know in a LT app note they use a dual matched thermistor. That was for higher frequencies, many Mhz.Subscribing to this topic.

author: Gyro
author_rank: Super Contributor
timestamp: April 16, 2016, 08:31:29 pm
content: I tried making one of these a few years back. I've just had a look and I can't see it, I'll post photos if I can find it.Dual heaters and sensors are the way to go, you want excellent isolation between the two andslightlylower isolation between each and ambient.I used a sandwich of two aluminium plates and two sheets of expanded polystyrene with the heater/sensors squeezed between them. I used 1206 resistors (200R) glued to the back of BAS16s (SOT-23). Connections were very fine 'magnet' wire (loosely twisted) inputs from one side, outputs from the other.Although I was only using an LM358, results were pretty good. Zero and tracking were ok, but I got horrible overshoot (hadn't worked out the opamp time constants correctly). I dropped it in favor of some other project and never got back to it.I've been thinking of having another go (your post is well timed) and have purchased some 1k2 0.1% 1206 resistors and BAV99s. The idea is to sandwich the BAV99 between two paralleled 1k2s to make 600R in. The BAV99 gives two series diodes, so twice the temperature coefficient, and connections on the same side of the SOT23 so better isolation (snip the center pin).Two 100R resistors would make a 50R input, not sure about frequency performance, that would depend on the terminations (as I said, twisted pair in and out).You probably want to look at Jim Williams's App note Lin Tech AN22, on the subject, he was using it for LCD CCFL backlight inverter evaluation and was using the LT1088 thermal converter. (edit:There's also) a rather good write-up somewhere comparing it with the technology used in the the HP3400A RMS meter.http://cds.linear.com/docs/en/application-note/an22.pdf

author: Gyro
author_rank: Super Contributor
timestamp: April 16, 2016, 08:41:28 pm
content: Ah, here's the other App note, LT AN61:http://cds.linear.com/docs/en/application-note/an61fa.pdfThe Thermal RMS bit starts on page 16, and the HP3400a comparison in Appendix A, page 28.It's quite a catch-all app note... It's also got his avalanche pulse generator on page 21.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 16, 2016, 08:53:22 pm
content: I see the LT1088 is discontinued and nearly $40 on ebay from sellers with not particularly excellent feedback

author: Gyro
author_rank: Super Contributor
timestamp: April 16, 2016, 09:08:22 pm
content: Sadly yes, but most of the app note info is equally applicable to home made heater-diode pairs.Edit: ...and SO much cheaper to repair if you burn it out, you wouldn't want to say goodbye to $40 each time!

author: Gyro
author_rank: Super Contributor
timestamp: April 16, 2016, 09:38:39 pm
content: I've found my original and grubby 'thermal module'. It's looking pretty sad, I realized that the 'few years' are more like 20 odd. The wires have all got broken in the mean time but as I said, I'm planning to re-do the elements anyway.I hope it might be of some help anyway. Pictures attached.P.S. Looking at it now, it could probably be made rather more compact!

author: timb
author_rank: Super Contributor
timestamp: April 16, 2016, 10:00:52 pm
content: There's an older Jim Williams app note before the LT1088 that discusses the construction of a thermal RMS meter with a Yellow Springs dual thermistor package. The idea being use two of them, thermally insulated. Since it's a dual thermistor, one heats and the other senses the heat. That part is no longer made either, but the principle is sound.I'll see if I can find the app note. Jim Williams had an obsession with thermal RMS stuff, as it appears in several app notes (and VF/FV circuits, but that goes without saying). It always puzzled me why, after the LT1088 died (which was his baby) that he wrote a whole app note on a new non-thermal responding RMS chip LT made, and never once mentioned thermal responding meters. It was like the LT1088 never existed. Sad.

author: Vgkid
author_rank: Super Contributor
timestamp: April 16, 2016, 10:15:53 pm
content: http://www.linear.com/solutions/1378Not the app note per se, but the relevant circuit.

author: amspire
author_rank: Super Contributor
timestamp: April 16, 2016, 10:43:36 pm
content: Matched dual resistor-sensor pairs are great at getting a real time output and eliminating 99% of the hassles of a 540B-type transfer standard, but it is at a big hit to the accuracy. Metrology is all about confirming everything. You cannot just trust in data sheets and hope the design has no unknown flaws.The transfer standard approach is slow, and you need very stable DC and AC sources. You need the sources stable to better then 0.001% over a period of several minutes. The Fluke procedure requires that you repeat the measurement cycle 3 times and all of the measurements have to agree within the intended tolerence. Also at least one of these sources has to be adjustable in increments of at least 0.001%. My original DC to AC design was actually intended for this purpose. So if this sensor is a go, it will make a useful pair with the AC/DC source design.Additionally, with the Fluke sensors, you can start to get thermocouple-type EMF voltage problems. They have a resistor reversing switch, and basically if your 540B starts to give different readings for different sensor directions, your 540B is cooked. Probably have to include a reversing switch in any thermal-based design.There is also the quality of the resistor to consider. There might be a significant voltage coefficient, or at low frequencies, the resistor temperature may be varying within a cycle. With the resistor's temp-coefficient, this could mean the resistance varies with the cycle. Best way I can think of to detect these problems is to attach the resistor in series with a stable resistor (like a vishay foil resistor) to a low distortion AC source (0.0001% distortion or lower) and check the distortion across the resistor.I am going to try and do a test today with my prototype sensor and match it to the Fluke 540B.

author: amspire
author_rank: Super Contributor
timestamp: April 17, 2016, 01:33:03 am
content: Quote from: Mickle T. on April 16, 2016, 03:29:52 pmThat looks like a neat effort for a first try. I want one!My design design for an AC source is much less ambitious. It is a calculable AC source, but the maximum frequency will probably be 10KHz initially. I can go to 10Khz very simply, and above that, the project escalates.I hadn't thought about using an oven, but it makes sense as a way to get extra stability out of standard parts.Did you calibrate it to get the 10.0000V AC and 0.9999V AC on the meters, or did you calibrate the DC reference only, and it correctly generated the right AC volts?Also, do you have any numbers on output stability? I can see a number of metal packages on your boards and I am guessing you have used hermetic Vishay foil resistors. Very nice!Richard.

author: Mickle T.
author_rank: Frequent Contributor
timestamp: April 17, 2016, 06:07:23 am
content: I did the calibration on the AC meter only. Short-term stability of the output voltage is about 0.01% and limited by the temperature fluctuation in the oven (~0.3C). 10 V frequency response linearity is ~0.05% (100 Hz to 100 kHz), ~0.16% (100 kHz to 1 MHz) - measured via Datron 1071. All critical feedback resistors are hermetic S5-61 metal-foil.

author: amspire
author_rank: Super Contributor
timestamp: April 17, 2016, 08:42:56 am
content: I have some initial results. I have a very jury rigged setup, but I have exactly the same signal going to the Fluke 540B and my RMS sensor.The sinewave source signal was 1Khz at close to 3V RMS. The DC supply was also 3V.I couldn't precisely adjust the DC to the match the AC voltage. The Fluke indicated that the DC was lower by 0.021%.Using the forward biased junction of the diode at 1mA on my sensor, I got:Cold temperature: 0.731195VAC volts: 0.637657V (The higher the voltage, the lower the reading)DC Volts: 0.637684VIt seems that a 0.01% difference causes approximately a 20uV change. So with a 37uV change, that means that my sensor indicates the AC source was about 0.018% higher then the DC voltage. That is only a difference of about 0.003%.I will test some more frequencies, and I will add a 0.01% switch to the AC source (it will lower the source by 0.01%) along with a vernier adjustment, so I can get a better match between the DC and AC sources.So far, it is looking good. Interesting thing is my sensor was only running at about 47 deg C above ambient which is a bit low. I will boost it up to about 60 deg above ambient next time and it should respond a bit quicker.Richard.

author: amspire
author_rank: Super Contributor
timestamp: April 17, 2016, 11:51:10 am
content: Did some more test at different frequencies. Ran at a higher temperature, but didn't make it any better.I found that for the initial measurement, I really had to leave it for about 30 minutes. I think the air in the jar containing the sensor has to stabilize. The sensor only warms the air in the jar very slightly, but until the air warms the right amount, none of the readings are stable. A smaller jar would probably be quicker. After it stabilizes, it is not to bad.I had a lot of problems with my Sig gen stability (down at the 0.01% level) but these are the results for the difference between my sensor and the Fluke 540B:100 Hz1KHz10KHz100KHz1MHz+0.003%+0.0006%-0.0005%+0.01%+0.01%It will only ever be of use as a calibration tool, and unless the sensor can be put in a vacuum tube, it only works as long as you follow a strict procedure. Might put the procedure into a micro-controller and so it will force you to wait until it is ready to measure. Starting off with the DC and AC as correct as possible will make the measurement much faster.Before I go further, I definitely need to set up stable DC and AC sources

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2016, 12:08:43 pm
content: For temperature measurement there is no need to use a 1 mA current through the diode - that is already a lot of current and thus heating. So I would suggest more like 10 µA - this still gives an 2.5 K output impedance. As a side effect the TK also gets a little higher at lower current.To make the thing react faster you would need less isolation, e.g shorter wires. So smaller is faster. A higher temperature does not help very much. If it is only 45 C this means you could use even higher voltage, so a larger range.The design that Gyro showed actually look quite good: two cells to compare and a reasonably thin (and thus fast reacting) isolation layer. The time it takes to stabilize goes up with the square of the thickness - so it can easily get to much isolation and thus to slow. Also the aluminum frame gives similar conditions for both sides.

author: amspire
author_rank: Super Contributor
timestamp: April 17, 2016, 12:14:51 pm
content: Quote from: Mickle T. on April 17, 2016, 06:07:23 amThat sounds like you are hitting the limits of the Datron accuracy. Sounds like you don't actually know how good your AC calibrator is. It is as good as the Datron at least.

author: amspire
author_rank: Super Contributor
timestamp: April 17, 2016, 12:34:47 pm
content: Quote from: Kleinstein on April 17, 2016, 12:08:43 pmI tried lower currents, but they make the diode impedance higher and so there is more AC noise that gets though to the diode voltage detector circuit. Also, the time constant of the 1mA diode current heating is faster then other time constants, so it makes little difference.QuoteThe lengths of the wires make no difference, but the sensor has to be kept away from any thermal mass. The wire length may make a difference for RF.The way the Fluke 540B-type sensors work is they rely on the sensor radiating heat. If I could put my sensor in a vacuum inside a black aluminium tube, it would work brilliantly. I cannot do the vacuum, but the sensor may end up in a cheap torch tube.QuoteIt is a very interesting design, but it achieves a complete different goal. It makes a useable real-time wideband RMS voltage measuring device, that possibly could get to 0.1% accuracy with a lot of work. 0.1% would be extremely impressive. But very hard to use it as a precision calibration tool.The sensor I am testing looks like it can probably manage 0.001% accuracy that makes it a candidate for calibrating other devices to 0.01%. The thing about this kind of calibration is you only use it, say, once a year. For a hobbyist, taking 30 minutes per voltage range calibrated is not bad. It is not like you have to be watching it the whole time. Once you have calibrated your best meter, you use that meter to calibrate everything else. So I don't want to make this transfer standard complicated - the simpler the better.

author: Gyro
author_rank: Super Contributor
timestamp: April 17, 2016, 02:04:53 pm
content: Your results look very impressive!I can understand that you want to keep it as simple as possible, but wouldn't you benefit from some ambient temperature compensation, ie. another unheated transistor operating at the same current, to compensate ambient changes during your run?

author: amspire
author_rank: Super Contributor
timestamp: April 17, 2016, 02:26:25 pm
content: Quote from: Gyro on April 17, 2016, 02:04:53 pmAbsolutely! For the next stage, I need to make a new sensor - and it will have a second unpowered transistor/diode. I will probably even glue a resistor on the top so that it has an identical thermal mass. I think that Fluke do exactly that in the 540B as well - I seem to remember that there are actually two thermocouple sensors, but only one is powered.

author: Gyro
author_rank: Super Contributor
timestamp: April 17, 2016, 02:47:26 pm
content: Quote from: amspire on April 17, 2016, 02:26:25 pmThat sounds like a good idea - I must study the 540B documentation and try the same myself. I plan to re-make both resistor-diode elements, but there's no reason that I have to apply power to the second one.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 17, 2016, 04:22:17 pm
content: Quote from: amspire on April 17, 2016, 02:26:25 pmIs this essentially the same thing as a bolometer?

author: amspire
author_rank: Super Contributor
timestamp: April 17, 2016, 07:51:38 pm
content: I think a bolometer is for measuring electromagneticradiation.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2016, 08:33:44 pm
content: You can consider the wires are low frequency wave guides - so in this way AC current is low frequency radiation, just not free space but near field and bound to the cables as waveguides.

author: branadic
author_rank: Super Contributor
timestamp: February 27, 2018, 08:12:29 pm
content: Hi,I was linked to multijunction thermal converter (MJTC by NIST) by a member. When I searched for it, I found:New AC Voltage Devices: Factor of Three Improvement in UncertaintyAC-DC DifferenceThis looks pretty much like what is known to be a thermal mass flow sensor:So I wonder, has anyone experience with thermal mass flow sensor elements such as FS2 (IST AG)or MFS02 (IST AG)or a discrete assembly with a hf usable resistor and two NTCs on a polyimide foil as a ac transfer standard? I do have realized such a thermal mass flow sensor only a few days ago, but had no idea up to now, that similar assemblies are used to measure ac rms voltage.-branadic-

author: Vtile
author_rank: Super Contributor
timestamp: February 27, 2018, 08:47:30 pm
content: Have anyone tried to use something like LTZ1000A or LM399 as the OPs diode & resistor pair. They both have the resistor? (or is it something else) and diode (which is not even thermally stable).

author: Vgkid
author_rank: Super Contributor
timestamp: February 27, 2018, 08:58:41 pm
content: Quote from: Vtile on February 27, 2018, 08:47:30 pmHere is a lm399 being used as a anemometer.http://www.gellerlabs.com/MGTA%20kits.htm

author: branadic
author_rank: Super Contributor
timestamp: February 27, 2018, 08:59:57 pm
content: Quote from: Vtile on February 27, 2018, 08:47:30 pmSeems like you haven't read the linked publications or no idea of how a thermal mass flow sensor works, otherwise you would have understood that this is nothing different. The only question is, to which degree this works, as a thermal mass flow sensor element is not optimized for hf, but the other parameters like good thermal coupling between heater and temperature sensor as well as poor thermal conductivity are similar.You compare apples with oranges.-branadic-

author: Vtile
author_rank: Super Contributor
timestamp: February 27, 2018, 09:32:16 pm
content: Yes I'm not too familiar with the thermal mass converter, resistive element thermally connected to temperature sensor. Preferably weightless, purely linearly resistive, without thermal lag, fully isolated from sensor etc. Used for obtaining real RMS values. ( ..But like said no I'm not expert of the area and didn't read this time the white papers. Plus reading from phone at the time I should already considering to get some rest for next day.)Isn't those integrated oven references at some level just that especially if modified (case removed) atleast they have better thermal coupling and less mass (bare chip) than OPs neat resistor and smd BOC device.. Just asking around since interested without equipment.

author: ap
author_rank: Frequent Contributor
timestamp: February 28, 2018, 10:00:44 am
content: A thermal mass flow sensor (TMFS) and a TVC work on different principles. Moreover, a TMFS is not specified for AC behavior and also not very accurate.If you need a DIY solution for a precision AC transfer standard, I would recommend Nikkohm devices.

author: branadic
author_rank: Super Contributor
timestamp: February 28, 2018, 06:46:32 pm
content: Quote from: ap on February 28, 2018, 10:00:44 amWell, no! The principle is not that different, but I agree that a TMFS element is not optimized for the use as MJTC application and already stated that especially the heater is not designed for hf. However, I don't agree that they are not accurate, as the TMFS like a MJTC are only sensor elements, thus both need to be calibrated within the complete system-branadic-

author: 2N3055
author_rank: Super Contributor
timestamp: February 28, 2018, 08:33:31 pm
content: Quote from: ap on February 28, 2018, 10:00:44 amHello!Would you by any chance know where Nikkohm LP34TW could be ordered...?Thanks!!Regards,Sinisa

author: ap
author_rank: Frequent Contributor
timestamp: March 01, 2018, 01:19:46 pm
content: Under 'contacts' the distributers are listed on their website. Shoudl you get a quote, would you publish this here please. I did request a quote for the TVC some time ago, it was arround 400 IIRC. The LP34 should be less.

author: 2N3055
author_rank: Super Contributor
timestamp: March 07, 2018, 04:06:32 pm
content: Quote from: ap on March 01, 2018, 01:19:46 pmI did send a request to few distributors in EU.. I got quotes ranging from 310 USD to 605 € for one piece....

author: Moon Winx
author_rank: Regular Contributor
timestamp: March 07, 2018, 08:07:09 pm
content: By the way, you can buy one of those NIST thermal converters from directly from NIST for a relatively dirt-cheap price (~$2k). Those guys don't get out of bed for less than $2k so actually getting a NIST-cal'd MJTC for that price isn't a bad deal.https://www.nist.gov/sri/standard-reference-instruments/sri-6002-multi-junction-thermal-converter

author: ap
author_rank: Frequent Contributor
timestamp: March 07, 2018, 08:53:02 pm
content: Quote from: 2N3055 on March 07, 2018, 04:06:32 pmThat price is for the LP34?

author: 2N3055
author_rank: Super Contributor
timestamp: March 07, 2018, 09:12:13 pm
content: Quote from: ap on March 07, 2018, 08:53:02 pmYep, LP34TW 50 Ohm .. That's dual, bridge mode converter..Regards,Sinisa

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 07, 2018, 05:11:16 pm
content: I've got a 540B and it seems to work fine. My understanding was that each individual unit was supplied with a correction factor and they aren't inherently as perfect as you'd think. I don't have that number for mine. Does anybody know more about this?When making sensors you'd be surprised at how much heat will conduct down even quite fine copper wire. Use constantan or manganin instead. The slight resistance is probably less of an issue than the conductivity of copper.I always wanted to play with the LT part, but cost prevented it then and availability prevents it now. If there had been more demand it would have survived, so we have to assume the market is small.

author: branadic
author_rank: Super Contributor
timestamp: May 07, 2018, 05:39:13 pm
content: I have sent some assemblies to Frank for testing.One setup is build of NTCs (0402) directly mounted on heaters/resistors (0603) of different values, but also NTCs and resistors assembled on a foil substrate very close to each other. Looking for the first results that Frank provides.-branadic-

author: Qmavam
author_rank: Regular Contributor
timestamp: August 17, 2019, 06:13:15 pm
content: Has this topic died or has anyone built a circuit we can copy.I recently purchased two HP400E AC voltmeters, I was happy when I got thefirst one, then when I got the second one the readings don't agree, by 15%.I would like a cheap way to calibrate it up to 2Mhz, but 10 would be great.I'd be happy with an accurate AC to DC converter, type circuit, rather than an AC reference.Thanks, Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: August 17, 2019, 06:56:40 pm
content: OK, sense I revived this thread, let me see if I have any understanding.In the very first post the OP thermally connected a 100 ohm resistor and aa C-B junction of a SMD NPN transistor.Then he says, "forward biased where you get about -2mV/C coefficient"I would ask, How much current needs to flow in the C-B junction to measure the -2mV/C?And, it would seem it needs to be very low to prevent internal heating.So how is that -2mV/C being measured?I'm thinking I can't just set my DVM to diode test, apply 1Vdc, take a reading,and the then switch to 1Vac and they would read the same.I think they should, except for the DVM supplied current.Help me out guys.Mikekbtw, might it be better to use a 50 ohm resistor to properly load a sig-gen.Maybe sandwich the C-B junction between two 100ohms resistors in parallel.

author: 2N3055
author_rank: Super Contributor
timestamp: August 17, 2019, 07:19:25 pm
content: Read the post 5 and 6, there is an explanation..

author: Qmavam
author_rank: Regular Contributor
timestamp: August 17, 2019, 08:06:57 pm
content: Ok, I read 5 and 6. He used a LM358 as an amplifier.I see no mention of how much current he used as bias in the diode.Anyone have an idea, my concern is self heating of the diode,Am I just being over critical or is that a concern?Do I have the theory correct, in that I can measure a dc voltage with good accuracy, I apply 1Vdc across the100 ohm resistor and read the diode drop voltage. Then I put AC across the 100 ohm resistor,and adjust the amplitude until it reads the same as the 1Vdc did. Now I have 1Vac.I don't understand the 100*C target, is that a maximum temp?Also this "we can easily get 100MHz+ opamps that should have a flat enough frequency response to get DC to 1kHz accuracies of 0.01% easily."Why 100MHz opamp, aren't we amplifying a DC voltage from the C-B junction?Thanks, MikekPS. I want to calibrate an AC meter up to 10 MHz, or at least 2MHz. (HP400E) I bought two and they don't agree.I was happy when I only had one! :-)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 17, 2019, 08:43:53 pm
content: For using a diode temperature sensor, a common current level is at 0.1 mA, for local use less current, like 10 µA is possible to.For a diode the differential impedance is at some 26 mV/ current. So 100 µA would result in 260 Ohms - so already a low impedance signal.100 µA * 0.6 V gives a power level of 60 µW. This is not much compared to 1 V at 100 Ohms -> 10 mW.

author: dietert1
author_rank: Super Contributor
timestamp: August 17, 2019, 08:58:21 pm
content: About 30 years ago i made a converter that still exists. It is based on a 10 or 20 KOhm NTC in a constant resistance regulator bridge (stability by self heating). If you can get that bridge to balance using an outer oven, then you have a very sensitive power detector that works by compensation. I mean the regulator will reduce self heating as much as you apply external heat. Mine has a conversion formula on its label: P = U² * 33 uW/V². So it works up to about 1 mW.It may not be a precision device, though. It has a 50 Ohm resistor glued to the NTC for AC/DC conversion. Don't know where the schematic went. Nowadays one could make the outer oven with a digital PID regulator.Regards, Dieter

author: 2N3055
author_rank: Super Contributor
timestamp: August 17, 2019, 09:24:39 pm
content: Quote from: Qmavam on August 17, 2019, 08:06:57 pmYou're supposed to read both application notes by Jim Williams. All theory and schematics for LT1088 are directly applicable...

author: Qmavam
author_rank: Regular Contributor
timestamp: August 17, 2019, 10:28:38 pm
content: Quote from: 2N3055 on August 17, 2019, 09:24:39 pmOff to do that, warning, that will probably generate more questions. :-)Thanks, Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: August 17, 2019, 11:13:20 pm
content: Quote from: Kleinstein on August 17, 2019, 08:43:53 pmAfter reading part of AN22, I see Jim Williams circuit uses about 5 milliamps of bias on the LT1088.Now we are up to a number (3mW) of power compared to 10mW of RF power.Any thoughts about that?But, what if I try to measure a 4mv ac signal, (is that possible) now my power across a 50 ohm resistorwould be 0.32 µW .I'm beginning to think this might not work well to check the calibration on a 10mV range of a AC voltmeter.Any thoughts.Can any on calculate heat rise of 4mv across a 50 ohm SMD with styrfoam as the insulator.I'm going down fast :-(

author: Qmavam
author_rank: Regular Contributor
timestamp: August 18, 2019, 01:54:43 am
content: Defining my problem.I just bought two HP400E's AC voltmeters.I drove then with my metered HP651A, so all measurements are relative to the HP651A accuracy.I measured each range from 1mV to 3 V.Unit A reads 9% to 19%Low, Unit B reads 6% to 13%High.This make me serious about building an RMS to DC converter.I'm not a designer, but I can build from a schematic, with pretty good knowledge of concerns about parasitics.I think I could do a pretty decent job building the resistor/diode in a insulated package.My concern is, I have a need to measure 4mV between 500kHz and 2MHz.What is needed to measure that low? It doesn't create me heat energy.If I used a 50 ohm resistor, 4mV would only create 0.32microwatts at the diode.Is that enough to heat the diode, to the point where I could get an accurate reading.I may need a several ranges to get to 3 volts.But to start, I could build just a 0 to 10 mV unit, to do the 4mv measurement I working towards.Any help appreciated.Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: August 18, 2019, 03:05:10 am
content: Hey guys, would it make any sense to use an LM35 and lap the cover down to a point where you are very close to the internal diode andthen mount your resistor. It has 10mV/C* output so starting with a gain of 20 and an inversion.I've been looking for a mask or internal layout of the chip. no luck.Would it be better to lap the to front or back of a TO-92 case.I thought the 8 pin case might be easier to work with but it also would have more thermal mass.Mikek

author: amspire
author_rank: Super Contributor
timestamp: August 18, 2019, 04:04:37 am
content: Quote from: Qmavam on August 17, 2019, 08:06:57 pmIt doesn't matter how much heat the diode generates as the amount of heat is exactly the same in both the AC and DC phases of the measurement. The heat completely cancels out. You could use 1mA or 0.1mA - whatever you find works best.QuoteYes.QuoteIn theory, the higher the better. Trouble is that with higher temperatures, unwanted things can happen. The SMD transistor package may only be rated at 125 deg C. Bonding glues could start to change. Also, if the resistor gets too hot, the long term resistance could start changing. This is all about getting very high stability - at least for a few minutes. If you can run it at 200Deg C and it is stable, then it would be easier to do the AC/DC transfer.QuoteThe thermal transfer depends on heat, and unless you have both AC and DC sources that are stable with a fairly high current load, then you need to buffer the input. It is pretty common to see the load of a direct thermal AC/DC transfer to cause the output of a signal generator to drift too much for an accurate transfer.The thing is, you do not want the AC performance of the buffer causing an error. If you want to measure AC up to 1KHz, then a 1MHz op-amp will have far too much error. The output could be down by 0.1% at 1KHz. The faster the op-amp, the less likelihood of errors due to the op-amp. If you want to do a 0.01% transfer, then you need errors in the amplifier of much less then 0.01%. You need well over a 10MHz op-amp speed and that is just for 1KHz.Richard

author: Qmavam
author_rank: Regular Contributor
timestamp: August 18, 2019, 04:29:27 am
content: Quote from: amspire on August 18, 2019, 04:04:37 amI'm sorry, I still don't understand the 100MHz. As I see it we are only amplifying a DC signal and fairly slow changing DC signal.Where does the AC come from?Ask another way, are we concerned about the AC characteristics of an opamp, if we are only amplifying DC signals?Thanks, MikekOh maybe the answer was in the previous paragraph."The thermal transfer depends on heat, and unless you have both AC and DC sources that are stable with a fairly high current load, then you need to buffer the input. It is pretty common to see the load of a direct thermal AC/DC transfer to cause the output of a signal generator to drift too much for an accurate transfer."So, are you say we need to buffer the input RF voltage that we are measuring?If that is the case, I don't thank that is a concern for me, all I want to do is drive/heat a 50 ohm resistor, just the load my sig/gen asks for.Mikek

author: amspire
author_rank: Super Contributor
timestamp: August 18, 2019, 05:07:06 am
content: Quote from: Qmavam on August 18, 2019, 04:29:27 amI mentioned 1KHz. For that, you want about 100MHz as I explained. What AC frequency are you wanting to measure? If it is 50/60Hz, then you can use a slower 5MHz opamp.Anyway, I am not sure what the problem is - fast op-amps are not expensive.https://www.aliexpress.com/item/32844836421.html?If you put a buffer amp in, both the AC and DC have to go through the same buffer amp to attempt to cancel out the buffer amp errors. Do you understand that the output error on a 1KHz signal through a 1MHz opamp has an error of at least 0.1% in amplitude? The 1Mhz opamp only has a gain of 1000 at 1KHz. The AD812 chip I linked to has a gain of 1,000,000 at 1KHz.QuoteI am not sure who is mentioning RF voltages - the thermal transfer method can work great with RF, but there are many more potential errors.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 18, 2019, 02:17:52 pm
content: Your sentence makes me think we are approaching the problem from two different solutions.I'm looking a using the thermal method with this circuit. I do note that is a high bandwidth device,But I don't understand why when you are only amplifying DC. But, I'm sure there is a reason.The problem is there are no LT1088 available, so another thermal input device is needed.I'm open to another method, do you have a circuit.Mikek

author: 2N3055
author_rank: Super Contributor
timestamp: August 18, 2019, 02:46:54 pm
content: LT1088 has transistor pair thermally isolated from each other, each with its own thermally coupled resistor. That is exactly what Gyro (and others) made from discrete components.On one resistor you connect your unknown voltage (AC or DC, or any combination). Electronics senses change in Vbe of transistor and starts changing voltage on other resistor until its temperature sensor transistor has exactly same Vbe. At that moment, second resistor will have voltage on it that will be exact DC equivalent of RMS voltage on first resistor.You can connect measured voltage to input resistor directly, or you might need (most likely ) some kind of front end buffer/amplifier/attenuator in front of it.For low frequency bandwidth it might be enough to calibrate the input with DC.

author: chuckb
author_rank: Frequent Contributor
timestamp: August 18, 2019, 06:20:16 pm
content: The LT1088 is available on Ebay.If you have an oscilloscope, does it have a 5V square output for calibration of the probes? That output can be 1% accurate on some scopes. I would put a small RC (1 kohm and 0.01 uf) to filter the edges of the square wave so the energy is inside the bandwidth of your voltmeter. If your meter errors are the same with the HP651A and with the filtered scope output, then use the HP651A to calibrate the meters.If the Voltmeter error is constant on all ranges and frequencies they may just need the meter movement gain adjusted.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 18, 2019, 07:19:09 pm
content: Quote from: chuckb on August 18, 2019, 06:20:16 pmI'll look at that, I have Tektronix 2465 that may have a good calibrator.Thanks, MikekEdit:I tried that, I have not seen a calibrator like the 2465 has, it changes frequency with the horizontal time base.Anyway it is designed to be able to drive 50 ohms at 1/2 voltage or 1M ohm at full voltage. (0.2vpp or 0.4vPP) New info to me.So I set it up without you filter because that was easy. With a 50 ohm termination the meter read 0.119V RMSwith supposedly 0.2Vpp signal in. I thought, well that messed up, but, this is a squarewave and the HP400E is an average responding, RMS calibrated meter, Making me think, the average of a squarewave is 50% so 50% of 0.2V is 0.1V.Can I get a confirmation?So it reads 19% high. The other unit reads 0.109 or 9% high.I put the filter in and (I removed the 50 ohm termination) the reading dropped low, so I decreased the cap by a factor of 100, this just makes me think I can get any value by picking the right cap. So I dropped the filter idea. FWIW, the scope reads 4.3% high as far as my eye can tell.Not sure I learned anything, MikekI have sent out for calibration quotes.

author: chuckb
author_rank: Frequent Contributor
timestamp: August 19, 2019, 07:01:32 pm
content: Quote from: Qmavam on August 18, 2019, 07:19:09 pmI don't know what the HP400E will read with the square wave input. I glanced at the Tek Manual, It said the probe calibration output was to be used with 1ms / division.One path is -Calibrate the scope with x1 probes (you will be working with small signals), 1 Meg input Z.Use the scope to calibrate the HP651 signal generator. 1 Meg input ZUse the generator to calibrate your meters.

author: IconicPCB
author_rank: Super Contributor
timestamp: August 20, 2019, 11:18:33 am
content: a miniature dual filament incandescent bulb could be used as a basic building block... two such bulbs could form a half of a self balancing wheatstone bridge.

author: 2N3055
author_rank: Super Contributor
timestamp: August 21, 2019, 06:47:34 am
content: Let's go back to original problem: how to check whether HP400E is reading correctly.You need relatively stable signal source. That means signal generator. Chinese FY6900 or similar is plenty good for that. If you have any other signal generator that use that.You need a RMS reading multimeter. Any handheld (even cheap ones) will be precise enough to verify 400E reading.Use sinewave, 400E is not thermal (true RMS) instrument. It says RMS, but it is average reading with RMS calibrated scale.Get 400E manual on Internet. Read it. There is a performance verification procedure.Despite being cool and all, 400E is not really any better at measuring things than, say, UNI-T 60E.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 21, 2019, 01:46:14 pm
content: I did a search for UNI-T 60E specifications, and didn't find it.I found the 61 and it's spec's only say 10kHz bandwidth.Does the UNI-T 60E have a 10 Megahertz bandwidth?Mikek

author: 2N3055
author_rank: Super Contributor
timestamp: August 21, 2019, 02:09:11 pm
content: Quote from: Qmavam on August 21, 2019, 01:46:14 pmSorry, my bad, I missed 500KHz to 2 MHz in your first post.In that case what ChuckB said..

author: Qmavam
author_rank: Regular Contributor
timestamp: August 22, 2019, 03:28:05 am
content: I sent the HP400E in for calibration today, I will end up spending more to have the unit calibrated that I paid for it.But at least I will have something I can trust, for a while.Mikek

author: babysitter
author_rank: Frequent Contributor
timestamp: August 22, 2019, 05:32:08 pm
content: You made me thinking of using a laser cutter for decapping, with a high speed setting and low power, tough.Black epoxi could be simple to remove. shiny metal and silicone reflects the beam so not much energy will stick.... SiO2 isolation layers might burn up, at least it was simple to engrave glas. Hm, ok, nevermind.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: May 30, 2022, 02:24:50 pm
content: Re-reading the topic came up with an idea. We need a vacuum, a heater, a temperature meter. This can all be found in thermostated crystal oscillators. Inside the vacuum flask there is a quartz crystal with a sprayed heater. Measuring the frequency deviation is not difficult.Similar vacuum resonators were used in precision generators. And I think now it's not very difficult to find them

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2022, 03:51:13 pm
content: The heater for a OCXO is quite slow to react to the crystal. In addition the crystals used in precision gear tend to be low TC and not really good for a temperature measurement. In addition the heater may not be a low parasitics resistor to high frequencies. A vaccuum is not absolute needed, it is just a way to get good isolation.

author: mawyatt
author_rank: Super Contributor
timestamp: May 30, 2022, 06:00:21 pm
content: An effective method to create an accurate AC waveform from a known DC value is to create as close to an ideal squarewave as possible with the AC amplitude Vpp of the Vdc value and a fast rise/fall time relative to the period with a 50% duty cycle. This waveform has an average DC value and RMS value of Vdc/2.A simple CMOS Flip-Flop (74AC74) creates a very accurate 50% duty cycle at low frequencies and swings the unloaded output to VDD and VSS. Make VSS ground and VDD a reference voltage, say 5.000 Volts, then the AC output will be 2.500Vrms, and the average DC value 2.500Vdc. You can buffer the FF output for a lower output impedance with a CMOS driver or discrete CMOS buffer. This works really well as we've done this quite a few years ago as an in-house AC reference.Of course the waveform isn't perfect with finite rise and fall times, however, as long as the waveform period >> rise/fall time the error is small. If our analysis is correct the error is ~ -6*Trise/Tperiod, with equal rise and fall times assuming linear slopes, so a 10ns rise/fall in a 100Hz squarewave is just 6ppm low.Another source of error is the meter bandwidth in reading the squarewave harmonics and the ideal harmonics fall off as 1/N. Since a squarewave contains waveform energy in the harmonics, this energy is attenuated by the meter bandwidth. For example assuming a meter has an simple low pass characteristic then the effective noise-bandwidth NBW is pi/2 times the BW, then the error will be the squarewave frequency F times /(2*NBW), or F/(2*NBW). So a meter with a 1MHz BW and a 100Hz squarewave will read 100Hz/(pi*1MHz), or 32ppm low.Edit: Math error corrections!!Anyway, the squarewave is easy to generate accurately and can serve as a low frequency AC reference is many cases where the ultimate precision isn't required.Best,

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 02, 2022, 03:09:56 am
content: Curious if a different way has merit. We can build a low THD sine generator, 0.001% or better. It should be possible to control the gain of the oscillator by comparing the waveform peak to a DC reference voltage. I don't see why this couldn't be done to an extremely fine degree. Not sure how fast it could be done, but it eliminates the harmonic issues. If you know the peak and it's a near perfect sine, you know the RMS.

author: 1audio
author_rank: Frequent Contributor
timestamp: June 02, 2022, 04:56:50 am
content: You just described the Fluke 510.https://xdevs.com/doc/Fluke/510A/510A_AA_imeng0000.pdfSet the reference voltage to 14.1214V and you get 10V out.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2022, 06:09:35 am
content: Quote from: Conrad Hoffman on June 02, 2022, 03:09:56 amSome (could be many) of the calibrators produce there AC reference that way.Measuring the peak voltage is still not easy.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 02, 2022, 12:03:19 pm
content: The Fluke circuit seems excessively complicated, but that's coming from somebody who's never done it. Need to think about how to do it with modern parts/methods. It just seems like this method is more accessible to the average volt-nut than thermal. I have a 540 transfer standard and have never trusted it. Also takes too many voltages to run it. I think they also came with a correction sheet of some sort, that I don't have. Anybody have an example of that?

author: mawyatt
author_rank: Super Contributor
timestamp: June 02, 2022, 01:52:40 pm
content: Would think a good high resolution DAC could produce a low distortion, low frequency sine wave that has accurate amplitude if derived from a good reference.One could also extend this to sampling the peak either by an ADC or analog technique and compare to a fixed reference for feedback like Fluke did. Since the sinewave is created by the DAC the peak should be known and thus the sample point. If an ADC is utilized to resolve the peak it could be "offset" and expanded to a small region around the expected peak amplitude. Control loop speed isn't an issue and a SD ADC could be employed with high preamp gain around the expected peak voltage.Best,

author: dietert1
author_rank: Super Contributor
timestamp: June 02, 2022, 02:00:08 pm
content: A modern part with extremely small distortions would be a 24 Bit Audio ADC. One could run the left channel on the sine and the right channel on the chopped DC reference. Those ADCs are pretty cheap and sample up to several hundred KHz.Regards, Dieter

author: 1audio
author_rank: Frequent Contributor
timestamp: June 02, 2022, 04:42:34 pm
content: The Fluke 510 is an old design. I have one and tweaked it down to around -110 dB THD. However implementing a modern version with IC's would not be too difficult. You need a 2 phase oscillator (state variable is the typical implementation) and a sample and hold triggered by the zero crossing of one phase to capture the peak of the other phase. One clever implementation used an ADC to sample the peak. Sample and hold chips are mostly obsolete today since the ADC's incorporate them now. There are other ways to capture the peak voltage. The reference circuit looks like Flukes standard FLU practice so its complicated as well.

author: Andreas
author_rank: Super Contributor
timestamp: June 02, 2022, 07:19:09 pm
content: Quote from: dietert1 on June 02, 2022, 02:00:08 pmBut look at the gain error. It might be surprisingly high. (typical in the 3-6% range).with best regardsAndreas

author: dietert1
author_rank: Super Contributor
timestamp: June 02, 2022, 08:44:56 pm
content: Yes, the audio ADC can still be useful to compare/transfer a sine or any other AC waveform to that chopped DC reference mentioned above. One could use the same ADC channel with a MUX and/or avoid channel imbalance by calibration.Regards, Dieter

author: Verticon
author_rank: Contributor
timestamp: November 24, 2022, 06:46:32 pm
content: I just came across this thread these days and because I fancied with a DIY ac standard already a long time I wanted to know what can be realized quick and dirty by using existing lab equippment. The setup is based on a method which was already described in this thread. I tried to measure the peak values of a pure sine wave by comparing these with a well known DC value. I connected the output of my home made state variable oscillator (low source resistance output) with the input of a 7A22 Differential Amp in an old TEK scope but with a precision DC source in series. By changing the polarity of the DC source the positive or negative peak value of the sine wave can be compensated to zero by observing it on the scope. Switching the polarity is neccessary to correct for a small DC part of the oscillator (Offest voltage of the Output Amp). The DC values have been mesured with a 3478 (on DC very close to the 34401A) and the calculated AC voltage (add the two unsigned DC values and divide the result by the two-fold suare root of 2) was compared with the measured value by my freshley calibrated 34401A.I have to admit that I was a bit surprised about the good and reproducable results of this quick test. A typical result: For the 1V AC range I' ve got a deviation to the directly measured values from the 34401A of approx. 0,03% (at 110Hz). This is well below the 90day spec of the 34401A and in the range of the measuring uncertainty noted in my calibration certificate. The pictures show a sketch of the setup, the oscillator (distortion < 0,001%) and DC source and a typical curve on the scope display to get a feeling about noise (1mV/div scale). I am pretty sure with some more effort on the oscillator output configuration and better shielding the accuracy can be significantly improved.

author: 1audio
author_rank: Frequent Contributor
timestamp: November 29, 2022, 01:02:20 am
content: That s the method Krohn Hite described for checking their function generators.