Affordable Useful AC & DC references

[Majorassburn]

It was suggested by other forum participants that I start a new thread to discuss specs and features of AC & DC voltage references that I make and sell on eBay (as SQWARREL. Sample Link https://www.ebay.com/itm/285789548856) where I kept things brief and non-specific. So, here is my attempt to do so. I appreciate your input and suggestions as this thread (and my device improvements) develop.

My intent is to make and sell very low-priced but USEFUL references for those of us who are not yet ready to resort to costly lab calibration. We may want to be able to perform basic, "ballpark" function checks of our DMM's to see if they're even working correctly or reasonably holding calibration.  And, we can't justify spending $150 or more for higher quality devices like the DMMCheck+ and others.

AC Voltage Reference:  Here's a device that generates a relatively pure, NON-FLUCTUATING, adjustable sine wave from 0 to 6VACrms @ 100Hz (fixed frequency) for those who do not have a quality signal generator or alternative. (100Hz was chosen as a DMM-friendly "sweet spot" for avoiding EMI-influence in the working environment.)

Remember, you can't do credible AC comparisons or checks by using isolation/step-down transformers, variacs or by sticking your probes into a mains socket because the line voltage is always jumping around and the sine waves don't look anything like sine waves because of all the noise and distortion.

The AC references can be pre-calibrated (buyer's choice) to between 100mV and 6VACrms @ 100Hz +/- 0.5% to cover near-full-scale readings of most popular DMM's, requiring only a single 12VDC regulated power supply or battery because they have an on-PCB ICL7662 inverter for the -12V rail.

There can be several variations (and prices) for such a device including lower sine wave amplitudes which allow using single supply op-amps and 9V batteries, etc., etc., but I would rather focus on minimum performance SPECIFICATIONS for device USEFULNESS at this time.

DC Voltage Reference: Rather than the untrustworthy cheap references, many using crappy components and USED AD586 parts and phony calibration stickers, I have developed a straightforward 10VDC reference based upon the LT1236 chip.

This DC reference currently uses a 14VAC 0.5A wall-wart transformer or other suitable 14-16VAC power supply to the input jack leading to an on-PCB 15VDC regulated power supply that then feeds the LT1236 circuitry. The power input can also be 18-24VDC through the same input jack and that also offers autopolarity protection due to the full wave bridge rectifier at the input on-PCB.

The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.

The DC reference trimming is done with the SDM3055 by powering on both for 24-48 hours before final trimming to 10.0000 which is as good as it gets until we buy a DMM6500 or similar in the near future.

However, after trimming one of the 10V references 2 months ago and keeping it powered on, and after re-checking the SDM3055 vs DMM6500 readings weekly, the 10VDC reference STILL reads exactly 10.0000, spot on, if that's worth anything.

Now, I realize that this whole discussion doesn't meet any volt-nut standards but, remember why we're here in the first place, as I set forth in my introduction.

I simply would like to improve the SPECIFICATIONS and USEFULNESS of these devices while keeping them low-cost to give the average DMM owner an alternative to spending hundreds of dollars and more on devices that may be of higher quality and accuracy but are of no more usefulness.

Thanks for any insight and help that comes to mind.  A few pix attached for general reference...
 
Modify message

[schmitt trigger]

Does it have distortion specs?

[alm]

The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.

The DC reference trimming is done with the SDM3055 by powering on both for 24-48 hours before final trimming to 10.0000 which is as good as it gets until we buy a DMM6500 or similar in the near future.

However, after trimming one of the 10V references 2 months ago and keeping it powered on, and after re-checking the SDM3055 vs DMM6500 readings weekly, the 10VDC reference STILL reads exactly 10.0000, spot on, if that's worth anything.

I would try to translate this to an adjustment uncertainty specification and long term drift specification (though measuring on a 2 month time scale may require very accurate and stable equipment). Compare this to measuring the drift of continents on the time scale of a year vs a century. You need way more accurate equipment to measure low velocities by comparing the position at two points in time. You are not trying to measure drift, you are trying to set an upper bound like the drift over 2 months is less than 100 uV (which might translate in an upper bound of 600 uV/year). Compare this to a datasheet values.

For adjustment uncertainty. To me the "brand new SDM3055" says "suspect and potentially high drift". Generally voltage references start to settle in after some months of operation. And from what I read, Siglent may not be doing a very good job burning in their references before putting them in instruments. But with the weekly comparisons to the DMM6500 you should be able to get some measurement of stability over time. The uncertainty might look something like this:

• DMM6500 absolute uncertainty from datasheet (1 year or 90 days since calibration?)
• Uncertainty of transferring from DMM6500 to SDM3055 (noise, temperature fluctuations / coefficients for both meters, repeatability)
• 1 week stability of SDM3055 (you can estimate this from repeated comparisons with the DMM6500 taking into account the uncertainties of the DMM6500 and the comparison)
• Uncertainty of measuring the voltage reference with the SDM3055 (noise, temperature fluctuations / coefficients for DMM and reference, repeatability)

For the ones that are not normal distributions (like specs from a datasheet), you multiply them by the factor from The Guide to Uncertainties in Measurement (see page 13 for how to rectangular distributions, e.g min/max specs). Then you add them all up (linear is the most conservative), and multiply by a coverage factor k=2. This should give you a best estimate for the uncertainty of the reference just after adjustment. Probably not all components will contribute equally. Feel free to simplify and use conservative estimates for those.

Then using the same procedure at different points in time, you can calculate an upper bound to the drift based on the uncertainty of each measurements. I would show a table with uncertainty calculations so anyone who doubts the figures can verify.

Ideally you would build a small thermal chamber that would allow to to characterize the temperature coefficient of the circuits. Temperature and time are probably the major variables affecting the output of the standard. Humidity would be next, but is difficult to control and also difficult to measure to to the large time constants involved.

[bdunham7]

The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.

I typically think of the errors rather than the accuracy, so I avoid terms like 'spot-on'.  To me 'spot-on' just means you don't have enough resolution to see the errors.  It's like someone asking me to machine something with a dimension of "one inch".  So I ask "to what tolerance?".  They reply "I want it exactly one inch".   I spend a week setting up my most precise measuring tools and another week lapping the surfaces so that they are 1 inch apart to a precision of 0.00001", the best I can do.  Customer comes in to pick it up, pulls out a measuring tape and says "yep, looks like exactly one inch". 

That's not a true story, of course, but it is sort of based on one.  The question I have first is about your 'comparison calibrating".  I'll presume you are not adjusting anything but rather just using a stable voltage source to take measurements with the two meters in parallel and comparing the results.  Correct me if that is wrong.  If that is the way you are doing it, are you saying that your SDM3055 always matches the DMM6500 exactly to the last digit?  And then your Fluke 87V matches the SDM3055 to the last digit, each and every time?  If so, that is truly exceptional.  If not, my recommendation would be to start by logging and stating the errors in each of those steps. 

[Majorassburn]

The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.

I typically think of the errors rather than the accuracy, so I avoid terms like 'spot-on'.  To me 'spot-on' just means you don't have enough resolution to see the errors. 

-snip-

If that is the way you are doing it, are you saying that your SDM3055 always matches the DMM6500 exactly to the last digit?  And then your Fluke 87V matches the SDM3055 to the last digit, each and every time?  If so, that is truly exceptional.

Both good questions.  First part, the display of "10.0000" on the SDM3055 is the limit of its resolution. So, all further errors (and there must be some) are masked.  However, four zeroes is probably as good as it's going to get with a $25 reference with today's technology.

More importantly, four zeroes is certainly adequate enough for the purposes outlined in my intro although certainly not anywhere near good enough for verifying the calibration of a 34465A or DMM6500.

The second point, is also a good question.  So, far the "comparison calibration" method I referred to is simply attaching both the DMM6500 and the SDM3055 to the same 10V reference (after warmup) and reading the displays. No intervention or actual calibration. The SDM3055 agrees with the DMM6500 to four zeroes. I have NOT recorded any further errors or deviation of the 10V reference revealed by the DMM6500's extra resolution during these comparisons.

As for the Fluke 87V in the photo, it agrees with the SDM3055 only SOME of the time because it's running in Hi-Res mode where the offset fluctuates +/- 1LSD. It can read anywhere from 9.999 to 10.001 with 10.000 most of the time. That's not good enough but it is a curiosity.

I'll buy either a 34465A or DMM6500 if these reference devices prove salable. If not, I'll stick with the SDM3055 for awhile as long as I have access to the new DMM6500.

So, considering that these are affordable, low-cost references, I think accuracy to four zeroes is adequate for the purposes which they are designed to fulfil. Maybe I'm missing something but offering free periodic re-cals of the 10V & AC references once further detailed specs are developed along the lines that poster ALM suggests above, might keep buyers happy and provide the value that is intended for the prices paid.

What I'm getting at is developing a spec for example, the 10V reference, that says something like, trimmed to four zeroes, plus or minus 2 LSD's over a 6 month period? Maybe?

[bdunham7]

What I'm getting at is developing a spec for example, the 10V reference, that says something like, trimmed to four zeroes, plus or minus 2 LSD's over a 6 month period? Maybe?

2 LSDs means 20ppm in this case.  If your device can meet that spec over 6 months under specified conditions, that would make it pretty good.  If they are that good, then a DMM6500 would be a good investment because you would be able to clearly see the errors, instead of just looking at that last digit and wondering if it is going to flicker or not.  My main suggestion, agreeing with alm, would be that you work on characterizing a set of them--at least 10--over a period of at least six months and over at least a 10C temperature swing.  You should also at least record the RH.  My second suggestion is that you change the IC socket and use this:

https://www.mouser.com/ProductDetail/Mill-Max/110-13-308-41-001000?qs=WZeyYeqMOWeYjIS4tXLt7Q%3D%3D

I know your BOM is tight, but I think you may eventually have issues with the style of socket you are using now.

[Majorassburn]

What I'm getting at is developing a spec for example, the 10V reference, that says something like, trimmed to four zeroes, plus or minus 2 LSD's over a 6 month period? Maybe?

My second suggestion is that you change the IC socket and use this:
https://www.mouser.com/ProductDetail/Mill-Max/110-13-308-41-001000?qs=WZeyYeqMOWeYjIS4tXLt7Q%3D%3D
I know your BOM is tight, but I think you may eventually have issues with the style of socket you are using now.

HA! I already have those great sockets. Gold flashed, low insertion force, beryllium copper spring retention, low profile and great K effects,  However, I opted to use the cheap tin plated plastic sockets because of the similarity of the plating on the leads of the IC and the sockets themselves possibly reducing K effects.

Somewhere, I read about the Nobility of Metals stuff. I like the construction quality of the round gold sockets much better but what do you think about the dissimilarity of the IC pin plating to socket junction effects?

[shapirus]

If you're going to make at least 5-10 of these references, why not design an actual PCB and order some at a fab house?

It will likely not be more expensive than the breadboards I see in the pictures, and it will definitely require way less work to assemble and will be less error-prone, not to mention professional appearance (especially if you pay a little extra for black solder mask lol), which may improve sales.

If you design a PCB, you can add holes/slots for inserting (with or without soldering) a metal shield that, in combination with the ground plane on the other side of the board, will cover the reference chip and all the signal traces to reduce noise in the output signal.

[bdunham7]

Somewhere, I read about the Nobility of Metals stuff. I like the construction quality of the round gold sockets much better but what do you think about the dissimilarity of the IC pin plating to socket junction effects?

My guess is that thermal effects will be minimal, mostly because the temperatures will pretty quickly even out and also because when you add them up they'll mostly cancel.  But this is something you should determine experimentally.

[schmitt trigger]

Asking again; does it have distortion specs?

A “relative pure sinewave” is not a spec.

[Majorassburn]

Asking again; does it have distortion specs?

A “relative pure sinewave” is not a spec.

Sorry, I missed your question earlier....
1) Where did you see "relative pure sine wave" ?  I don't recall using that term in the AC Reference description.
2) Distortion is less than 10%. Depending on the designs I've used (phase shift, dual integrator, etc. and the type of components and amplitude designed for, it can be less than 1%. The newest designs have about 1-2%.

When designing these references for DMM checking, the frequency, stability and amplitude are much more important than a few percent distortion. These are fixed, low frequency sine wave generators and they aren't aimed for audio use. I have a line of  low distortion 1KHz-20Khz sine gens for that.

How about you guys with much more sophisticated measuring setups buy a few of these AC & DC references and actually put them to the test? I would make a special price of 50% of my cost to you and after you evaluate and report, I'll give you a free sample from the first production run. Partners anyone? 

[J-R]

Some thoughts:
- Have a look at the sites of Doug/Russ/Ian for ideas of the information that might be worth adding to your listings: https://voltagestandard.com | https://dmmcheckplus.com | https://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-2-mini-detail
- A real PCB with a revision number seems critical.
- Ship some free units out for reviewers to check out, but talk to them first.  (mjlorton just came back after a year off and may be looking for stuff, dunno)
- Protect the trim pots or lock them in place with some adhesive or etc.
- Enclosure option?

[Kean]

Sorry, I missed your question earlier....
1) Where did you see "relative pure sine wave" ?  I don't recall using that term in the AC Reference description.

Your top post, third paragraph.

AC Voltage Reference:  Here's a device that generates a relatively pure, NON-FLUCTUATING, adjustable sine wave from 0 to 6VACrms @ 100Hz (fixed frequency) for those who do not have a quality signal generator or alternative.

[EC8010]

When designing these references for DMM checking, the frequency, stability and amplitude are much more important than a few percent distortion.

I'm afraid you're wrong there; you do need low distortion. If you look at mains voltage, it has flat tops (caused by capacitor input supplies) and a true-RMS meter gives a very different answer to a mean reading meter calibrated RMS of sine wave. Total harmonic distortion on mains with those flat tops is typically 3%.

[IanJ]

You need a proper Pcb if you are manufacturing to sell IMHO. Here's some points:

- Looks more professional, better branding.
- Better control/elimination of noise induced by crosstalk etc.
- Better control/tracking over batches & Cal certs using Serial No's / Pcb versions.
- Much faster to manufacturer.
- Your sales will go up.

Also,
- The test instruments you use to calibrate need to be an order of magnitude or better resolution/stability of the device you are trying to Cal, and have their own traceable Cal standard test certs.

Hope that helps.

Ian.

[Majorassburn]

Wow!  I want to say Thank You to all who have made suggestions and offered advice in this thread. This is a great forum and a great learning tool. I love it because of all you guys who take time to weigh in on topics like this.

As you know, I'm in the early stages of developing a few very low-cost products to see if they are even salable and useful to low-end DMM & Scope buyers (hobbyists, DIY, Homeowner, Students, Field Repair, etc.).  I saw that this explosively growing market had a gap of available, affordable (very low cost) but still useful "ballpark" references to do some function testing of their low cost devices.

These typical low-cost devices do not warrant professional calibration and, in most cases, have no calibration adjustments, anyway. So, for those low-end buyers who don't already have high-quality DMM's, etc. to compare to, an inexpensive reference device might be the only way to gain a little confidence in what their Low-end device is telling them. So, the gap.

We would all probably agree that a DMM Check+ and the like would be overkill for most of that low-end market so I would make no attempt to duplicate or impinge upon the typical DMM Check+ buyer/user demographic. I'm strictly at the very low end with specific focus on the most common basic functional tests.

Why all this preamble?  Because I appreciate all your inputs.  So, please don't think I'm ignoring your recommendations if I don't address them individually or implement them right away. You guys are helping me LEARN and grow. My dilemma is keeping the sale price LOW while providing VALUE to the buyer. It's a tough balance. Your input helps me move towards achieving those goals, while on a strict budget.

Thanks again.
Major

[Majorassburn]

Sorry, I missed your question earlier....
1) Where did you see "relative pure sine wave" ?  I don't recall using that term in the AC Reference description.

Your top post, third paragraph.

AC Voltage Reference:  Here's a device that generates a relatively pure, NON-FLUCTUATING, adjustable sine wave from 0 to 6VACrms @ 100Hz (fixed frequency) for those who do not have a quality signal generator or alternative.

Thanks for proving that my age is creeping up on me! 
My reply should have said that I never used that type of term in a device description or ad.
Of course, in this thread, my statement that my AC reference generates a "relatively pure sine wave" was meant for forum discussion. What does "relatively pure" mean to me? Almost, but not quite perfect, in shape. 

[shapirus]

What does "relatively pure" mean to me? Almost, but not quite perfect, in shape. 

You should be able to characterize that using a sound card and some software spectrum analyzer.

[Majorassburn]

Asking again; does it have distortion specs?

A “relative pure sinewave” is not a spec.

Sorry, I missed your question earlier....
1) Where did you see "relative pure sine wave" ?  I don't recall using that term in the AC Reference description.
2) Distortion is less than 10%. Depending on the designs I've used (phase shift, dual integrator, etc. and the type of components and amplitude designed for, it can be less than 1%. The newest designs have about 1-2%.

When designing these references for DMM checking, the frequency, stability and amplitude are much more important than a few percent distortion. These are fixed, low frequency sine wave generators and they aren't aimed for audio use. I have a line of  low distortion 1KHz-20Khz sine gens for that.

How about you guys with much more sophisticated measuring setups buy a few of these AC & DC references and actually put them to the test? I would make a special price of 50% of my cost to you and after you evaluate and report, I'll give you a free sample from the first production run. Partners anyone? 

I want to re-address, clarify and correct my reply:
1) All AC references that I have advertised and sold on eBay in the past year have output pure sine waves  at fixed frequencies at controlled amplitudes, as each sold device indicated.
2) Distortion specifications for all such outputs are: Less Than 1%. (My previous reply had typos and brain-farts in it  ) Below photo.
3) The most important benefit (in my mind) of owning one of my AC references is that it outputs a STEADY, NON-FLUCTUATING, NON-NOISY, NON-DISTORTED, SAFE sine wave that you can actually measure with a high degree of accuracy, AS OPPOSED TO the more common method of trying to guess at any derivative of measuring dirty, unstable mains voltages (like direct probes or step-down transformers, variacs, etc.). That's if you don't have a nice signal generator.

[Majorassburn]

When designing these references for DMM checking, the frequency, stability and amplitude are much more important than a few percent distortion.

I'm afraid you're wrong there; you do need low distortion. If you look at mains voltage, it has flat tops (caused by capacitor input supplies) and a true-RMS meter gives a very different answer to a mean reading meter calibrated RMS of sine wave. Total harmonic distortion on mains with those flat tops is typically 3%.

I agree with you but disagree with you??? 
Yes, the mains are very dirty and distorted. About the only thing you can depend on is that the frequency is usually very close while voltage fluctuates and so-called sine waves are flat-topped and almost unrecognizable due to the influences that you pointed out.

BUT: When using one of my AC references at 6VACrms at 100Hz, with less than 1% distortion of the sine wave output, the frequency, stability and amplitude are much more important than sub-1% distortion concerns. And, the typical audio and RFI distortion concerns aren't really relevant when checking an under-$100 DMM with a 100Hz, 6V sine wave as long as the sine wave is a sine wave.

You won't even be thinking about distortion if the amplitude and frequency of the sine waves are jumping all over the place, right?

Please check the attached photo and let me know what you think of that compared to sticking your probes into a mains socket.

[bdunham7]

2) Distortion specifications for all such outputs are: Less Than 1%.

How are you measuring that?  You probably won't see 1% harmonic distortion on a scope trace.  1% might matter if you were intercomparing TRMS and average-responding meters.  For TRMS to TRMS comparisons, it might not matter very much especially at the handheld level of precision.

[Majorassburn]

Just an aside....

With a source of a precise, 50% duty cycle square wave, 12.000VDC @ 100Hz, crest factor of 1, in theory, that should equate to a precise 6VACrms display on a properly calibrated true-RMS meter, right.

Interestingly, the display on our new Siglent SDM3055 shows "6.0004" but on all of our less-than-1-year-old Fluke 87's the display is "6.011 to 6.013" on their 6V range (They have about 10% overrange capability).

It is my understanding that they both use similar TRMS conversion protocols. Comments?

[bdunham7]

With a source of a precise, 50% duty cycle square wave, 12.000VDC @ 100Hz, crest factor of 1, in theory, that should equate to a precise 6VACrms display on a properly calibrated true-RMS meter, right.

Interestingly, the display on our new Siglent SDM3055 shows "6.0004" but on all of our less-than-1-year-old Fluke 87's the display is "6.011 to 6.013" on their 6V range (They have about 10% overrange capability).

It is my understanding that they both use similar TRMS conversion protocols. Comments?

I believe they both use AD chips, but different versions.  What is the "precise" source of your sqware wave and how precise is it specfied to be?

A TRMS meter should do a reasonable--but not perfect--job in this instance.  Again, I concentrate on the source of errors and there are some additional sources of error when using a non-sinusoid as your test stimulus.  The specifications for the Fluke 87V allow for up to 64 counts of error (1% + 4 counts) for signals within 1kHz BW and then 2% for signals above 1kHz BW.  To begin any analysis of the errors involved with non-sinusoid signals, calculate the contribution of the harmonics beyond the 1kHz point, in this case the 11th harmonic and up.  Or just Google it. 

[Majorassburn]

With a source of a precise, 50% duty cycle square wave, 12.000VDC @ 100Hz, crest factor of 1, in theory, that should equate to a precise 6VACrms display on a properly calibrated true-RMS meter, right.

Interestingly, the display on our new Siglent SDM3055 shows "6.0004" but on all of our less-than-1-year-old Fluke 87's the display is "6.011 to 6.013" on their 6V range (They have about 10% overrange capability).

It is my understanding that they both use similar TRMS conversion protocols. Comments?

I believe they both use AD chips, but different versions.  What is the "precise" source of your sqware wave and how precise is it specfied to be?

A TRMS meter should do a reasonable--but not perfect--job in this instance.  Again, I concentrate on the source of errors and there are some additional sources of error when using a non-sinusoid as your test stimulus.  The specifications for the Fluke 87V allow for up to 64 counts of error (1% + 4 counts) for signals within 1kHz BW and then 2% for signals above 1kHz BW.  To begin any analysis of the errors involved with non-sinusoid signals, calculate the contribution of the harmonics beyond the 1kHz point, in this case the 11th harmonic and up.  Or just Google it.

That was also my understanding that they both use similar AD chips. Of course, the Fluke 87V's are all well within specs. In fact, they appear to be within 0.25% rather than the currently published 0.7% in the User Manual (as compared to the 1% in the Cal Manual) according to my calculations. What caught my curiosity is that all 8 of the 87V errors agree so closely. That tells me it is by design.

I tested a few of the Flukes on sine waves rather than square waves and their errors decreased by about 50%. No big deal, just a heads up that there is a possible benefit to sourcing sine vs. square waves when checking TRMS DMM's.

[ebastler]

I saw that this explosively growing market had a gap of available, affordable (very low cost) but still useful "ballpark" references to do some function testing of their low cost devices.

Frankly, I don't see that market need. I already have a "ballpark reference", namely the digital multimeter I bought. If I were to spend time and money to obtain another reference on top of that, I would want something a bit more definitive.

"I have always wondered whether I can trust my meter. Nevermore! Now I wonder whether I can trust my cheap homebrew voltage reference."

[Majorassburn]

I saw that this explosively growing market had a gap of available, affordable (very low cost) but still useful "ballpark" references to do some function testing of their low cost devices.

Frankly, I don't see that market need. I already have a "ballpark reference", namely the digital multimeter I bought. If I were to spend time and money to obtain another reference on top of that, I would want something a bit more definitive.

"I have always wondered whether I can trust my meter. Nevermore! Now I wonder whether I can trust my cheap homebrew voltage reference."

Funny. But, there are 30+ eBay buyers who would disagree with you. Check my feedback.

[EC8010]

My point was that oscilloscopes are very poor for revealing distortion and that by the time you can see distortion on an oscilloscope (such as flat topped mains), it can be pretty bad, and that some distortions (flat topped mains) cause significantly different measurements between different methods of measurement. Sadly, just switching between a sine wave and a square wave does not reveal all the measurement errors. I found a Rigol oscilloscope that measured cycle RMS acceptably when switched between square and sine, but when I varied the mark/space ratio it got it wrong. Clearly, its internal calculations were wrong, yet it's not hard to calculate cycle RMS perfectly for any waveform provided that your sampled measurements are correct. When I attempted to use it to measure noise it was hopeless - a factor of three out if I remember correctly. To sum up, you need lowish distortion (<0.1%) to be certain that the errors of your generator are insignificant compared to the meter to be tested, and no oscilloscope is good enough to verify that. (Despite their puff, most oscilloscopes are roughly 8 Effective Number Of Bits, and that's because it's good enough.)

By the way, your oscilloscope trace doubled the errors achievable because you only exercised half of its ADC's range, throwing away a bit of resolution. And even as shown, it's not a smooth sine wave - it has visible steps.

There are no excuses in test and measurement. If you want to calibrate even a 3 1/2 digit DMM, then you need something ten times better, so you can't verify your tester with an oscilloscope. You either need your tester to be better by design or verified by something better. A recording quality soundcard is a very good start.

[Majorassburn]

My point was that oscilloscopes are very poor for revealing distortion and that by the time you can see distortion on an oscilloscope (such as flat topped mains), it can be pretty bad, and that some distortions (flat topped mains) cause significantly different measurements between different methods of measurement. <snip>

By the way, your oscilloscope trace doubled the errors achievable because you only exercised half of its ADC's range, throwing away a bit of resolution. And even as shown, it's not a smooth sine wave - it has visible steps.

There are no excuses in test and measurement. If you want to calibrate even a 3 1/2 digit DMM, then you need something ten times better, so you can't verify your tester with an oscilloscope. You either need your tester to be better by design or verified by something better. A recording quality soundcard is a very good start.

Here's some better resolution for you. See any flat tops or bottoms or weirdly shaped curves other than what looks like perfectly shaped sine waves? The "steps" you see are from the digital oscilloscope's low resolution screen recreating & displaying the sine wave. BTW, I didn't quote distortion specs in my ads based upon viewing an oscilloscope screen. I agree, that scopes don't agree nicely with DMM measurements. I just view them as guidelines/approximations.

[shapirus]

Here's some better resolution for you. See any flat tops or bottoms or weirdly shaped curves other than what looks like perfectly shaped sine waves?

Even if you don't see them, it doesn't mean that the wave isn't distorted and there are no harmonics. It looks good, but we still can't say how actually good it is without a proper spectrum analysis.

[ArdWar]

Here an easy test, can you tell whether the bottom waveform is a pure sine wave or not?

[tggzzz]

My point was that oscilloscopes are very poor for revealing distortion and that by the time you can see distortion on an oscilloscope (such as flat topped mains), it can be pretty bad, and that some distortions (flat topped mains) cause significantly different measurements between different methods of measurement. <snip>

By the way, your oscilloscope trace doubled the errors achievable because you only exercised half of its ADC's range, throwing away a bit of resolution. And even as shown, it's not a smooth sine wave - it has visible steps.

There are no excuses in test and measurement. If you want to calibrate even a 3 1/2 digit DMM, then you need something ten times better, so you can't verify your tester with an oscilloscope. You either need your tester to be better by design or verified by something better. A recording quality soundcard is a very good start.

Here's some better resolution for you. See any flat tops or bottoms or weirdly shaped curves other than what looks like perfectly shaped sine waves?

That's as convincing as putting a voltmeter on a battery and asking whether the displayed voltage is wierd.

I agree, that scopes don't agree nicely with DMM measurements. I just view them as guidelines/approximations.

If the instruments are in cal, they should "agree". That's "agree" within the limits of their specification, of course.

Anybody selling calibration tools realluy ought to know that instinctively. It shouldn't need to be said.

[Majorassburn]

My point was that oscilloscopes are very poor for revealing distortion and that by the time you can see distortion on an oscilloscope (such as flat topped mains), it can be pretty bad, and that some distortions (flat topped mains) cause significantly different measurements between different methods of measurement. <snip>

By the way, your oscilloscope trace doubled the errors achievable because you only exercised half of its ADC's range, throwing away a bit of resolution. And even as shown, it's not a smooth sine wave - it has visible steps.

There are no excuses in test and measurement. If you want to calibrate even a 3 1/2 digit DMM, then you need something ten times better, so you can't verify your tester with an oscilloscope. You either need your tester to be better by design or verified by something better. A recording quality soundcard is a very good start.

Here's some better resolution for you. See any flat tops or bottoms or weirdly shaped curves other than what looks like perfectly shaped sine waves?

That's as convincing as putting a voltmeter on a battery and asking whether the displayed voltage is wierd.

I agree, that scopes don't agree nicely with DMM measurements. I just view them as guidelines/approximations.

If the instruments are in cal, they should "agree". That's "agree" within the limits of their specification, of course.

Anybody selling calibration tools realluy ought to know that instinctively. It shouldn't need to be said.

OK, Guys. Like most threads, the helpful responses tend to occur in the first page(s) or so. After that, the latecomers, nay-sayers and perpetual-post-pessimists and nit-pickers seem to weigh in and dredge up all kinds of minutia and picayune reasons that everything discussed prior to their late arrival is either worthless, misstated, or wrought with technical shortcomings. Enough.
If I listened to that kind of unhelpful crap, I'd never produce a product.

I'm appreciative and grateful for the input from those who have reached out with constructive criticism and helpful suggestions, so.....

Here's my preliminary spec for the AC Reference Module:
Adjustable Sine Wave Output 0-6VACrms @ 100Hz, into 10MegOhm Non-capacitive Load, Less Than 1% THD, 6 Month Accuracy 0.5%, 76 Degrees F, 60% Humidity, Free Periodic Re-Cals.

Here's my preliminary spec for the 10VDC Reference Module:
10VDC, Trimmed to 10.0000 +/- 2LSD, into 10MegOhm Non-capacitive Load, 6 Month Accuracy 0.1%, 76 Degrees F, 60% Humidity, Free Periodic Re-Cals, All Additional Specs Per LT1236-10 Data Sheet.

How's that for a start? Suggestions? Questions? Comments? 

Remember, these are $20 or so devices aimed at non-professional folks who own low-priced test equipment and not the DMMCheck crowd.

Thanks in advance.

[bdunham7]

My point was that oscilloscopes are very poor for revealing distortion and that by the time you can see distortion on an oscilloscope (such as flat topped mains), it can be pretty bad, and that some distortions (flat topped mains) cause significantly different measurements between different methods of measurement.

In practical terms, discussing actual devices that might reasonably be used with the sort of reference mentioned here,  I think that distortions not visible on an oscilloscope trace are unlikely to cause significant errors when comparing meters that use a TRMS conversion system.  This is assuming a fundamental of 100Hz as shown.  Things would change if one of the meters was a reasonably accurate average-responding system.  Unfortunately I don't have time or bench space to spare at the moment, but a simple experiment with an AWG could show this quite easily.

[bdunham7]

If I listened to that kind of unhelpful crap, I'd never produce a product.

Here's my preliminary spec for the AC Reference Module:
Adjustable Sine Wave Output 0-6VACrms @ 100Hz, into 10MegOhm Non-capacitive Load, Less Than 1% THD, 6 Month Accuracy 0.5%, 76 Degrees F, 60% Humidity, Free Periodic Re-Cals.

You should be able to put the "unhelpful crap" into perspective with mathematical arguments or experimental demonstrations.  For example, how much difference between TRMS meters might you see with a signal that has 1% THD?  (probably not much) Also, which harmonics are present in your signal?  (It matters, possibly a lot) 

As for your AC specs, I would observe that 10M "non-capacitive" is probably a grossly insufficent spec for an AC source since many meters have much lower impedances, like 1M + 150pF.  You'd need to know your output source impedance (it's probably much, much lower than 1M) and figure out what errors you might have with various loads.  You might also want to explain how you're substantiating the 1% THD spec and the 6-month accuracy spec.  If the answer is "it's just my best guess, but hey they're cheap", well I suppose that'll have to do.

[Majorassburn]

My point was that oscilloscopes are very poor for revealing distortion and that by the time you can see distortion on an oscilloscope (such as flat topped mains), it can be pretty bad, and that some distortions (flat topped mains) cause significantly different measurements between different methods of measurement.

In practical terms, discussing actual devices that might reasonably be used with the sort of reference mentioned here,  I think that distortions not visible on an oscilloscope trace are unlikely to cause significant errors when comparing meters that use a TRMS conversion system.  This is assuming a fundamental of 100Hz as shown.  Things would change if one of the meters was a reasonably accurate average-responding system.  Unfortunately I don't have time or bench space to spare at the moment, but a simple experiment with an AWG could show this quite easily.

Thank You for putting this distortion discussion in such practical terms. You nailed it.

[Majorassburn]

If I listened to that kind of unhelpful crap, I'd never produce a product.

Here's my preliminary spec for the AC Reference Module:
Adjustable Sine Wave Output 0-6VACrms @ 100Hz, into 10MegOhm Non-capacitive Load, Less Than 1% THD, 6 Month Accuracy 0.5%, 76 Degrees F, 60% Humidity, Free Periodic Re-Cals.

You should be able to put the "unhelpful crap" into perspective with mathematical arguments or experimental demonstrations.  For example, how much difference between TRMS meters might you see with a signal that has 1% THD?  (probably not much) Also, which harmonics are present in your signal?  (It matters, possibly a lot) 

As for your AC specs, I would observe that 10M "non-capacitive" is probably a grossly insufficent spec for an AC source since many meters have much lower impedances, like 1M + 150pF.  You'd need to know your output source impedance (it's probably much, much lower than 1M) and figure out what errors you might have with various loads.  You might also want to explain how you're substantiating the 1% THD spec and the 6-month accuracy spec.  If the answer is "it's just my best guess, but hey they're cheap", well I suppose that'll have to do.

Excellent points.
1) Thank You. My source impedance is an Op-Amp 20mA short-circuit protected output, 10uF capacitor-coupled to a 10KOhm, 20-turn Cermet pot to Ground, center wiper tap. So, plenty of juice to play with at 1megOhm. I will change all of my specs to "into a 1MegOhm + 150pF Load" because it is also inclusive of most handheld and bench DMM's as well as scopes.
2)  Thank You Again. The 6-month accuracy spec is by actual testing units for almost 2 years. In fact, the tested units are actually well within a 0.25% spec. since power-cycled and constantly powered during that period. Of course, they vary with ambient temp so it is hard to be more exact and that's why I opened it up to 0.5%.

As for the 1% THD, that's my experimenter's best guess, having studied many app notes and circuitry specs.. I have a hunch that it might be much lower than 1%. But for what these references are designed to do, 1% is plenty adequate. I'll let someone with a $10,000 Spectrum Analyzer prove me wrong. 

Your last comment about the harmonics has me confused. With substantial output low-pass filtering built in to my modules, I didn't think harmonic content would matter much based upon the suppression provided by the LP filtering.    Comments?

[CatalinaWOW]

I actually agree there is a need for the type of product you are trying to provide.

Here is a relatively off the wall suggestion.  There has been much discussion of the cheap Chinese references.  Several people have found them both stable (I know that requires definition, but remember the thread we are on) and with outputs matching the attached sticker.  I am one of those.  But there is clearly wide variation in the product as indicated by a large number of other reports.

Some of the negative comments against these references are actually not well founded.  A used LM399 is not necessarily a pejorative, but an indication of a well aged part.

It seems there might be a business opportunity in providing a screening service for these devices.  With your existing equipment you could screen them to hobby levels of precision.  Whether this would be profitable would depend on two factors.  First, the yield of "good" devices.  And second, the marketability of the screened devices given the reputation these things have developed.  The latter would be positively affected if you provided a traceability trail for your results.   This doesn't necessarily involve a formal calibration, but would require you to analyze the tests you have performed to "confidence test" your equipment and provide a justifiable set of error bars on your results.  This analysis would be beneficial to you regardless of whether you take on this screening activity.

[bdunham7]

I have a hunch that it might be much lower than 1%. But for what these references are designed to do, 1% is plenty adequate. I'll let someone with a $10,000 Spectrum Analyzer prove me wrong. 

Your last comment about the harmonics has me confused. With substantial output low-pass filtering built in to my modules, I didn't think harmonic content would matter much based upon the suppression provided by the LP filtering.    Comments?

It wouldn't be that difficult to measure your THD to the levels you need with a PC soundcard input or a decent oscilloscope with FFT.  No SA needed and in fact most won't even work for this purpose.  I wouldn't take a victory lap yet because I would think that a good portion of your customers might actually have average-responding meters where even the lower harmonics matter.

The harmonic content matters because some TRMS meters have very low BW--1kHz or even less in some cases--and if a substantial portion of your THD is out-of-BW harmonics, then the resulting error will be larger.  Put another way, I'd expect different TRMS converters to respond to distortion that is within their specified BW in much the same way, but if the distortion is out-of-BW for one of them then they would differ.

[Kleinstein]

I don't see the distortion that critical. The accuracy of the cheap meters is not that high and they should still respond reasonable with a slightly distorted sine. The BW is usually up to at least 1 kHz - just the specs may not be for a higher frequency. The upper limit for the specs is different from a -3 dB bandwidth limit.  Part of the limitation can be a slew rate limit and thus not an issue for moderate low harmonic content, but a big issue with the square wave.

It would still make sense to check the distortion to have some relible specs, e.g. with a sound card. If one wants to use the scope, that in combination with a notch filter to suppress the base frequency. This is the old style method.

Driving the output directly from an OP-amp can be an issue. The current is sufficient, but op-amps don't like driving capacitive loads. E.g.  150 pF (1.5  m of cable or a DMM input)  can be enough to make an TL07x wired as a buffer oscillate. Even those OP-amps that are specified to drive quite some capacitance are not not really happy about capacitive loading.

[Majorassburn]

I don't see the distortion that critical. 
<SNIPPED>
Driving the output directly from an OP-amp can be an issue. The current is sufficient, but op-amps don't like driving capacitive loads. E.g.  150 pF (1.5  m of cable or a DMM input)  can be enough to make an TL07x wired as a buffer oscillate. Even those OP-amps that are specified to drive quite some capacitance are not not really happy about capacitive loading.

Thank you for your input.  Attached is a simplified schematic showing the output driving a capacitor. No oscillations have been observed in prototypes yet. Comments/suggestions?
Also attached is a TI Note regarding Op Amp compensation for driving capacitive loads. Such a method doesn't seem necessary at my low frequency unless I'm missing something?

[miro123]

I am a little bit confused.
1. From one side , the TI application note clearly defined two components - Cf and Riso. But I don't see it them back in your circuit.

2. Another issue with your circuit is output potmeter creates two problem - your output impedance extremely high for any calibration instruments.
What is your attenuation driving nominal specified capacitive load ? And what about specified ppms?
3. What is the transfer function vs temperature oof LPF - 5,1K /1u. How this applies to AC accuracy tests
4. What about the polarity of capacitors.
5. What about stability of  initially set of potentiometer ratio?

Can you provide the specified accuracy spec within 6months? - if you take into account my remarks 2, 3, 4 and 5

[Kleinstein]

In the last circuit the ouput is not directly driven by the OP-amp. Except for 100% of the amplitude the pot at the output will isolate the external load from the OP-amp. However the resistance from the pot will also add to the output impedance and the resistance is thus depending on the amplitude setting.  This is not good at all. In AC mode some DMMs are one 1 M input resistance and there upt to 5 K of output resistance would be a detectable error of up to 0.5%.

The more normal way for AC outputs is to have a 50 ohm output impedance from a resistor after the amplifier. 50 ohms would be OK with a 1 M input impedance meter, except for the high end and if constant one can still correct for this.
The need to isolate the amplifier from capacitive load does not depend on the signal frequency, but the choice (faster one tend to be more sensitive) of the amplifier.

The oscillator is a square wave followed by a relatively simple low pass filter. With LM358 OP-amps the amplitude may not be very stable, as the ouput swing of the OP-amp is not very stable (e.g. it can depend on the temperature). Chances are also that the -12 V part may not be that stable. The LM358 also tends to produce quite some cross over distortion - often already visible at 100 Hz.

[Alex Nikitin]

As much as I like to do things in a purely old fashioned analogue way, an accurate, reasonably stable and inexpensive AC source is best to be made by digital synthesis with a microcontroller and a DAC. There are microcontroller chips made with internal DACs, band-gap references and even look-up tables for sinewave generation. All you need to add is an output buffer/filter and you'll get a reference AC source with better than 0.1% accuracy and stabiltiy.

Cheers

Alex

[Majorassburn]

As much as I like to do things in a purely old fashioned analogue way, an accurate, reasonably stable and inexpensive AC source is best to be made by digital synthesis with a microcontroller and a DAC. There are microcontroller chips made with internal DACs, band-gap references and even look-up tables for sinewave generation. All you need to add is an output buffer/filter and you'll get a reference AC source with better than 0.1% accuracy and stabiltiy.

Cheers

Alex

Can you suggest a few part numbers for us to look into? Thanks.

[Majorassburn]

I love this particular Metrology forum because I'm in the company of many highly talented electrical engineers and advanced hobbyists. Therefore, I learn.

However, I'd like to re-post here what I started out saying when I opened this thread:

"My intent is to make and sell very low-priced but USEFUL references for those of us who are not yet ready to resort to costly lab calibration. We may want to be able to perform basic, "ballpark" function checks of our DMM's to see if they're even working correctly or reasonably holding calibration.  And, we can't justify spending $150 or more for higher quality devices like the DMMCheck+ and others.

AC Voltage Reference:  Here's a device that generates a relatively pure, NON-FLUCTUATING, adjustable sine wave from 0 to 6VACrms @ 100Hz (fixed frequency) for those who do not have a quality signal generator or alternative. (100Hz was chosen as a DMM-friendly "sweet spot" for avoiding EMI-influence in the working environment.)"

I'm mainly a marketing guy. I realize that engineers and dedicated electronic theoreticians may not appreciate that but here's my philosophy regarding my original intent in this thread:

!)  A low priced electronic product's salability decreases proportionately to the degree of electronic quality and design sophistication built into it.

What I mean by that is, if the goddamned thing works well enough to satisfy the target market that it's aimed at, build it and sell it!  Continued attempts at perfecting it will lead to fewer and fewer sales! 

2)  You've all heard the old phrase "Build a better mousetrap and the world will beat a path to your door."

Well, I'm not building mousetraps and I don't want the world at my door, either! There are enough bill collectors and tax men there already! 

I'm trying to offer products that are affordable and useful and, by virtue of those two objectives, are necessarily filled with shortcomings of many kinds. My stuff will never appeal to knowledgeable, experienced and talented electrical engineers because those professionals exist on a much higher plane.

But, for the unsophisticated hobbyist, student, homeowner, DIYer and "electronically curious", my stuff is attractive and provides solid value to them. 

Just look at Chinese electronics today. They are, in some cases, vastly superior to American made products (the dwindling few) and are usually much less expensive to acquire. Are they perfect? Hell, no! BUT, that doesn't stop people from buying them in mass quantities, does it. 

Compare that to Keysight, Fluke, Tektronix, etc. and other truly high-quality brands and you will see that these superior brands and products are much LESS salable to the masses because they are truly outstanding in design, capability, accuracy, durability, etc., and MORE EXPENSIVE, right?

3)  If you want to sell a lot of stuff, you need to have just about the lowest selling price of any product in that category or very few people are going to buy it! That's an indisputable fact of consumer behavior in today's world, like it or not. Just look at stuff on eBay for proof!

And, the more you try to educate and inform the masses that your product is technically better by design, the more likely they will tune you out quickly and buy something that is presented more simply and is cheaper!  Today's low end buyers do not want to be confused by FACTS! 

Okay, I got that off my chest, so....

I really appreciate everybody's input to my various threads and the help that provides me as I try to balance my offering a low-cost alternative to the other calibration-checkers out there.

If anyone has more suggestions or input, please continue to weigh in with an eye towards what I reiterated above.   
Thanks again.
Major

[tggzzz]

...
I'm mainly a marketing guy. I realize that engineers and dedicated electronic theoreticians may not appreciate that but here's my philosophy regarding my original intent in this thread:

Several people have, belatedly, begun to smell that. It would have helped everybody if you had made that clear earlier.

What I mean by that is, if the goddamned thing works well enough to satisfy the target market that it's aimed at, build it and sell it! 

A key part of marketing is to give sufficient information to determine what a product won't do.

It would have helped if you did that explicitly, rather than leaving it to people to ask questions and make inferences.

We've all seen claims/patents for better mouse traps and apple corer/peelers. Almost all aren't any such thing.

[artag]

not to mention professional appearance (especially if you pay a little extra for black solder mask lol), which may improve sales.

I *despise* black solder mask. I want to see where the traces go, even if I'm not repairing it, or have the plots available. I'd regard it as 'wannabe-cool' which is most definitely not the same as 'professional'.

[shapirus]

I *despise* black solder mask. I want to see where the traces go, even if I'm not repairing it, or have the plots available. I'd regard it as 'wannabe-cool' which is most definitely not the same as 'professional'.

the remark regarding the black solder mask was obviously a (half) joke.

[Majorassburn]

...
I'm mainly a marketing guy. I realize that engineers and dedicated electronic theoreticians may not appreciate that but here's my philosophy regarding my original intent in this thread:

Several people have, belatedly, begun to smell that. It would have helped everybody if you had made that clear earlier.

What I mean by that is, if the goddamned thing works well enough to satisfy the target market that it's aimed at, build it and sell it! 

A key part of marketing is to give sufficient information to determine what a product won't do.

It would have helped if you did that explicitly, rather than leaving it to people to ask questions and make inferences.

We've all seen claims/patents for better mouse traps and apple corer/peelers. Almost all aren't any such thing.

Ya know, I've never been a fan of your arrogant, self-serving, hyper-critical posts in my own, and several other threads. Guys like you NEVER have anything good to say unless you're touting your own imagined success so, your remarks and criticisms and admitted paranoid suspicions as to where I'm coming from are all wasted hot air on me.

I've noted previously how your baselessly superior attitude tends to embarrass and chase away those participants who you deem to be less learned than you and, therefore, not worthy of your respect to at least be courteous toward their requests for help in some of your replies.

Do me a favor and put me on your ignore list and I'll do the same for you. That way, other participants won't have to witness your desperate attempts to throw a wet blanket on any more of my posts.

EDIT: deleted an asinine reference I made to his homeland. Sorry. Temper got the best of me for a moment.

[mawyatt]

Here's quick drawn schematic of a simple Wein-Bridge 100Hz (Wien or Wein) Oscillator using a simple gain limiting for amplitude stability, followed by 2nd Order Active LPF to reduce harmonic content. Bridge Oscillator should produce a respectable sine-wave output if not forced into excessive limiting, so LPF may not be required.

BOM for just electronic components should be between $0.34 and $1.56, so should suffice as a Cheap Sine-Wave AC source.

Anyway, we haven't built this, someone give it a try and would expect this to outperform the schematic shown earlier. If anyone builds this, please report back results.

In the meantime we are considering a very stable and accurate Sine-wave source (another completely different design) that has somewhat performance guaranteed by design rather than direct calibration (performance verified by indirect means). If interested (serious) contact us, we may move forward with this.

Edit 100Hz not 100KHz
 
Best,

[bdunham7]

What I mean by that is, if the goddamned thing works well enough to satisfy the target market that it's aimed at, build it and sell it!  Continued attempts at perfecting it will lead to fewer and fewer sales! 
If anyone has more suggestions or input, please continue to weigh in with an eye towards what I reiterated above.   

First, R&D can and should result in better performance and lower costs.  Both are reasonable goals for improvement.

Second, your concept of 'well enough to satisfy the target market' is a pretty modest goal simply because your target market seems to explicitly be users without the means to critically evaluate the product in any meaningful way.  They're relying on you!

[Majorassburn]

an island off the coast of a real continent, which island is known to be populated by many of corrupt morals, conversational hyperbole, achievemental dysfunction and an affinity for inbred and, in several cases, immoral so-called "royalty" admiration syndrome.

wtf have I just read.
does it really belong here?

It's all about his attitude toward forum participants. Take some time to read his posts! But, you are right. It doesn't belong here and I deleted it. Maybe you can too.

[Majorassburn]

What I mean by that is, if the goddamned thing works well enough to satisfy the target market that it's aimed at, build it and sell it!  Continued attempts at perfecting it will lead to fewer and fewer sales! 
If anyone has more suggestions or input, please continue to weigh in with an eye towards what I reiterated above.   

First, R&D can and should result in better performance and lower costs.  Both are reasonable goals for improvement.

Second, your concept of 'well enough to satisfy the target market' is a pretty modest goal simply because your target market seems to explicitly be users without the means to critically evaluate the product in any meaningful way.  They're relying on you!

I don't disagree with your comments at all.

I fully agree that your first comment on R&D and that it should be an ongoing effort. But, marketing a viable product in the meantime should not be prohibited until "all" R&D efforts have been exhausted.

Your second comment is 100% on target! Those buyers would be relying on my integrity and my products to do everything (and more) of what the ads say they will do. That is sacrosanct with me.

However, in the case of these low cost devices, there is no concomitant obligation on my part to educate the buyer as to how, why or where the buyer should go to achieve an even better result than that which the buyer will receive by using my devices.

It's all about value, which includes suitability for the purpose the device is designed for and functionality to provide what the ads claim the device will do. Of course, complete safety is intrinsic and of utmost importance.

The perpetual fault-finding and pursuit of the device's limitations in presenting a low cost device such as mine accomplishes nothing positive for the potential buyer (except not to buy) and nothing positive for folks like me that would like to offer a low cost alternative (albeit with limitations) to that which is already available at much higher cost.

I am not suggesting that product DEFECTS, which prevent the accomplishment of the purpose for which the device is designed, are acceptable in any way.

[schmitt trigger]

Here's quick drawn schematic of a simple Wein-Bridge 100Hz (Wien or Wein) Oscillator using a simple gain limiting for amplitude stability, followed by 2nd Order Active LPF to reduce harmonic content. Bridge Oscillator should produce a respectable sine-wave output if not forced into excessive limiting, so LPF may not be required.

BOM for just electronic components should be between $0.34 and $1.56, so should suffice as a Cheap Sine-Wave AC source.

Anyway, we haven't built this, someone give it a try and would expect this to outperform the schematic shown earlier. If anyone builds this, please report back results.


Best,

Which opamp part numbers would you suggest to employ? LM358, TL072? Other?

[mawyatt]

Here's quick drawn schematic of a simple Wein-Bridge 100Hz (Wien or Wein) Oscillator using a simple gain limiting for amplitude stability, followed by 2nd Order Active LPF to reduce harmonic content. Bridge Oscillator should produce a respectable sine-wave output if not forced into excessive limiting, so LPF may not be required.

BOM for just electronic components should be between $0.34 and $1.56, so should suffice as a Cheap Sine-Wave AC source.

Anyway, we haven't built this, someone give it a try and would expect this to outperform the schematic shown earlier. If anyone builds this, please report back results.

Best,

Which opamp part numbers would you suggest to employ? LM358, TL072? Other?

Anything should work, TL is a little better than LM358, both are dirt cheap!! We've just ran a sim (if interested) the 1.5K was changed to 2.2K 2.4K and back to back zeners (in series) seemed to also work a little better than diode bridge and single zener, and 1K changed to 100.

Edit: Actually both the back to back zeners and the bridge diodes with single zener work well in sims, 2.4K and 1K seem to produce a low distortion sine-wave at ~100Hz. Of course these values should be adjusted based upon FFT results to "see" the harmonics, and minimize such!! Note the zener isn't the 1N751A, couldn't find models for these, and we used a OP07 Op-Amp since it's in standard library of LTspice, and didn't want to fiddle around looking for the LM358 model.

Best,

[thermistor-guy]

...
However, after trimming one of the 10V references 2 months ago and keeping it powered on, and after re-checking the SDM3055 vs DMM6500 readings weekly, the 10VDC reference STILL reads exactly 10.0000, spot on, if that's worth anything.
...

That is a good result for a reference in a plastic package, in Florida.

Your wet season is coming up (May-Oct). It would be interesting to check how well the unit performs over the next six months,
with elevated humidity.

[Kleinstein]

The wien bridge oscillator can get a good sine quite well, but the amplitude stability is usually not that great. In the version with just the simple rectifier and zener to reduce the gain the amplitude will not be very stable amplitude. E.g. there are 2 diodes in series to the zener and the capacitor ratio will also effect the needed gain. So I would expect something on the order of -8 mV_pp/K  or close to -0.1%/K for the amplitude stabilty.

There are better ways to stabilize to amplitude, e.g. with an active rectifier or measureing the peak voltage and than actively regulate the oscillator gain. The LPF after the oscillator is only a thing to improve on the harmonics and this is no longer needed with a regulated (e.g. with a JFET) amplitude.

[mawyatt]

The wien bridge oscillator can get a good sine quite well, but the amplitude stability is usually not that great. In the version with just the simple rectifier and zener to reduce the gain the amplitude will not be very stable amplitude. E.g. there are 2 diodes in series to the zener and the capacitor ratio will also effect the needed gain. So I would expect something on the order of -8 mV_pp/K  or close to -0.1%/K for the amplitude stabilty.

There are better ways to stabilize to amplitude, e.g. with an active rectifier or measureing the peak voltage and than actively regulate the oscillator gain. The LPF after the oscillator is only a thing to improve on the harmonics and this is no longer needed with a regulated (e.g. with a JFET) amplitude.

This was an attempt to keep the parts count/cost same or lower than OP concept utilizing similar cheap components. Of course better amplitude stabilizing techniques are available, but back to back zeners is pretty simple!! BTW if one uses back to back 6.2~6.8 zeners, then TC of the zener is ~ 2mv/K in reverse breakdown and the other zener is in forward conduction is ~ -2mv/K (of course this all varies with current so just a very crude approximation), one might not expect the overall to be as bad as -8 mv/K.

If one is seeking a stable and low TC AC RMS measurement source, the technique mentioned below is hard to beat for simplicity/cost/stability/precision. A couple days ago we were checking our seven 6 1/2 digit DMMs (3-KS34465As, DMM6500, 2-AG/HP34401As & SDM3065X) and quickly verified the AC RMS readings, spot on as usual

https://www.eevblog.com/forum/testgear/ac-rms-dmm-tests/msg3940957/#msg3940957

Edit: Just updated with measurements 4/16/24

Best,

[PlainName]

I'm mainly a marketing guy. I realize that engineers and dedicated electronic theoreticians may not appreciate that [...]

Belatedly commenting, although I've been lurking from the start, so not exactly one of those late-coming naysayers

As a marketing chap I am surprised you don't see the benefit of having this on a pukka PCB. The first time I saw it in the buy/sell area I just passed straight over on the basis it was an amateur lash-up, and I suspect there will be a number of potential users who have similarly done so. IMO, making it look the part would be a better improvement than improving the specs so far as sales go.

Perhaps the problem is that you're really a marketing guy and don't have the apps, or experience, to create PCBs. If that's the case, the solution may be to ask someone to do them for you. They are not very complex and should be pretty simple to do. If you then order just five, you'll get a professional-looking PCB at less than a dollar, possibly half that if you happen upon a coupon or something. As others have noted, that would improve the reliability in both building and ongoing use, and make it quicker and simpler to assemble. And at similar, or lower, hard cost than the boards you're using.

[tatel]

...
I'm mainly a marketing guy. I realize that engineers and dedicated electronic theoreticians may not appreciate that but here's my philosophy regarding my original intent in this thread:

Several people have, belatedly, begun to smell that. It would have helped everybody if you had made that clear earlier.

What I mean by that is, if the goddamned thing works well enough to satisfy the target market that it's aimed at, build it and sell it! 

A key part of marketing is to give sufficient information to determine what a product won't do.

It would have helped if you did that explicitly, rather than leaving it to people to ask questions and make inferences.

We've all seen claims/patents for better mouse traps and apple corer/peelers. Almost all aren't any such thing.

Ya know, I've never been a fan of your arrogant, self-serving, hyper-critical posts in my own, and several other threads. Guys like you NEVER have anything good to say unless you're touting your own imagined success so, your remarks and criticisms and admitted paranoid suspicions as to where I'm coming from are all wasted hot air on me.

I've noted previously how your baselessly superior attitude tends to embarrass and chase away those participants who you deem to be less learned than you and, therefore, not worthy of your respect to at least be courteous toward their requests for help in some of your replies.

Do me a favor and put me on your ignore list and I'll do the same for you. That way, other participants won't have to witness your desperate attempts to throw a wet blanket on any more of my posts.

EDIT: deleted an asinine reference I made to his homeland. Sorry. Temper got the best of me for a moment.

I found there's quite a few people on this forum that seems to suffer from a mix of Tourette's and Asperger's syndromes. It also seems to me there's a good amount of unused testosterone. And lots of ego. I myself have fallen on that at least once. I have to say, you are just one more cat in that bag, sorry.

But, I have learned, if you post here, better you previously made sure you got a thick enough skin. Particularly if you are asking for advice, more over if you are in some way promoting any product, and still even more if you are asking for free advice to make your product better.

So yeah, tggzzz's comments can be not always the most adequated ones, but at least I have seen him stating he had been made aware he was wrong. That's something I have still not see coming from you. I'm sure you are able to simply state your goals, letting aside any aggresivity, probably originated from any perceived attack to the quality of your product. I for one would greatly appreciate that. Otherwise I'll stick with the people that isn't selling anything. just saying.

And, if there's still some unused testosterone getting the best of you, please go play some union football. Preferably with a "2" on your back. Even in a local league, that will use all of that testosterone, make any skin grow a lot thicker, put any ego exactly where it should be, and vanish any unneeded agresitivity.

[Majorassburn]

I'm mainly a marketing guy. I realize that engineers and dedicated electronic theoreticians may not appreciate that [...]

Belatedly commenting, although I've been lurking from the start, so not exactly one of those late-coming naysayers

As a marketing chap I am surprised you don't see the benefit of having this on a pukka PCB. The first time I saw it in the buy/sell area I just passed straight over on the basis it was an amateur lash-up, and I suspect there will be a number of potential users who have similarly done so. IMO, making it look the part would be a better improvement than improving the specs so far as sales go.

Perhaps the problem is that you're really a marketing guy and don't have the apps, or experience, to create PCBs. If that's the case, the solution may be to ask someone to do them for you. They are not very complex and should be pretty simple to do. If you then order just five, you'll get a professional-looking PCB at less than a dollar, possibly half that if you happen upon a coupon or something. As others have noted, that would improve the reliability in both building and ongoing use, and make it quicker and simpler to assemble. And at similar, or lower, hard cost than the boards you're using.

Thank You and I share your views completely. I have not yet ordered PCB's because the "final" designs for my devices are not yet finalized.    But, I WILL be using PCB's very soon.
BTW, I have been making (taping, etching, plating, drilling, punching, assembling, blah, blah) PCB's for more than 40 years and I certainly appreciate the requirement that these devices must be offered on professionally assembled PCB's in order to appeal to buyers like you and me.

[Majorassburn]

REPLY TO TATEL:

Thanks for the heads up.    I didn't quote your epistle because it was so lengthy. But, I hear you and you have offered me some valid criticism. I appreciate that.
Now, I have to go grow another layer of skin, so pardon my temporary absence.... 

[tggzzz]

So yeah, tggzzz's comments can be not always the most adequated ones, but at least I have seen him stating he had been made aware he was wrong. That's something I have still not see coming from you. I'm sure you are able to simply state your goals, letting aside any aggresivity, probably originated from any perceived attack to the quality of your product. I for one would greatly appreciate that. Otherwise I'll stick with the people that isn't selling anything. just saying.

Where I'm not reasonably sure of something, I'll either not comment or I'll add caveats. When I realise I've made a mistake, I don't try to revise history, and I do try to apologise. IMHO constructive conversations cannot occur without that.

I'm very tolerant of beginners asking information, provided they listen to the answers, and think. Watching someone's capabilities improve is a profound pleasure.

We all make mistakes; that's part of learning. However, as I taught my daughter, "let's make new mistakes".

I'm less tolerant of people who don't listen[3], or don't learn, or choose not to improve, or do who misrepresent other people's position[1], or who continue to misrepresent their product[2] (typically to ignorant managers).

I've spent a lifetime being plagued by salesman/companies who make impossible claims for their products. I know others feel the same!


[1] often in the form of chosing to omitting relevant context, or with strawman arguments

[2] e.g. if their product does what they claim, then they've solved the Byzantine General's problem or the split brain problem, or broken the laws of thermodynamics etc.

[3] possibly due to the Dunning-Krueger syndrome

[Gyro]

...
[3] possibly due to the Dunning-Krueger syndrome

Please stop spouting that bloody stupid term, it encourages copycat use on the forum that takes a while to die down (or at least has in the past). BTW, you spelt Kruger wrong.

[tggzzz]

...
[3] possibly due to the Dunning-Krueger syndrome

Please stop spouting that bloody stupid term, it encourages copycat use on the forum that takes a while to die down (or at least has in the past). BTW, you spelt Kruger wrong.

Looks like I did misspell it, probably not for the first time.

Since it seems you want a discussion (which you think will probably take a time to die down) about the D-K syndrome....

If you have a better shorthand term, let us know. If none, I will continue to use it when I think appropriate.

[Gyro]

About the only thing it means, by current thinking is that people tend to think that they are above average, and possibly that, in their eyes, incompetent people don't know enough to know how incompetent they are. Neither part seems useful in the current circumstances. What about 'competence bias' [Edit, as a less confrontational term].

[tggzzz]

I don't think it means that.

 "Competence bias" is a term that is too ambiguous. Additionally, the pejorative nature of the "D-K syndrome" term is useful in that it encourages critical examination of whether a person is sufficiently knowledgeable to express an opinion/statement.

No, I don't think all opinions are equally valid. (Does any engineer?)[1]
Yes, I do apply that to myself

[1] e.g. too many teachers confidently state that, using the normal laws of arithmetic, 1+2*3=9.

[Gyro]

Maybe you're not applying it to yourself quite enough  - Yes, I'm finding you a bit OTT these days too. Possibly a time for quiet reflection.

[tggzzz]

You chose to bring the subject up. That ensured it didn't die.

[Majorassburn]

So yeah, tggzzz's comments can be not always the most adequated ones, but at least I have seen him stating he had been made aware he was wrong. That's something I have still not see coming from you. I'm sure you are able to simply state your goals, letting aside any aggresivity, probably originated from any perceived attack to the quality of your product. I for one would greatly appreciate that. Otherwise I'll stick with the people that isn't selling anything. just saying.

Where I'm not reasonably sure of something, I'll either not comment or I'll add caveats. When I realise I've made a mistake, I don't try to revise history, and I do try to apologise. IMHO constructive conversations cannot occur without that.

I'm very tolerant of beginners asking information, provided they listen to the answers, and think. Watching someone's capabilities improve is a profound pleasure.

We all make mistakes; that's part of learning. However, as I taught my daughter, "let's make new mistakes".

I'm less tolerant of people who don't listen[3], or don't learn, or choose not to improve, or do who misrepresent other people's position[1], or who continue to misrepresent their product[2] (typically to ignorant managers).

I've spent a lifetime being plagued by salesman/companies who make impossible claims for their products. I know others feel the same!


[1] often in the form of chosing to omitting relevant context, or with strawman arguments

[2] e.g. if their product does what they claim, then they've solved the Byzantine General's problem or the split brain problem, or broken the laws of thermodynamics etc.

[3] possibly due to the Dunning-Krueger syndrome

TO TGGZZZ:  Please either contribute something sane, meaningful, helpful and relevant to this thread discussion or else take your thread-clogging demented crap to some other thread. Thank you.

[tggzzz]

So yeah, tggzzz's comments can be not always the most adequated ones, but at least I have seen him stating he had been made aware he was wrong. That's something I have still not see coming from you. I'm sure you are able to simply state your goals, letting aside any aggresivity, probably originated from any perceived attack to the quality of your product. I for one would greatly appreciate that. Otherwise I'll stick with the people that isn't selling anything. just saying.

Where I'm not reasonably sure of something, I'll either not comment or I'll add caveats. When I realise I've made a mistake, I don't try to revise history, and I do try to apologise. IMHO constructive conversations cannot occur without that.

I'm very tolerant of beginners asking information, provided they listen to the answers, and think. Watching someone's capabilities improve is a profound pleasure.

We all make mistakes; that's part of learning. However, as I taught my daughter, "let's make new mistakes".

I'm less tolerant of people who don't listen[3], or don't learn, or choose not to improve, or do who misrepresent other people's position[1], or who continue to misrepresent their product[2] (typically to ignorant managers).

I've spent a lifetime being plagued by salesman/companies who make impossible claims for their products. I know others feel the same!


[1] often in the form of chosing to omitting relevant context, or with strawman arguments

[2] e.g. if their product does what they claim, then they've solved the Byzantine General's problem or the split brain problem, or broken the laws of thermodynamics etc.

[3] possibly due to the Dunning-Krueger syndrome

TO TGGZZZ:  Please either contribute something sane, meaningful, helpful and relevant to this thread discussion or else take your thread-clogging demented crap to some other thread. Thank you.

Noted, so even if you "rewrite the history" of your own posts again, the trail won't disappear. For example...

...
EDIT: deleted an asinine reference I made to his homeland. Sorry. Temper got the best of me for a moment.

(Kudos is due for making the change explicit )

Hint: your not liking something doesn't make it invalid. The name of Al Gore's movie springs to mind: "An Inconvienient Truth".