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."
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?
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?
I agree, that scopes don't agree nicely with DMM measurements. I just view them as guidelines/approximations.
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.QuoteI 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.
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.
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.
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.
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.
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?
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.
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
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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:
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!
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'.
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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.Quote
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.