Electronics > Metrology

An Experimental AC Voltage Calibrator

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REW has a good help section.  REW has many functions and options, so it can take some effort to read all the relevant descriptions, and defer reading about features that you initially won't be interested in trying.  You would typically use the Generator and RTA functions for manual observation, and use the Measure function for automated sweeps.  The Preferences function is the starting point for selecting the soundcard and driver (hopefully there is an ASIO driver available) and the soundcard channels you want to use as output and input.   The https://www.avnirvana.com/ website has an REW forum, and is also where the software is available for download.  Some forums have a strong use of REW and threads such as https://www.diyaudio.com/community/forums/software-tools.123/.  Googling well known soundcards (like EMU0404, and Focusrite 2i2) brings up posts/threads where the soundcard has been tested using REW (and others) and indicates how they have been used and setup.  Googling tutorial may also be worthwhile - I don't look at the youtube posts but I can imagine there are a few helpful ones.

The 2019 review link (https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-asus-xonar-u7-mkii-adc-dac-hp.8165/) indicates your card does have an ASIO driver available, but it sounds like there are issues, and the ASUS site only shows software up to 2016.  Hopefully you have a Mk2 as that has later software.  If you can get ASIO with operation at 192kHz at 24bit then that is great start.  Some soundcards are just fair performers, in which case you may need to spend time to characterise their performance for DAC and ADC paths to appreciate how to get a suitable performance for your application and to appreciate any limitations (eg. https://www.avnirvana.com/threads/measuring-frequency-response-and-distortion-of-amplifiers-%EF%BC%884x150w-rms-and-4x80w-rms%EF%BC%89.9764/).

If you are lucky enough to have a soundcard and ASIO driver that operates 'norminally' with REW, then that can open up a lot of measurement functions that imho far exceed legacy test equipment like scopes and distortion analysers and signal generators and impedance analysers/LCR meters, within the bandwidth constraints of typically 2Hz to 96kHz.

As an aside, I recently did a quick impedance check on a 1-to-9999 ohm 1930's 0.1% tolerance resistance decade box that I just restored, and due to the bifilar wind of the manganin coils and layering it seems like phase shift is fairly low out to 100kHz (circa 10deg at 30kHz).  So I hope to do some more investigation on how practical it may be to normalise for effectively zero phase shift up to 100kHz - all with the help of my soundcard and REW.

I installed REW on my Win10 PC and set the sinewave gen to 1KHz 1.0v out on the ASUS Xonar U7 soundcard and monitored the signal. Ambient temp was 28C.

The amplitude-time stability was poor compared to even my worst analog sig gen. It dropped by about 0.6mV per minute and showed no sign of leveling out even after 30 min.

Unless I am doing something drastically wrong, a PC soundcard source is not good enough for this project.

Different sound- cards can be quite different in there performance. A super stable amplitude is normally not the main traget there, more like low distortion and stable frequency.

David Hess:
Audio DACs can be very drifty as well.  Gain and offset precision is not a requirement for audio.

You would need to dive into the DAC implementation etc. to get a good sense of the causes of amplitude stability. I'll check a few later today (I have a number to check). Most audio DAC's derive their reference from the main supply. They may have an internal reference but its not super precise and may well be temperature sensitive and not compensated.  E.G. https://www.es.co.th/Schemetic/PDF/AK5394A.PDF  If you look the reference is sort of accessible but only for bypass caps. There may be an option for an external reference in the DAC on the Xonar card. Really good AC source stability may require a different approach (like the anal0g solution). My Boonton 1120 uses a state variable oscillator + sample and hold + Ref02 to get a stable source. The Fluke 510 also has a pretty carefully designed reference. It simpler and may be something that lends itself to the eBay oscillator board. https://xdevs.com/doc/Fluke/510A/510A_AA_imeng0000.pdf


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