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| Question: Cheap pure sine wave genertor 1khz? |
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| Marco:
--- Quote from: trobbins on January 14, 2020, 03:49:53 am ---Circa 130dB plot taken whilst having fun with the harmonic null technique: https://dalmura.com.au/static/Loopback%20distortion%20trim.JPG --- End quote --- You are correcting for the combined non-linearity of the DAC/ADC this way, which means you are adding real harmonics which the ADC can't "see". Still, it proves the DAC/ADC are stable enough to get 130 dB of consistency in between calibration ... and there are ways to separate out the ADC non-linearity from the DAC non-linearity. It works over a small frequency band, but as I said last time analogue switches are cheap (and have vanishingly low distortion when feeding an non inverting opamp buffer, a couple 100 milli-Ohm of channel modulation in the switch does nothing with a >10^9 Ohm load). |
| MasterT:
--- Quote from: trobbins on January 15, 2020, 05:27:30 am ---... For a 15 year old soundcard, that noise floor level is circa -130dB below the fundamental, but it appears you don't have confidence in that measurement process (and have yet to explain that succinctly imho). --- End quote --- I already says, thd -n magnitude & phase are variables. They vary with temp, voltage , input level, freq., and coupe hundreds other parameters. It's not clear, how long do you track your equipment to get this down to -130, but I think long time accumulation approach is not right way. Instantaneous, sporadic deviation would not be visible on long term statistical data. Fast, 1--2 msps >16bits adc offers much faster loopback control. I tried with CS4344 & CS5361 last summer, thought sigma-delta would get me better linearity than SAR AD798X, but for some reasons - bread-boarding, adc driver or volt/temp instability, I hardly could get -90 despite whatever they say in DS. Also possible, I got fake chips. |
| rhb:
A few points: 1 bit = 6 dB But you need to use ENOB, not the output word size. I don't recall seeing 24 bit sound cards 15 years ago. 130 dB requires an ENOB of 21.67 bits which in turn requires a 24 bit ADC. In seismic we measure and correct for ADC skew, location accuracy, channel to channel gain and phase error. All these things are being controlled as tightly as possible. But if the work requires more accuracy it can be done in post processing. That's a very powerful concept. A common correction these days is water depth changes of fractions of an inch because of tides, currents and subsea topography. Those maps you see showing the topography of the deep ocean seafloor? Those are made by measuring the bumps on the surface of the sea by bumps in the seafloor from satellites. If your equipment is floating in the water you can't dynamically correct for changes in the water depth. The correction is done in processing by shifting everything to a common datum. Once the ADC and DAC errors have been measured using the loopback I outlined earlier, the input sine wave WAV file has a Wiener inverse filter applied to predistort the waveform so that analog errors are corrected before they happen. This is fundamental to modern cell tower transmitters. The ADC errors have their Wiener inverse applied after acquisition to ensure that the DUT actually sees a pure sine wave. You don't need a better sound card to measure a sound card. just do the loopback test I mentioned and the algebra will remove the sound card errors once you characterize them. If anyone wants help implementing the sound card correction send me a PM. I have no interest in doing it myself right now as I need to build a 16' x 12' screened room for a new lab area first. I do hope to work my way through Jim Williams' paper some day. To do that in a meaningful way I have to measure the THD at each iteration of the design. I do not want to buy an HP distortion analyzer to do that. I don't care all that much about audio equipment beyond using it. I have been playing guitar for 50 years. Just for fun, I checked and I have directed people to "Max Wien, Mr. Hewlett and a Rainy Sunday Afternoon" in 16 posts on EEVblog. Like many of my contemporaries, I revere Jim Williams. Reg |
| trobbins:
March 2006 product entry, so circa 14 years (although the software took some time to mature): http://ixbtlabs.com/articles2/proaudio/emu-0404-usb.html |
| rhb:
Boy, I bet those were $$$. I bought a Korg 1600 Mk II DAW circa 2004-2005 after despairing of finding a USB device which would work with Linux. So I had been looking at audio interfaces a lot just before those appeared. I bought a Tascam USB unit, but the software was so flaky I couldn't make it work. So I bought a packaged system that did work even if the UI was agony for ~$1300. 16 bit 44.1 kSa/s only. |
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