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Question: Cheap pure sine wave genertor 1khz?
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hamster_nz:
I think this will be my next project - a < $10 low distortion 1kHz generator.

Thinking of using a Wein Bridge, but with an LED coupled to an LDR to replace the classic light build to control gain. Bit worried about that being non-linear though, and spoiling the result.
Gyro:
The best ones do seem to use LED+LDR. There's effectively zero voltage modulation of the LDR resistance.
MasterT:

--- Quote from: trobbins on January 13, 2020, 09:04:30 pm ---...
Using my EMU0404 USB as DUT with loopback, I did some testing last year and had 0.0006% 2nd and THD at 389.6Hz test tone when limiting bandwidth to just include 2nd harmonic. I can then null that out (0.0001%) with -104dB of added 2nd H with 90 deg phase shift.  I effectively did the same for higher order harmonics. 

This method will work for any typical soundcard, so the generation of a very low harmonic distortion waveform is not limited to the raw performance of the soundcard itself.  As such, a cheap or even zero-cost, and quick setup can be prepared by most.

--- End quote ---
This trick IS limited by performance of the adc. I don't know what kind of chipset inside EMU0404, but I doubt it has THD+Noise level below 0.0001% (-120dBc).  I'm working on similar project, doing software correction THD by magnitude and phase tracking in antiphase on stm32F767 dual dac DDS generator. The problem I have is non-linearity of AD7984, about -110 dBc, so can't break this barrier and my DDS always has THD above this flour.
MasterT:

--- Quote from: Gyro on January 13, 2020, 09:51:44 pm ---The best ones do seem to use LED+LDR. There's effectively zero voltage modulation of the LDR resistance.

--- End quote ---
Page 35, distortion.
trobbins:

--- Quote from: MasterT on January 13, 2020, 10:00:47 pm ---This trick IS limited by performance of the adc.
--- End quote ---
The technique also has to contend with the measurement null being the result of both DAC and ADC, and not just DAC, so some deduction needed to confirm that DAC result by itself is nulled.

Noise level will depend on a few factors, so not straightforward to report on.  The mid-band noise level for that EMU soundcard is down around -130dB from full-scale signal, and uses 24-bit 192 kHz AKM AK5385A ADC and AK4396 DAC.

Still, the trick can also be used to null out any additional harmonic distortion from added devices up to the point of DUT connection, so could be applied to an add-on signal amplifier if needed for the DUT and test environment.  The power of the technique is simplicity and ability to make a mid-range silk purse out of a sows ear.
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