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Question: Cheap pure sine wave genertor 1khz?
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schmitt trigger:
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Grandchuck:
Thread revival time.  Got some new ICs: OPA2810.  Time to try another low distortion oscillator circuit and used an old and simple one based on a circuit from a 1994 Elecktor magazine.  The modified schematic is shown here along with the THD reading.  I changed R3 from 2.05 k to 2.2 k and eliminated C3 and C4 (I might add them later and see if it makes any difference).  I also added a 5 k potentiometer on the output.

The construction is also shown with a close view of the IC and its adaptor.  There is an 8-pin dual-inline socket on the board so it is easy to pop in other ICs such as a TL0821 (tried one and it works, but with a bit more distortion).

The actual THD is below 0.001% because the 8903B residual noise and distortion are too high for this measurement.  I see a new rabbit hole coming  :-DD
pcprogrammer:
Very interesting stuff about the 1KHz sine generation here :-+

I wrote code running on an STM32F303 for making a very good sine wave. I myself do not have the equipment to check the quality, but a friend of mine, whom I wrote it for, told me the distortion is very low.

One can check it out here: https://github.com/pecostm32/STM32F303_Sine_Square_Generator
Grandchuck:
Thanks.  Lots of bits makes for low THD:
     THD(%) = 1/(2^N) x 100 (where N is the number of bits)
That is the ideal case.  In practice, the THD is higher.  i.e. my Siglent 14 bit generator gives about 0.008% (ideally, 0.006%).
David Hess:

--- Quote from: Grandchuck on June 27, 2021, 12:45:30 pm ---Thanks.  Lots of bits makes for low THD:
     THD(%) = 1/(2^N) x 100 (where N is the number of bits)
That is the ideal case.  In practice, the THD is higher.  i.e. my Siglent 14 bit generator gives about 0.008% (ideally, 0.006%).
--- End quote ---

That assumes that the linearity is commensurate with the resolution, which is rarely the case, and even achieving better than 0.01% requires attention to the audio path after the DAC.  Typically at least 2 bits are lost to non-linearity of the DAC but it is 3 or 4 bits with some devices which is revealed by integral non-linearity specifications or 8 or 16 counts.

Testing of 16 bit and greater ADCs is a real challenge and where the various "parts per billion" oscillator designs come into play.
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