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World-class sine wave generator for cheap?
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toli:
A little while ago I've assembled a test setup for audio stuff at home (for hobby use).
I'm using an EMU 0404 USB as the generator/recording device, a DIY pre-amplifier as the interface, and for extreme needs I'm using Victor's oscillator that was mentioned here.

I've ended up building a small PS for the oscillator to fit inside its case, and modding the EMU soundcard to improve THD at higher frequencies, and designed the measurement pre-amplifier to fit my needs. I've shared some of it here on the forum, but everything can be seen on my blog: www.tolisdiy.com
You can take a look and see if some of it is useful to you. The EMU is an excellent solution for DIY measurements on the cheap, with fairly good performance. I got mine for close to nothing on eBay.
If you are using a desktop PC and not a laptop, you might have even nicer options since you won't have to use a USB solution.
basinstreetdesign:
OK, Chapter 2 in the tale of making a REALLY low distortion audio oscillator...
At the end of the tale before at https://imgur.com/a/Buj8WfD  the oscillator works fairly well but had two issues.

First, there was some 60 Hz power line crap leaking into the signal and was being displayed on my HP 3580A spectrum analyzer.  I had to find out where this was coming from and do something about it.

Secondly, the THD of the oscillator was so low that it completely disappeared below the noise floor of the SA 90 dB down.  So I couldn't tell just how far down it was.  The solution to that, for me, was to make a notch filter to pass the signal through to suppress the fundamental of the signal.  That would allow me to crank the sensitivity of the SA up to bring the distortion products into view.  But the filter had to be tuneable and tuneable over pretty much the whole audio range.

Here is my attempt at solving those two issues:
https://imgur.com/a/6C434zj

I might as well say it again, but my motivation in building this was more academic than pragmatic.  Yeah, I'll use it if when (or if) I build another stereo amp but, mostly, it was just an intellectual exercise in seeing what I could do with glory-box proceeds and little to nothing in the way of out-of-pocket spending.
trobbins:
One issue you encountered is ground loops with multiple equipment powered from the mains AC.  That gets worse when the EUT is mains powered too.  Battery powering everything but the EUT is an easy-out option.

Do you reach any overload/damage level if the notch gets accidentally changed when you are at -70dB setting ?  I can see an overload indicator.
TheUnnamedNewbie:
This might be too much of the RF guy in me talking, but what is wrong with making a mediocre oscillator, and just having a boatload of bandpassfilters after it to isolate the desired tone? This is how most of the ultra-pure RF generators work.
srce:

--- Quote from: TheUnnamedNewbie on May 25, 2020, 07:35:07 am ---This might be too much of the RF guy in me talking, but what is wrong with making a mediocre oscillator, and just having a boatload of bandpassfilters after it to isolate the desired tone? This is how most of the ultra-pure RF generators work.

--- End quote ---
How do you make the filters? The non-linearity of just resistors and capacitors can introduce distortion at these sort of levels (i.e. 140dB), no?
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