World-class sine wave generator for cheap? - Page 1
Forum section: electronics / projects

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: February 15, 2020, 07:40:11 pm
content: Back when building a tube stereo amp I needed a fairly good sine signal to test distortion but I didn't have such a thing. So building one went on my long-term ToDo list. I've heard about oscillators with THD at -120dB and I thought it would be cool to have that. But how? They cost mucho$ (Audio Precisions' System One). Building a digital source needed an elaborate DDS system, parts I didn't have and probably SW so didn't attract me. So I thought maybe it could be done with a good 'ol Wein bridge oscillator.I posted a question about this project some time ago. Thanks for all who responded to it but I ended up choosing a more low-class solution to my problem than what I had originally hoped.Here is a picture story documenting progress...https://imgur.com/a/Buj8WfDIt's not finished, but,what do you think?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 15, 2020, 08:34:55 pm
content: Nice! The bulb solution works well but usually limits the THD at low frequencies. I assume you've read all the historical stuff on the HP200A that Bill Hewlett did his thesis on. I've used the oscillator in the Bob Cordell spectrum analyzer article as inspiration.http://www.cordellaudio.com/papers/build_a_thd_analyzer.shtmlIt's very inexpensive to build and does about 0.0002% if well tweaked. There are some threads about ultra low THD oscillators at the DIYaudio forum too.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 15, 2020, 09:14:33 pm
content: The cheapeast, simplest and the best one is Victor's ultra low distortion oscillator.

author: duak
author_rank: Super Contributor
timestamp: February 15, 2020, 11:19:44 pm
content: Interesting design and build. Thx for sharing!If you're having trouble with the EM field from the transformer, a copper strap around the winding and core can help. I can't find an image of exactly how it's done, but the attached image should give you a good idea. It doesn't go through the core otherwise it would form a shorted turn. I can't remember if its supposed to be grounded or not. I expect it should be. It is a complete loop with the ends electrically connected together.

author: mcovington
author_rank: Regular Contributor
timestamp: February 15, 2020, 11:52:54 pm
content: Wien, not Wein.Pronounced 'veen.'

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: February 16, 2020, 12:10:32 am
content: Quote from: mcovington on February 15, 2020, 11:52:54 pmOops, my mistake. Thanks

author: mcovington
author_rank: Regular Contributor
timestamp: February 16, 2020, 01:46:10 am
content: It's wrong in a lot of textbooks -- don't feel bad. I looked up Wien's original 1890 (?) article in the Annalen der Physik. He was experimenting with an "optical telephone," which was an earphone with a mirror with light reflecting off it to make a primitive oscilloscope. He was one of the first people to see waveforms.

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2020, 04:33:50 pm
content: See the classic Jim Williams National Semiconductor app notes on HP 200 and ultra low thd oscillators.Jim RIP, was the expertJon

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: April 01, 2020, 12:52:55 am
content: QuoteYeah I've read one that a few times.There were a couple of issues with the last effort.First there was some power-line crap leaking into the signal that I saw on my audio spectrum analyzer, a HP 3580A that I re-calibrated a while ago. I had to track that down. I didn't want my box spewing that out.Secondly, the oscillator worked so well that the distortion products would not show up above the noise floor of the SA. I couldn't turn the input sensitivity of the SA up to bring them into view or else the fundamental would overdrive it and I would see nothing BUT LOTS of harmonics. So a notch filter for the signal was in order.Now I have worked through those issues and herehttps://imgur.com/a/6C434zjis the result. Except for the op-amps I didn't have to buy anything.What do you think?

author: trobbins
author_rank: Frequent Contributor
timestamp: April 01, 2020, 04:11:00 am
content: I reckon you would enjoy using an external USB soundcard interface, like a 2i2 for low cost, or if your lucky a decade old EMU. You should be able to set that up to better your SA by 10-20dB, and it can lead you in to a wide range of tests that weren't available 10-20 years ago (due to software development). At least you would appreciate that any form of test setup requires attention to shielding and ground loops, and the advantage of most modern equipment like soundcards and PCs is that they can be battery powered to negate most forms of ground loop.

author: toli
author_rank: Frequent Contributor
timestamp: April 01, 2020, 01:28:40 pm
content: 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.comYou 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.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: May 25, 2020, 06:35:13 am
content: OK, Chapter 2 in the tale of making a REALLY low distortion audio oscillator...At the end of the tale before athttps://imgur.com/a/Buj8WfDthe 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/6C434zjI 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.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 25, 2020, 07:29:30 am
content: 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.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 25, 2020, 07:35:07 am
content: 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.

author: srce
author_rank: Regular Contributor
timestamp: May 25, 2020, 09:28:05 am
content: Quote from: TheUnnamedNewbie on May 25, 2020, 07:35:07 amHow 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?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 25, 2020, 09:37:58 am
content: I was thinking (air) inductors and capacitors. I am not aware how linear these are. I know that MLCC will be atrocious for this, but thought film or mica capacitors could offer very good performance?I am not an expert in this field myself, but I do know that this is how at work we tested some amplifiers for a hobby project of one of my colleagues. Started with a simple signal generator, and then just had filter banks we could tune to eliminate the harmonic. I think we got -120 dBc or better with that method.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: May 25, 2020, 08:38:47 pm
content: Quote from: trobbins on May 25, 2020, 07:29:30 amThe overload indicator on the SA indicates that, any higher signal level will distort internally and render the display full of harmonics. Now, the HP scope is damage-proof from high signal levels by limiting the signal if too large (clipping). The overload indicator comes on just short of actual distortion-inducing internal clipping, though. So I can increase the input sensitivity up to the point where that light JUST comes on (about 4 dB above the top line of the display) but internal distortion has not yet occurred. So if the input signal were to be accidentally changed upwards in level by a bunch of dB while the scopes input sensitivity is set quite high, (-70 dBV) there will be no damage.