Entry level signals generator

[ginetto61]

Hi i would start with some measurements on line level buffer and preamp
So the max Vout from the units under test would be 3-4 Volts ( i have two low noise sound cards to try on the receiving end just to start)
But i would like to buy a standalone signals generator
I wonder which would be the price for something decent (i.e. low THD)
I see some units but i cannot understand their real performance

https://www.amazon.it/YIWENG-Generatore-arbitrarie-campionamento-memorizzazione/dp/B0CP58XNVG/ref=sr_1_57?crid=30DKIZFY2IUG2&dib=eyJ2IjoiMSJ9.Yz-ADEB7520kupg1GfSit1pm0QbliAuqk1XafbbUKOAVN8-gqXbMerZ8MiUVA7NZ_Jc5Z3SugzYtCN-G1zyoGFiK89UDOuuZbfCZtGKxw52vLy2XYb_cv1gACfePQAdr5VyOngxqZvjZlwqx9M4Fdwi63VqLNLkEo2XrY5KhEBSCihnHdyLOkJ4RIUHfeozRND_ZNmIB6etexmZ1KWLf1BkmWmdCKnlkGzpkpa3lM9NjdNRzpkLuF4l6nWA1gWxG5UsedpskOdz3XrmZoSq16SfWOkLmAJVig0hHrg46l5Y.mAQ7ZClWtKKu-uUk5nHDhDpiJ8RIDQwsmC4nRccuElM&dib_tag=se&keywords=generatore+di+segnali&qid=1719653548&refinements=p_36:6800-&sprefix=generatore+di+se,aps,257&sr=8-57&th=1

https://www.amazon.it/Generatore-Contatore-Controllo-Sinequenza-Ac100-240V/dp/B0CSDZ3VT8/ref=sr_1_92?crid=30DKIZFY2IUG2&dib=eyJ2IjoiMSJ9.eb34Ik6wXn-CoZU6FrlBGS-X1U2l_ecDWcGBaMnreFP-Zc7B4KXSewv1zZWEpcEsYpogH4yM8VAS2VFWymXuRXdYbOTf1af1xGG3IAbJ-Aa9ueyPI_f2wlTjxDak4BnFdZIHcmlFTm-fhUnK1JY-i4X7nq5Am1C1kueIOGYMpv0VV4ZANDpjKPA5SsNG4a_WfOxCuszLq15QdG6-hz5phJ7ifggL_bvxHSiK7cf8miRyMsdRLKFsphoXrG5chshcouM5HnJaCmM8BJjdINyBH7XgZd1MeRZD4-VqjnT0VB4.rvEk96AKTC2mT27RApA0b1SMlyJJoz_xbGAPxgLrmGY&dib_tag=se&keywords=generatore+di+segnali&qid=1719653615&refinements=p_36:6800-&sprefix=generatore+di+se,aps,257&sr=8-92

Would these cheap units clearly a better option than a soundcard plus SW ? i am mostly interested in sine and square waves
Thank you very much to all

 

[Mooly]

My thoughts...

Any cheap signal generator that is essentially a function generator like these will not deliver low THD. The sine is artificially synthesised and distortion can be quite high.

What you really need (imo) is an old fashioned Wien Bridge type generator for low distortion audio work and also a Function Generator for general purpose work giving sine/square/triangle output. Audio generators have gone out of fashion though and I don't see any decent new ones.

This is a typical function generator:
https://cpc.farnell.com/gw-instek/sfg-1003/function-generator-1ch-dds-3mhz/dp/IN07959

but the distortion is quoted in dBc which I suspect is a way of covering up how bad it is (in relative terms). I found an online calculator to convert db to percent. dBc I assume is C weighting and I'll bet that is done to make results look even better than they otherwise are. This is not a bad example, its typical.

Data sheet for it:
https://www.farnell.com/datasheets/1691990.pdf

Over the audio range that generator is -55dBc which for -55dB is 0.18%

This type of thing is much better distortion wise (not that I'm recommending this although it may be fine)

https://www.ebay.co.uk/itm/17640832...Qu6t8ivfo0wLznDExtv642SSGzzP|tkp:BFBM0LDAtoxk

 

[ginetto61]

Good morning Sir ! thank you very much indeed for your always kind and precious advice
I will buy one immediately I like a lot also the rotary knob Very very nice
I am quite sure the THD from the generator will surpass the one from the circuit under test
But i need one signal generator standalone The price is very very good
Thanks again

 

[Mooly]

hope you like it

I use one of these

https://www.diyaudio.com/community/threads/belco-sig-gen-age-info-etc.126467/

And one of these:

The sine produced on the function generator is not totally pure as it is synthesised from the basic triangular waveform the generator produces by using diode shaping. Although it looks OK on a scope you can actually hear that its not quite pure when compared to a true sine, and if you look really carefully on a scope you can see a very tiny glitch 'like a join' at the top and bottom of the sine. Its microscopic but its there and you can hear it.

 

[bucks bunny]

With your soundcards and REW you already have a low THD and very stable tone generator.
No need to buy that old stuff.
Furthermore buy a decent used notebook like lenovo X220i3 for about 100€
that you can use exclusively as your audio measurement station.

 

[MarcelvdG]

dBc normally means decibel with respect to the carrier. It's a quite normal term for RF engineers. -55 dBc is about 0.18 %, as Mooly already wrote.

 

[Mooly]

Thanks Marcel I could only think of SPL weighting curves tbh.