Mini DIY signal generator

[PRR]

Many links have rotted. Here is the project on Merlin's site:
http://www.valvewizard.co.uk/siggen.html

[PRR]

> What *does* determine the maximum ƒ?

When the open-loop gain is not >> than required closed-loop gain. Because of the resistance ratios, you may need a really fast opamp to get out of the audio band.

However Merlin has far more hands-on experience with this plan and may have further insight.

[tubegeek]

Thanks, PRR. I think it's a TL072 per the schematic, maybe try a 4558? Certainly swapping a chip is within my skill set. I'll look at the op amp comparison shootout page in The Art for clues....

[tubegeek]

... seems like the TL072 is not the worst choice based on the features? "High" slew rate of 16, not sure what I'd find that'd better. I hope Merlin has a thought....

[merlinb]

I didn't measure level BTW, just sent it into my laptop so I could count peaks per second and determine ƒ's. I was using a 96K sampling rate in my audio interface so I should have seen something above 15K if it was there, I guess.

Hmm I'm not convinced a standard laptop soundcard is going to have a bandwidth beyond 20kHz?

[tubegeek]

It's an external (USB) interface, only a cheap Behringer 2-in model but I'd expect it to go higher than 15K with a 96K sample rate. It's a reasonable question though - I'll hook the mini sig gen up to a scope (or run a different sig gen into the same interface) to see if I can narrow that question down. And using the scope I can get a reasonably accurate voltage measurement too.

[PRR]

'4558 is *slow*. Errors (lack of excess gain) will mount above 1kHz. (It will "pass audio" much higher, but that's an easier job.)

I've seen $13 USB sound "cards" pass 19KHz fine, but I agree it is a point to check. A classic method is to jack line-out to line-in and run tones. I have used RMAA but it may be getting old. PassMark SoundCheck is reputable but I'm not sure what tests it does.

[tubegeek]

A classic method is to jack line-out to line-in and run tones.

Great suggestion. Tried that with the computer playing tones out through the interface in one program, jacked into the interface and recording in another program. 50K was no problem, that's where I stopped.

With a fresh battery, I was able to get the Mini Sig Gen as high as 23K and record it but after that the signal again dropped precipitously. Does that tell us anything?

[Kevin Mitchell]

Here's a layout of the bare bone version.
Link to PDF with transfer layout and bottom view



-KM