Single turn BFO audio oscillator

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tedsonoma

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Joined 2010
#1
Hi,
I am looking to buy or build a low distortion audio oscillator that has a single turn 20Hz to 20kHz dial - useful for speaker rub and buzz testing. Onsoku in Japan still makes these but thousands of dollars. I have tried a few $60 units but too small and unstable at low Hz.

I believe one method to get a single turn dial is to make a Beat Freqency oscillator like the old B&K and Genrads.

Has anyone seen a simple design for a solid state low distortion BFO audio oscillator? When I say low distortion, I mean maybe under 0.1% since the ear can be sensitive to distortion in pure sine testing. Not testing amps here.

Thanks
 
#2
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#3
AGREE that PC-based sweeps can do <<0.1% and are "the" way to go today.

NOT a ready-to-cook project, and not low-THD as far as it went, but I have been staring at this old idea for a few days.

https://open.library.ubc.ca/media/download/pdf/831/1.0103232/1
An Audio Frequency Sweep Generator, Bennett, 1959
Related to:
Functional Circuits and Oscillators (Herbert J. Reich) 1961 "Seven League Oscillator", book page 406

Related to H-P 207A wide-range oscillator (which is conceptually ready-to-steal, except you can't get that monster tuning-cap today).
http://hparchive.com/Manuals/HP-207A-Manual.pdf
http://hparchive.com/Journals/HPJ-1957-01.pdf

My suspicion on the Bennett paper: his calcs slipped a cog on the C-R network. The poles are not evenly spaced as you would suspect. I may very well be fuzzy on what is supposed to happen.
 
#5
I may have double posted - - but thanks to everyone for all the leads. I had forgotten the 207A was a 3 decade single range dial - and I owned one in the 70's! But I was hoping for something solid state stable, and I need a few, maybe 10, so buying used is out. And PC based is not conducive to the how these are used, but I will look at the papers.

Thanks
Again,
Ted
 
#6
I had an HP207 in the far distant past. The tuning cap was not that special. The associated network was. http://hparchive.com/Manuals/HP-207A-Manual.pdf The rest was essentially an HP200cd. It would be in interesting but really challenging project. you had to back off on distortion to get reasonably fast sweeps without the agc (light bulb) getting lost.

Some function generators can sweep three decades (Krohn Hite for example). With tweaking you can get close to .1%

Conceivably a couple of Exar function generators chips could give you the BFO. The oscillators can be square waves and a good mixer and LP filter will get low distortion audio (the reason for the GR BFO).

Still software may be easier lower distortion and quicker.
 
#7
tedsonoma asked for 20Hz to 20kHz. The EXAR chips will do 2000:1 range in 0.01Hz to 1MHz. Not sure why we would need to beat them.

"All" large variable caps are now "special", even single-gang 356pFd crystal-radio tuners. The 207A had a large dual (actually 4-gang but parallel) which filled half the box. While we could hack-up a 207A today, the chore would delay the speaker testing. And tedsonoma suggests a need for 10 of these.

A WAV file on $29 music-players might do almost 20-20K at 0.1%.
 
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