Function Generator Sine Wave Bad

[roberts67]

Hello! I just got a used function generator. GF8056 Elenco. The sine wave is pointy at top and too rounded at bottom. I can’t seem to dial in a smooth sine wave. Any advice appreciated! Robert

 

[brig001]

The offset on the function generator is set fully negative. Does the clipping go away if it is centered?

Brian

 

[JRE]

The offset knob is rotated full negative, but it is pushed in. That should set the offset to some fixed value set inside the device somewhere. That is, the knob has no effect when pushed in. You have to pull it out to change the offset.


I don't think that's the problem - unless the knob is broken.

 

[roberts67]

Thanks for responding. I have tried popping out and adjusting and that does not fix the issue. Thanks for the help!

 

[rayma]

Does the distortion remain the same, independent of the setting of the amplitude knob,
either in its rotation, or the pull/push setting of the -20dB switch?

How about the triangle waveform (from which the sine is created), does that look good?

 

[roberts67]

I tried that and that does not change it.

 

[PRR]

Call/write Elenco. This is a triangle-wave machine with a "sine" converter. My first FT job included trimming the converters on synths. It isn't too tricky, except it should not be THAT far out. You need specific guidance on this machine to check power rails and then find the several sine-converter trims. It would be in the service manual, but Elenco does not post that.

 

[roberts67]

Thanks for the advice! I have reached out to them.

 

[duncan2]

"Pointyness " in a non high end function generator isn't unusual and isn't always a fault as many function generators have sine wave distortion values that would not be acceptable in relation to THD testing .


Rounding at the bottom that's a different matter , I do not like those "sine wave convertors " as I can and have built sine wave generators with 0.001 ( or less ) .


I do agree the "shaping " needs to be adjusted.

 

[Mark Tillotson]

For audio distortion measurements the triangle->sine conversion method is orders of magnitude too poor for use.


If you can see a distortion to the wave on a 'scope, its really bad already. Several percent usually...

 

[PRR]

For audio distortion measurements the triangle->sine conversion method is orders of magnitude too poor for use. ...

THD testing is a specialized application. A lot of audio design repair and testing can be done with any repeatable roundy-wave.

 

[roberts67]

Got it adjusted and now have a clean signal. Trim pot #46 adjusts the distortion for the sine wave on these units.