[EFM] Offset and oscilloscopes?

[Harry Bissell]

Hi Mitch:  (et al)

I don't quite understand the VCA5b design philosophy. I'd expect only one
offset pot...
and that is how I built mine.  If you want to use the inverting input, then
you put the pot
on the non-inverting input. Short the inverting to ground, adjust the pot
for zero volts out.
Offset done. Or switch inverting and non-inverting above, do the same
procedure.

If you want to offset, I'd use a regualar op-amp stage rather than a VCA. If
the VCA
has a DC offset, it will THUMP with a fast attack or decay at the control
voltage input.

But IMHO, only one offset pot is required.  Don't know why there are two.
Tom ???

Also (as noted by the doctor) there is no offset happening in your o'scope
drawings.
What you expect is what you should get...

H^)  harry

The offset will vary a little with the amount of bias current

Mitchell Hudson wrote:

>     A friend of mine just loaned me an oscilloscope. Its very eye
> opening to be able to look at the wave forms through the scope. It also
> raised some questions.
>
>     I am feeding some white noise through the offset on Tom G's VCA5b. I
> am expecting that the noise wave form will stay the same shape but move
> up or down realative to the center of my scores 0v. Instead the wave
> just gets a little fuzzier.
>     So Fed a triangle through the offset to see what it looked like. It
> seems that the line on the scope looks kinda thicker when the triangle
> is fed through the scope. As I turn up the offset the line gets thicker
> and and fuzzier still while the peaks of the waveform come together and
> transpose themselves when the offset is turned all the way up. Could
> someone explain whats going on here?
>     I can't take a picture of the wave but a made a little diagram to
> describe what is happening at:
>
> http://www.webdevils.com/schematics/question.jpg
>
> Thanks
>
> --M