[sdiy] Re: Sawtooth phase shifter

[harrybissell]

Magnus Danielson wrote:

> My old site... if it doesn't start with http://rubidium.dyndns.org don't expect
> it to be maintained.
>
> Anyway, it looks as the one I have on my page. I don't recall needing to change
> it.

OK thats good.  I'll check that page instead. But as you say, its probably the same
file. If its four resistors, it is.

> First op-amp is the comparator and second is the summer, and the negative
> feedback looks OK.

Yes I agree with that.

> OK, now for a *really* stupid question. Did you hook up the + and - inputs of
> the second op-amp correctly? From experience nobody dare to ask an engineer
> about these things, but I have recent experience that exactly that which nobody
> beleives is the problem is most probably exactly the problem.

Yes I got them right. Also presuming there might have been a typo... I swapped them.
Of course the second opamp should have negative feedback (as you drew).  If you
swap the inputs it does not make any sense (positive feedback would assure that it
would be more comparator than summer :^)

> When I look at the schematic it looks as I intended. However, it was always
> nagging me that I had swapped R3 and R4 or R1 and R2, but I did try to
> recalculate it and it came out alright. I may very well have swapped them on
> both locations. Thus, I am not even assuming that I did it right. ;O)

What I did on my version is add a resistor from the positive input to ground...
in effect making the last stage look more like a diff amp than a simple summer.

> OK, I really should build this during the weekend. ;O)

Let me know if it works.  Like I said, its more important to me to find out
why it will not simulate correctly.

If anyone else plays with 'spice' sims... give this circuit a try and see if it works
???

H^) harry