[sdiy] Re: Sawtooth phase shifter

[Magnus Danielson]

From: harrybissell <harrybissell at prodigy.net>
Subject: Re: Sawtooth phase shifter
Date: Thu, 15 Sep 2005 23:29:25 -0400
Message-ID: <432A3C13.92EFA6EF at prodigy.net>

> Magnus Danielson wrote:
> 
> > Both these designs is fairly easy to understand if one draws the waveforms,
> > make notes about voltages and is able to sum properly. Simulation is trivial
> > actually. If your simulation tool fails, toss it since by brain obviously beats
> > it. Not a good benchmark, but anyways...
> 
> I'm not sure that I agree.  I have several sawtooth phase shifter designs.
> They all simulate correctly.  Yours doesn't.  That does not mean that yours
> does not work... I just want to
> 
> 1) be sure that the .pdf is correct. I googled "sawtooth phase shifter" and got
> you link at
> home.swipnet.se/cfmd/synths/ schematics/sawphaseshift.pdf

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.

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

> 2) understand 'why' the simulation is failing.
> 
> I have a design with five resistors and two opamps (same as yours) that
> does simulate correctly...  In both cases I'm assuming that the TL07x is capable
> of swinging about 13.5V with a 15V supply.  I verify that I have CV of +/-5V
> and sawtooth input of +/- 5V.  The comparator gives the correct PWM and shifts
> correctly... but the output stage gives a pwm wave as well... no hint of sawtooth
> at all (let alone phase shifted saw).
> 
> Sure... I have had simulations fail before. But never one this simple...

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.

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)

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

Cheers,
Magnus