[sdiy] equations up

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: Re: [sdiy] equations up
Date: Mon, 16 Jun 2003 11:40:41 -0600

> Magnus, et al. --
> 
> At 04:35 PM 6/15/2003, Magnus Danielson wrote:
> 
> > > >You formula (2) for the CCO is inaccurate. It misses out the 
> > dependence on V+
> > > >for which also I0 depends.
> > >
> > > The assumption that goes into Eqn. (2) is that any drift in the CCO 
> > core is
> > > "tuning drift".  I probably should have been more explicit about that, but
> > > since the emphasis of the document is on correcting converter inaccuracies
> > > I don't see this as an important issue.
> >
> >You fail to see my point I think.
> 
> Yes, you need to actually say what your points are. I can't read your mind.

I fail to articulate them you mean... which I might agree with.

> >  The point is, the V+ dependence of the
> >reference current and the V+ dependence of the reset-voltage reference (a
> >20k over 10k divider in the ASM-1) neatly cancels out... if you care to do the
> >math as I propose.
> 
> No need to do any math -- it's obvious.  :-)

The correct wording is "it's trivial" (meaning, it has been solved before).

Since I dont' think it is as obvious, why not make it "show" when talking about
sufficiently close matters?

> >Start to see my point?
> 
> Now that you actually say what it is, yes.  Simply put, you are saying that 
> the exp and lin driving paths have a different dependence on PS voltage, so 
> there is no simple way to compensate both paths for PS voltage drift.

... given that the CCO actually depends directly on V+, yes!

> I ignored that in my derivation for three reasons.  First, I don't see 
> drift via the lin input as a critical issue.  If you do, then yes, it needs 
> to be worried about.  Second, I assumed stable power supplies etc., to 
> focus on the main points of scale and tuning drift.  Third, I always use 
> stable local regulation for critical voltages.  In other words, your V+ 
> above is replaced by a stable local source (actually, 6.9V, 20 ppm/K).

A statement is true under its assumptions. Why not make the assumptions clear?
Sometimes you have to motivate the assumptions in order to make issues clear.

> Yes, it's very cute that V+ cancels in your first term, but, as you are 
> pointing out yourself, relying on that cancellation leads to other 
> problems.  But you're the one that is relying on that, not me.  :-)

Well, I was not saying your derivations where wrong, but rather lacking other
aspects and that it would not be bad to cover that ground while being at it.

> > > I also assume that well-regulated, stable supplies are used for critical
> > > voltages.  I would never use the AMS1 design for a serious VCO without
> > > extensive modifications.
> >
> >Please elaborate... without motivation it remains strong but empty words, and
> >if there is to learn, then I am awaiting to see what is to be learned.
> 
> I have had an improved version of that oscillator on my web site for years.

I have had my comments on compensations on my web site for years.
This is meaningless. You still could have motivated it, if even breifly, and
then referenced your webpage.

> > > Again, regulated sub-supplies for critical voltages are a must.
> >
> >Indeed, but why are you fighting of making them explicit when there isn't that
> >difficult to do?
> 
> I'm not fighting anything.  If you think I should say in my doc that I 
> assume well regulated stable power, then I will.

I think you indeed should say that you assume stable powersupply, and give a
short motivation why.

The CV side remains uncompensated for V+ and V- dependencies through pots.
Another annoying fact of life.

> >What part of the design did you change the heating on?
> >
> >The PSU as well?
> >
> >Did you change the line voltage?
> >
> >Did you change the load on the PSU?
> 
> Well, you just keep saying the same thing. You are concerned about PS 
> stability. OK, use local regulation.  It costs next to nothing.

I agree it costs close to nothing, but the reasoning giving this fact is still
worth giving or else it is lost.

> Thanks for clarifying you question.

Thanks for taking the time to understand my armwavings...

Cheers,
Magnus - slightly intoxinated