[sdiy] equations up

[Ian Fritz]

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.

>  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.  :-)

>However, other signals which does not depend on the V+ to
>ground potential difference does not cancel the same way.
>
>While your investigations concentrate on the temperature compensation alone,
>my point was that the mear fashion other things where short-handed a specific
>issue where being missed out. The assumption of stable supplies leaves a 
>lot of
>questions and really no answers. There is ways to remove the dependence of
>low-frequency (downto DC) variations, but due care has to be taken or else the
>improvement is lost, since both the reference-current and the comparator
>reference-voltage must track or the cancelation effect turns into an error
>induction path.
>
>OK, CCO equation for the ASM-1 VCO is:
>
>        3 * I
>             in
>f    = -------
>  osc   V  * C2
>         +
>
>The expo-converter voltage is
>
>               V                  V     V
>                lin     (...)      +     lin     (...)
>I   = (I    - ----) * e      = (---- - ----) * e
>  in     ref   560k              1.5M   560k
>
>put these together and you get...
>
>             3 * V            3 * V
>                  +                lin       (...)
>f    = (-------------- - --------------) * e
>  osc    V  * C2 * 1.5M   V  * C2 * 560k
>          +                +
>
>for this case out falls the V+'s of the first part, where as Vlin/V+ does not
>cancel as well on the second. Compensate just one of the V+ from another 
>source
>and you are dependent on it. The bad thing about this form of V+ dependence is
>that it only works for low frequency issues, where as the Iref V+ dependence
>integrates through the C2 cap before it hits the reference, so it is not 
>ideal.
>
>If you have a different way of acheiving the reference voltage for the
>comparator your story might be much different.
>
>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.

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).

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.  :-)

> > 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.

> > 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.

>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.

Thanks for clarifying you question.

   Ian