SV: Re: [sdiy] New Thomas Henry VCO-1 Page Up

[Magnus Danielson]

From: karl dalen <dalenkarl at yahoo.se>
Subject: SV: Re: [sdiy] New Thomas Henry VCO-1 Page Up
Date: Mon, 28 May 2007 01:44:47 +0200 (CEST)
Message-ID: <94801.52122.qm at web27611.mail.ukl.yahoo.com>

> You are talking about the saw core here dont you?

Yes.

Cheers,
Magnus

> Reg
> KD
> 
> --- Magnus Danielson <magnus at rubidium.dyndns.org> skrev:
> 
> > From: Neil Johnson <neil.johnson97 at ntlworld.com>
> > Subject: Re: [sdiy] New Thomas Henry VCO-1 Page Up
> > Date: Sun, 27 May 2007 20:18:06 +0100
> > Message-ID: <13011C9D-9B34-4D38-A6C7-465CB574DB22 at ntlworld.com>
> > 
> > Neil,
> > 
> > > Ian Fritz wrote:
> > > > In 2000 I made additional improvements, including using good modern  
> > > > opamps (OPA27 for the CV summer, OPA134 for the CV servo and the  
> > > > Schmitt trigger).  At the same time I added added temperature  
> > > > compensation for linear temperature drift and tweaked up the scale  
> > > > factor drift.
> > > >
> > > > My version of this VCO is now reasonably stable, although the use  
> > > > of the supply rails for critical voltages is something I would  
> > > > avoid in a new design.
> > > 
> > > It is a continual source of amazement to me that so much effort and  
> > > expense is put into the choice of fast comparators, low-offset  
> > > opamps, esoteric temperature compensation schemes, etc, only for the  
> > > design to then rely on the supply rails for reference or other  
> > > critical voltages.  Especially given the low cost and ease of use of  
> > > even half-decent bandgap reference diodes available these days.
> > 
> > Well, the frequency tracking does actually not depend on it, since the expo-
> > pair reference current dependence balance out the reference voltage
> > dependence.
> > I was also thinking like you until I derived the whole thing myself.
> > Modulations on the powersupply will however leak-through, but this is a high-
> > frequency phenomenon due to the integrating part and that DC wise it cancels.
> > 
> > The feature that does change with supply voltage is the amplitude.
> > 
> > If you want to use a bandgap reference or similar, you need to treat both the
> > current reference and the reset-reference voltage. If you do not treat both
> > but
> > only one, then you have actually made things worse, since then you actually
> > brought the balance out of operation and you *will* depend on the supply
> > voltage.
> > 
> > This explains how we have been able to handle it fairly well as it is.
> > 
> > There is however a voltage dependence you can run into, that you acheive by
> > having the tune-knob work between the positive and negative supply if these
> > does not track each other. This is actually one of the hidden smart things in
> > the Oberheim SEM, where the negative supply is made to mirror the positive
> > supply. Even if the resistors isn't perfect match, that is not a problem for
> > the tracking.
> > 
> > So, while it is not ideal, it is actually better than one can first assume.
> > 
> > Cheers,
> > Magnus
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> > 
> 
> 
> 
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