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

[karl dalen]

You are talking about the saw core here dont you?

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
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