[sdiy] VCO voltage reference

[Magnus Danielson]

From: Antti Huovilainen <ajhuovil at cc.hut.fi>
Subject: [sdiy] VCO voltage reference
Date: Tue, 11 Apr 2006 22:58:23 +0300 (EEST)
Message-ID: <Pine.GSO.4.61.0604112255370.21709 at vipunen.hut.fi>

> Hi
> 
> Would this list cover all the points which should be fed from stable local 
> voltage source in a typical VCO?
> 
> Comparator reset threshold
> Expo amp collector voltage
> Tune pot V+ & V-

Yes.

The comparator reset threshold and the expo-amp collector voltage (which
generates the reference current) cancels out, however since the reference
current integrates in the capacitor the cancelation does not work on
modulations so you want a stable voltage for that reason, but not necessarilly
an exact voltage.

A change in the reference current will change the frequency linearly, since
the expo will multiply the reference current with the exponential of the
input voltage multiplied by q/kT. The input voltage is scaled down and adjusted
for the T part with a TEMPCO typically.

The tune pot V+ and V- needs a stable voltage, but not necessarilly an exact
voltage such that the pot setting does not change the sum voltage for the
pitch, an unstable voltage would modulate pitch.

Voltage references are fairly cheap and accessable, so why not use them.
Slow shifts in voltage is acceptable.

Cheers,
Magnus