[sdiy] ASM-1 VCO: Sensitivity of linear FM Inputs?

[Magnus Danielson]

From: Michael Zacherl <sdiy-mz01 at arsprototo.at>
Subject: [sdiy] ASM-1 VCO: Sensitivity of linear FM Inputs?
Date: Sun, 18 May 2003 15:32:53 +0200

> Hi List,

Hi Michael,

> I just went through the description of the ASM-1 
> VCOs and found follwing note:
> 
> "The 560 kohm input resistors for linear FM can be 
> made whatever value you want, with smaller 
> resistance allowing for wider FM sweeps. 560 kohms 
> is just copied from Electronotes."
> 
> What's the sensitivity of this inputs with this 
> 560k resistor? How many Hertz per Volt (appr.)?

This is not an easy question to answer, since it depends ;O)

The actual output frequency comes from the total output current, and the
linear FM part sums with the reference current before being multiplied by the
exponential part similar to this:

               V          qV
                lin     - --
I    = (I    + ----) * e  kT
 out     ref   R
                lin

(I actually think there is a subtraction between the Iref and Vlin part, but
it's available on the page if you look it up, I'm just too lazy to do it since
I just came in from sitting in the sun reading about LISP.)

So, the Hz/V depends on the logaritmic input (normal CV) sum. Also, you in
practice depends on the positive to ground voltage as seen by the comparator,
a handfull of other frequency setting components etc. I've tried to have
sufficient of the math available online for people to be able to understand it
better.

If you desperately need a unscaled linear CV input, you must hook up on the
output of the expo-converter and input of the integrator-assembly. Be *REALLY*
carefull not to introduce DC offsets when this is not the specific intent, or
the oscillator will be very hard to keep in tune.

Cheers,
Magnus