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

[Michael Schulze]

I too ran into this - the circuit as designed only produces a narrow sweepo,
and I am interested in classic Buchla style complex FM.  Does anyone know
how many octaves the Lin FM input on a Buchla will sweep with the audio
signals as they are configured in THAT system?

> From: Magnus Danielson <cfmd at swipnet.se>
> Date: Sun, 18 May 2003 17:28:23 +0200 (CEST)
> To: sdiy-mz01 at arsprototo.at
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] ASM-1 VCO: Sensitivity of linear FM Inputs?
> 
> 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