[sdiy] VCO reset time

[Don Tillman]

   > Date: Tue, 01 Jun 2004 23:27:32 -0400
   > From: harrybissell <harrybissell at prodigy.net>
   > 
   > What is a good ramp reset time ?   

Hey Harry,

If you want to spec the required accuracy and the highest frequency
you care about, it's pretty straightforward to calculate how much a
given reset time will detune an oscillator.

If we talk about a "detuning ratio", a number slightly less than 1.0,
I believe the equations are:

  Rdetune = Fdetune / Fintended

  Rdetune = 1 / (1 + Fintended * Treset)

  Fdetune is the detuned oscillator frequeny in Hz.
  Fintended is the intended oscillator frequency in Hz.
  Rdetune is the 1.0 based ratio.
  Treset is the time to reset the sawtooth in Seconds.
  
So a VCO with a 1.0 uS reset time, running at 10.0 kHz, will be
detuned by 0.9901, or down 1%, which is down 17.2 cents.  That could
be enough to be annoying.  (For comparison, a "stretch tuned" piano
may be sharp 30 cents around 4.0 kHz.)

That assumes a fixed reset time, which may not always be the case.
Since the reset is working against the VCO control current, it may be
that the reset time is longer at high frequencies.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com