[sdiy] VCO reset time

[Czech Martin]

Integrated VCOs sometimes have two saw cores, thus
hiding the discharge ramp.
You get some glitches, but the discharge time 
will have almost no influence and can be as long as 
nearly one half of a cycle plus safety.

And you need no bipolar current source, but switches.

In the discrete world we do not have matching for the capacitors
or other components, so timing differences betweend the two
cores will somehow influence the shape, perhaps this
can be circumvented if both saws and a square are combined
to give a fundamental frequency saw, slightly different
slopes in both halves should not matter. A clever setup of
switches will get rid of most of the artefacts.
And we can select capcacitors, or add some capacitance at will
which integrated solutions can not (or not so simple).

m.c.

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Don Tillman
> Sent: Freitag, 4. Juni 2004 07:25
> To: Magnus Danielson; uzs159 at uni-bonn.de; harrybissell at prodigy.net;
> synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] VCO reset time
> 
> 
>    > Date: Wed, 02 Jun 2004 21:21:56 +0200
>    > From: Rene_Schmitz <uzs159 at uni-bonn.de>
>    > 
>    > > What is a good ramp reset time ?  How fast have you seen
>    > > ramp reset be ?
>    > 
>    > Zero. (A special triangle core ;-))
> 
> 
>    > Date: Thu, 03 Jun 2004 00:14:30 +0200 (CEST)
>    > From: Magnus Danielson <cfmd at bredband.net>
>    > 
>    > I consider triangular cores a very interesting concept and they
>    > might compete very well with sawtooth cores IMHO. 
> 
> 
> I've been questioning the whole point of sawtooth cores for a couple
> years now, and I'm thinking that triangle cores are a lot better.
> 
> Here are three advantages of triangle core VCO's:
> 
>   1. No reset detuning.
> 
>   2. You can get each of the basic waveforms in just one step:
> 
>      triangle -> saw
> 	      -> square
> 	      -> sine
> 
>      With the sawtooth you need two steps to get to the sine wave (and
>      possibly the square wave depending upon how you want PWM to
>      work):
> 
>      sawtooth -> triangle -> sine
> 	                  -> square
> 
>   3. Any glitches in the triangle-to-sawtooth conversion will be less
>      noticable because the sawtooth has more harmonic content to mask
>      them, while any glitches in the sawtooth-to-traingle conversion
>      will be more noticible as there is less harmonic content to mask
>      them.
> 
> The only advantage I can see to a sawtooth core is in visualizing the
> waveshaping.  That is, your waveshaping transfer function looks like
> your output waveform.
> 
> Of course a trapezoid core VCO is better still.  :-)
> 
>   -- Don
> 
> -- 
> Don Tillman
> Palo Alto, California
> don at till.com
> http://www.till.com
> 
>