vco

[Magnus Danielson]

From: chnowak at web.de
Subject: vco
Date: Sun, 30 Jul 2000 12:08:07 +0200

> hi folks,

Hi Chris!

> i'm in the process of designing and implementing my first synth diy 
> project and have just finished the design of a first very simple vco. 
> i would be very happy if some of you experts would have a look at it 
> and tell me if this design is ok. i haven't yet calculated proper 
> values for the resistors and the capacitor, though.

Expert or not, I will share my thougths...

> you can find it at:
> http://www.crosswinds.net/~chnowak/chnvco_rev_a.gif

Ah, I haven't seen an attempt for a triangle oscillator for quite some time,
we usually see sawtooth oscillators and a bunch of waveshapers and triangle
usually pops out of one of them. Nothing wrong, by no means, but unusual.

CV mixers seems fine. you could make it simpler by moving the scale trimpot
over to the op-amp feedback loop. Also, hook the (+) input to ground through
a resistor about equal in size as the total source impedance seen on the
(-) input for good noise preformance, this is just a simple thing usually but
not allways done. Minor details/comments. No real designchange required.

You get a good grade for putting resistors in each end of the scale trimpot in
order to reduce the trimpot range, this is how it should be done in far more
places! A gold star for that.

Ah, you use a integrator curcuit (the op-amp with capacitor in feedback loop),
this is normally seen these days. The thing is, you have set up the transistor
pair which seems like a diffrential voltage source. However, I think the
integator solution is a bit unortodox (yeat again, not necessarilly a bad
thing) but I sit here and think that the +15 V resistors act as a discharge
resistors to the caps. It will probably kind of work, but I fear a error term
in the frequency tracking from them. I will not say it will not work since I
haven't analyzed it that thorough. I would simulate this in Spice or something.

I do like your idea of using a transistor pair (in basically a multiplier
setup) in order to divert the expo current in diffrential. I see how your
comparators will create the inverse phase squarewaves which feeds the trannies.
Neat idea.

> i also have a little problem with the exponential converter. for 
> voltages between -15v and 0v, i'd say it is indeed exponential, but 
> between 0v and +15v rather not. it would be very nice if someone 
> would have a look at it and tell me if this curve looks normal to him:
> 
> http://www.crosswinds.net/~chnowak/cursrc.gif

Your curve is what one can expect of the solution you have, an tanh curve!
Maybe you should start of with looking at some expo converters from for
instance the ASM-1 (http://home.swipnet.se/cfmd/synths/friends/stopp/) or
from the ASM-1 derivate V/Oct -> V/Hz converter
(http://home.swipnet.se/cfmd/synths/schematics/). The expo converter in those
schematics will deliver a current pull just as your schematic requires.

One of the things to consider about the expo converter is its temperature
stability. This is the main reason for temperature drift. There are ways to
reduce this problem, tempco resistors, heated transistors (owen setup),
sence transistor compensation etc. Many of these migth even be combined.

> anyway, thank you very much in advance for your comments and 
> suggestions !

BTW. I admire your attitude in trying to design your own curcuit in your FIRST
DIY project. Most seems to at least borrow the basic design from somewhere.

Cheers,
Magnus