[sdiy] 4069 VCO to get started? My answer

[Scott Gravenhorst]

Nicolai Czempin <nicolai.czempin at alcatel.de> wrote:
>Wow, I'm really looking forward to building and fully understanding that 
>4069 cicuit one day. That day may not be that far away :-)
>
>What puzzled me was not the general idea of an integrator or 
>Schmitt-Trigger. What did so was that the circuit showed the building 
>blocks simply as digital inverters. And I was wondering how on earth an 
>inverter can be one thing in one part of the circuit and another thing 
>elsewhere.
>So perhaps to make it clearer the symbol in the schematics could be 
>changed to reflect the functional usage.

Ah, well, these markings, such as the use of dotted lines to enclose the
function of the schmitt trigger or the integrator will be found in the more
elementary texts as visual aides.  These are not included in normal
engineering schematics because it is expected that you can visually
recognize them.  Do yourself a favor and pick one block, like the schmitt
trigger, look that up in the books you have and see if you can identify it
in the schematic you're trying to understand.  At least you already know
that this VCO has the 3 components mentioned, you should be able to draw
these functional dotted lines on the drawing.  

One thing about the 4069UB VCO, it's integrator is not a "simple"
integrator.  It is an integrator that is modified using Franco (de Franco?)
compensation.  This compensation consists of an extra resistor and a diode
and is present to correct high frequency response being slightly musically
flat because the VCO's reset time is not zero.  This compensation is not
necessary to make the circuit oscillate.  In fact, for my use of it, I found
the compensation unnecessary, so I removed it.  

See http://home1.gte.net/res0658s/fatman/VCO_pwm_tri_suboctave.html

The heavier line drawing is the VCO with PWM.  The VCO part is the upper
loop with 3 gates.  The integrator is the leftmost gate (really just an
amplifier) with the cap from input to output, the schmitt trigger is the
rightmost 2 gates with the two resistors, one on the input, one providing
(positive) feedback.  You can actually eliminate the current source which in
this case is linear and connects to the left side of the loop.  To eliminate
the current source (really a current sink), remove the source connection and
replace it with a variable resistor from the loop to ground.  Also include
some fixed resistor, perhaps 5K or 10K in series  with the variable one
which I would put at 100K, or 500K or even 1 meg.  twisting the knob should
control the frequency.  The resistor becomes a rather crappy current sink,
but works well enough that this circuit will oscillate.

I have stripped the drawing down to the absolute minimum components at
http://home1.gte.net/res0658s/fatman/SimpleOscillator.gif which will
oscillate, but is not voltage controlled.  I have also drawn lines around
the building blocks (which were, in fact, already marked in the original
drawing).  When you look at it's output with an Oscope, you will notice that
it does not produce a straight ramp and then reset, rather, it will be a
curved hump and a reset.  This curved hump is because the constant current
sink is replace with a resistor.  Musically useful VCOs use a current source
or current sink to force the hump into a straight line.  This makes the
circuit give a linear frequency response with respect to it's input CV
(really converted to a constant current by either an expo or linear current
sink).


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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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-- FatMan: home1.gte.net/res0658s/fatman/
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