[sdiy] Adding "sync" to a Through-Zero Sinewave Oscillator design

David G Dixon dixon at mail.ubc.ca
Sun Oct 13 21:35:12 CEST 2019

Hello Willi,

I'm not sure this message was directed to me, but yes, I finished it.  It's
called the Intellijel Rubicon (now the Rubicon 2) and it's a pretty popular
eurorack module.

The "Doc Sketchy" trick for TZFM is as follows:

In my standard tricore VCO (the Intellijel Dixie), Linear FM is achieved by
putting a linearized 2164 VCA in the current loop of the tricore oscillator
(between the squarewave-generating comparator and the triangle integrator).

For TZFM, one simply replaces this linearized VCA with a balanced modulator
(which can also be efficiently built from one or two linearized 2164 VCAs).
However, since this changes the polarity of the oscillator mid-period, one
has to add some logic to switch the polarity of the triangle wave summing
into the comparator, or else the comparator will get stuck at one of the
rails.  This is very easily accomplished by putting the control signals of
the balanced modulator VCAs into a comparator and using the output to drive
a JFET-driven +1/-1 amplifier.

None of this involves the V/Oct current.  That is unchanged whether it's a
regular VCO or a TZFM VCO.

Also, sync is done in the usual way for both regular and TZFM VCOs.
"Resetting" sync is accomplished with a JFET across the integrator
capacitor.  "Reversing" sync is accomplished by feeding the sync master
signal into the tricore comparator.

I've never designed a sawcore TZFM VCO -- I don't like sawcore VCO design --
too fussy for me.  Tricore is way easier to get right, and shaping a
triangle into a saw is a piece o' cake.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of
Sent: Sunday, October 13, 2019 11:29 AM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Adding "sync" to a Through-Zero Sinewave Oscillator


just read in SDY list and found you worked on a TZ-VCO design. How you
perfom the multiplication for V/Oct current with linFM (Carrier/Modulator).

We also did it and designed a saw core TZ-VCO. Sounds on

Do you finished it?


willi sauter

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