[sdiy] Triangle to saw conversion in triangle-core VCOs

David G Dixon dixon at mail.ubc.ca
Wed Nov 13 16:54:51 CET 2019

Here's how I do it (and how it has been done in the Dixie and Rubicon):

First, cut the triangle in half (through a gain-of-0.5 inverter) and lift it
into positive territory by summing a trimmed constant voltage of exactly
half of the triangle's peak-to-peak amplitude.

Second, send this halved and lifted triangle to a "Positive-Negative" unity
gain opamp circuit.  This is an opamp with the input connected to both the
positive and negative terminals through resistors of the same value as the
feedback resistor, and with a JFET switch on the positive input connected to
ground so that when the JFET is "On" the opamp is an inverter, and when it
is "Off" the opamp is a follower (the negative input is offset by the
positive input at a gain of 2 -- this requires 1% resistors for accuracy).
This JFET is controlled by the square wave which is generated by the
tri-square core comparator (through a diode to eliminate the positive

This technique gives a decent sawtooth waveform.  If the "halving and
lifting" opamp is replaced by a simple inverter, then this circuit gives a
sawtooth wave at twice the frequency.  This is also used in the Rubicon.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of
Florian Anwander
Sent: Wednesday, November 13, 2019 5:10 AM
To: synth-diy at synth-diy.org
Subject: [sdiy] Triangle to saw conversion in triangle-core VCOs


can somebody tell me how the triangle to saw conversion in 
triangle-core-vcos works?

I am thinking especially about the CEM3340. In the datasheets 
blockdiagram the converter is a kind of a blackbox with an comparator 
outside that controls something in the box, but that is - at least for 
me - not enough to infer from it the detailled function of the converter.



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