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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Adam,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>To answer your question, let’s talk
about triangle-core VCOs for a moment, since this is what I have the most
direct experience with.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>The typical tricore VCO is a “tri-square”
core – a comparator compares the sum of the triangle wave and a voltage-limited
(i.e., with back-to-back zeners) square wave (generated from the comparator)
with ground. When this signal passes through ground, the comparator and
the square wave flip states. This changes the direction of current to the
triangle integrator, and defines the voltage of the triangle apices. That’s
how a basic tri-square oscillator works, such as you would find in a Dixie VCO.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>To get Linear FM on this beast, a linear
VCA is inserted in the loop between the square-wave generator and the triangle
integrator. If the gain of this VCA is taken to zero, then the oscillator
stops. If the gain of this VCA is taken negative, what happens? The
triangle and the square wave are now not in the correct phase relationship, so
that their sum will move away from ground, and will get stuck at one of the
voltage rails. Again, the VCO will stop because the comparator will never
flip. In order for the VCO to continue oscillating under this condition,
the triangle signal being summed at the comparator input must be inverted. This
requires a bit of logic to read the polarity of the Linear FM VCA’s gain
(or some other convenient indicator of the triangle current’s direction) to
drive an optional inverter.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>When this is all done correctly, the VCO
will continue to oscillate normally even though the Linear FM gain has gone
negative (i.e., has passed “through zero”). If the TZFM
circuit has been designed well, then there will be no perceptible “glitch”
when the FM passes through zero, and the resulting waveform will pass smoothly
and symmetrically through a slope of zero. This is what is achieved with,
for example, a Rubicon VCO.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>If Linear FM gain is broad and passes through
zero this gives a much deeper and richer-sounding FM. It is the basis of
bells, mallots, and all sorts of other strange and wonderful effects. You
generally do not find TZFM on Roland, Korg or Sequential synths. There was
(is) nothing fundamental preventing these makers from incorporating analog TZFM
– they just chose not to. When I finally get around to building my
own dream monosynth, it will definitely have TZFM built in (it’s actually
pretty easy, and getting easier – I’ve got it down to almost no
additional circuitry beyond the conventional VCO core – maybe two or
three additional opamps and a JFET – thanks to my beloved 2164). Hopefully,
that will happen sometime later this year.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Dave<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<div>
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<hr size=2 width="100%" align=center tabindex=-1>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>Adam Inglis<br>
<b><span style='font-weight:bold'>Sent:</span></b> Saturday, January 04, 2020
6:25 PM<br>
<b><span style='font-weight:bold'>To:</span></b> ColinMuirDorward; Synth DIY<br>
<b><span style='font-weight:bold'>Subject:</span></b> [sdiy] Thru-Zero, was
as3340 PW/tuning issue</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Thanks Colin<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Someone in that thread says they would love a "through-zero
version”.<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Seems to be a lot of talk about TZ all over these types of forums -
why? What’s the big deal? How are these different from the VCOs in my
Roland, Korg and Sequential synths?<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Sure I could google but I know I’d get better education here ;-)<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Adam<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><br>
<br>
<o:p></o:p></span></font></p>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>On 5 Jan 2020, at 6:26 am, ColinMuirDorward <<a
href="mailto:colindorward@gmail.com">colindorward@gmail.com</a>> wrote:<o:p></o:p></span></font></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div>
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<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Just a note to say thanks to everyone contributing to this
conversation. I've applied lots of what I learned here to my design, and I've
got the PWM/pitch interaction down to what I think is an acceptable minimum.
Anyways, it's nothing special, but I've started a <a
href="https://www.muffwiggler.com/forum/viewtopic.php?f=17&t=225670">build
thread over at muffs</a>, for anyone who needs yet another low cost 3340 vco. I
need several, so thought I may as well go public in case anyone else does, too.
I'll be using JLCPCB assembly for all the resistors and caps, and the rest will
be through hole at home. I have lots of as3340s, so can include with the PCB if
anyone needs. 3340 is running on +10/-5, so the circuit should be at home in
12v or 15v setups. I'm in <st1:place w:st="on"><st1:City w:st="on">Yukon</st1:City>,
<st1:country-region w:st="on">Canada</st1:country-region></st1:place>.<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Thanks,<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Colin<o:p></o:p></span></font></p>
</div>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>On Fri, Jan 3, 2020 at 12:39 PM David G <st1:City w:st="on"><st1:place
w:st="on">Dixon</st1:place></st1:City> <<a href="mailto:dixon@mail.ubc.ca">dixon@mail.ubc.ca</a>>
wrote:<o:p></o:p></span></font></p>
</div>
<blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;
margin-left:4.8pt;margin-right:0in'>
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<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=2 color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'>The Intellijel Rubicon (and Rubicon-2) gives the option of
generating pulse waves from Triangle, Saw, or Double-Saw (twice the
frequency). It’s not a whole synth, just a TZFM VCO.</span></font><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=2 color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'> </span></font><o:p></o:p></p>
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face="Times New Roman"><span style='font-size:12.0pt'>
<hr size=2 width="100%" align=center>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'><o:p> </o:p></span></font></p>
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</blockquote>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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