[sdiy] Transistor questions

[David G Dixon]

Hello Spiros,
 
Yes, I have only designed and built triangle core VCOs before now, for the
very reasons to which you allude -- the "Dixie" and the "Rubicon" were the
results of my efforts -- both of which track very well.
 
The bidirectional current source is very easy to achieve, and back-to-back
zeners (or, in my case, a "zener bridge") ensure accurate levels.  Also, if
you apply balanced AM to the square wave, you can achieve through-zero FM.

  _____  

From: Spiros Makris [mailto:spirosmakris92 at gmail.com] 
Sent: Wednesday, March 05, 2025 10:57 PM
To: David G Dixon
Cc: Roman Sowa; synth-diy mailing list
Subject: Re: [sdiy] Transistor questions


[CAUTION: Non-UBC Email]	
An interesting observation in regards to the sawtooth core: 
Its operation has an inherent non ideality that can be optimised but never
eliminated (you will never switch "instantaneously" no matter what); the
apparent simplicity comes at the cost of balancing transistor capacitance,
conduction resistance, trace inductance, comparator hysterisis and delay
etc. The reset portion of the waveform will last a constant amount of time,
which eventually becomes comparable to the adjustable ramp portion, and that
messes your tracking, as well as the waveform itself (although I'd argue you
won't be able to hear that difference in texture).
In contrast, the triangle core oscillator does not have this inherently
unachievable feature, since the discharge/charge currents are not required
to ever become infinite. Of course, the comparator propagation delay,
frequency dependence of hysteresis, capacitor leakage will create a set of
constraints in operation. However, it is admittedly easier to make a very
fast accurate comparator than making an adequate reset circuit for a
"wideband" tracking sawtooth core. The cost you have to pay is a
significantly more complex bidirectional current source, which cannot be
realised easily with discrete components.
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