[sdiy] VCO tuning philosophy re-visited

[Ian Fritz]

Hi Mike --

>It was a discrete transistor design, quite complex for the era. It has a
>grounded capacitor fed by a current source, buffered by a FET. The FET
>connected to two comparators, which in turn set and reset a bistable. The
>bistable controlled some kind of current mirror in the charging circuit,
>(this is the bit I can't recall properly). The core wave produced was a
>triangle. It climbed  to the trigger voltage of one comparator, which then
>set the bistable, switching on the current mirror, and discharging the
>capacitor to the second comparator trigger voltage, where the bistable
>reset, and the sequence continued.

Similar to my Tri VCO:
http://home.comcast.net/~ijfritz/sy_cir4.htm
Also similar to the EN TZ VCO in using a bistable. I believe this method 
does have a reset error -- the wave overshoots its target a bit while the 
comparators switch. Anyway it definitely seems quite different from the 
charge balance oscillator.  (See the LM331 literature for the details of 
the operation).


>The same designer began with a voltage controlled astable using bipolar
>transistor cuurent sources, in a FET astable, claiming a 10^7:1 frequency
>range. Tim Orr used that VCO as the driver for his VCO3, a distant ancestor
>of Ian's 'Double Deka'; originally a 6 step sequencer, but with the note
>that the range extended up to audio..

I did a headslap when I saw that a few years back. His method was a bit 
simpler than mine.  BTW, the original DD goes back to the same time era. It 
used (still does, actually) an optimized OTA-based Tri VCO to track up to a 
fundamental of 20-30kHz.

Ian