There’s been some chat in the CAKE thread about Through Zero FM - to oversimplify, FM that stays in tune. Turns out that this is actually possible with our beloved 3340, with “a bit of extra circuitry”.
I found this as an example:
from here: Juergen Haible
It’s a bit rich for my blood, but I thought I’d share it as a point of interest for anybody looking to explore this more.
At the risk of blowing past the topic in the second post, a couple of op amp through zero designs are these:
http://ijfritz.byethost4.com/sy_cir16_teezer.htm
https://www.haraldswerk.de/VCO/Trapezoid/Trapezoid_VCO.html
(and another version of the latter here)
Gerbers and everything, nice!
The “thru zero” bit is the rectifier and the zero crossing circuitry (two comparators and a couple of XOR gates) in the lower right corner.
(so you could make this entirely optional, maybe even via a daughterboard).
