[sdiy] Through Zero FM VCO

[Stewart Pye]

Hi,

I've drawn up a schematic for a triangle core through zero FM VCO with 
part of the circuit based on the EN129 VCO (namely the 4013 flip flop to 
control the integrator current direction). 
electronotes.netfirms.com/*EN129*.pdf .  The rest of the circuit is 
quite different.

I don't have all the parts I need to breadboard it here, and I was 
wondering if anyone has used the EN129 circuit or something similar, and 
what kind of results they had. On his website Donald Tillman mentions a 
problem with using a triangle core VCO for through zero FM:

"If the control voltage goes through zero at the same time the triangle 
wave hits a reference voltage then it shouldn't be necessary to switch 
current sources at all.  If the wrong decision is made when the triangle 
wave hits a reference voltage, the triangle voltage could get lost 
beyond the reference voltage."

The 4013 should take care of this as long as the pulse from the upper 
and lower trip point comparators is long enough, as the 4013 clock input 
is not effective when the SD or CD are held high. I'd be interested to 
hear if anyone's tried this type of circuit...

Regards,
Stewart.