linear FM : was RE: [sdiy] Temperature Compensated VCO attempt - help?

[René Schmitz]

At 18:05 04.02.03 +0100, Magnus Danielson wrote:
>Actually, what I said was than when you reach EITHER of the two limits you
>want to swap the direction, regardless of which limit and which direction you
>currently have in your "storage element" or "state machine".

Right.

>You are correct that there is an issue with meta stability. You always have
>this issue creaping up on you.

I once proposed a circuit which does this. There should be quite a bit about it 
in the archives because we did discuss here this back then. It was in '99. The 
subject was "thru zero VCO schematic".
However the issues with forbidden states or metastability weren't resolved, so I 
ended up with the design which is at my page. Which is saving the rectification 
of the modulation signal, compared to the EN design, but still has a nonsteady 
transition because the sign of the FM input must still be sensed.

>One way of solving the issue is to have the linear FM bandwidth limited (to
>say 20kHz or something) and toss in a slight hysteresis for the upper and lower
>limit comparators, enought to ensure you get long enought pulses. Then you
>should basically be home free.

The problem lies in the nondeterminism: What do you do if the FM signal changes 
its sign during that pulse?

Cheers,
 René


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159