thru zero VCO questions

[Rene Schmitz]

Hi Jürgen!

Hm, why switching at the zero crossing ?! To me it makes only sense when I
rectify the FM input to put it into the expos ref. input. But one can also
directly integrate the FM input. Much like in two opamp LFOs. As I see it I
have to toggle the direction when either the upper or the lower threshold
is exceeded. When the FM input changes sign, so does the integration
direction, I don't need to switch anything for that. I only need the FF to
remember the last direction. When either of the thresholds is met, then the
polarity of the FM input signal is inverted. 

So my idea looks like this:     
							
FM in -> polarity switch -> OTA integrator (Iabc = expo I) -> 2 comparators
->ORd together into clk of toggle D-FF, output controls the polarity switch.

>For more recent thru zero VCOs I have abandoned this concept, however.
>The major problem is that you might loose a pulse when your modulation
>signal just touches zero and only goes slightly negative. This can add some
>"dirt" to the signal at small modulation depth. Not always unpleasant, but
>not very precise either. I have not made direct comparisons with VCOs
>of the other method (up / down switching with the sign of modulation
>voltage) - I've built them, but for different application.

This circuit has a similar problem, when the integrator exceeds the
threshold voltage, and the FF fails to toggle, then it locks up.

>But think of it: What you really need is a *switch* function to change the
>VCO spinning direction, an what you have is a *sign* function of the
>modulation signal, so that's exactly what you need. Why make a double
>pulse from the sign signal, and restore the switch function by toggling
>a flipflop with this double pulse ? (The 3340 *needs* this, but a discrete
>solution doesn't.)

Since I don't want to do that, I'll answer with an other question. 

Why fullwave rectify the modulation signal and then restore it via a
switched optional inverter. In other words, its not neccessary to put the
FM input into the expo convertors linear FM input. A normal integrator is
already a "thru zero" integrator, but the hysteretic switch doesn't respond
right.

Bye 
 René

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