I find that perhaps it's effect is like open loop5 g x, Y- _( O' Y1 N
modulation. I mean that the average vco input voltage is fixed by another negative feed back loop # S6 o6 l+ A. k2 _( e1 Z) bbut add some small ac voltage into the vco input node to get like Frequency modulation : I/ Y; X9 x- `- L& Y& y(spread spectrum effect) the above is my concept % q: Q, v6 Q: s6 {* g6 p! Pjust provide you to think about it