I find that perhaps it's effect is like open loop 9 i3 w0 n0 d& \2 Jmodulation. I mean that the average vco input voltage is fixed by another negative feed back loop1 U, X/ G6 l. H5 q& J
but add some small ac voltage into the vco input node to get like Frequency modulation; c; x5 ~' J6 {8 F; b- ^6 `
(spread spectrum effect) the above is my concept 7 R' n8 E$ p5 y7 Z
just provide you to think about it