I find that perhaps it's effect is like open loop9 r# ]# z' h4 e0 L" G
modulation. I mean that the average vco input voltage is fixed by another negative feed back loop : w: ] w4 y: j: b. H7 T3 Mbut add some small ac voltage into the vco input node to get like Frequency modulation& |. S! |# \& H1 O# Z
(spread spectrum effect) the above is my concept 8 Z/ r4 E0 p f! S8 ~( Pjust provide you to think about it