I find that perhaps it's effect is like open loop 3 c/ b# E5 _( T9 c/ b; ^modulation. I mean that the average vco input voltage is fixed by another negative feed back loop % N) \* L+ C5 J/ Hbut add some small ac voltage into the vco input node to get like Frequency modulation$ r: O/ g6 M+ P% D; H
(spread spectrum effect) the above is my concept / d0 N+ e( \. n2 }( x) v! Yjust provide you to think about it