How to explain negative gain of VCO(Kvco)

[B_rok]

I am currently designing differential ring oscillator. I managed to get the oscillation and now trying to vary the Vctrl to get the output frequency. After changing the voltage i managed to plot my frequency vs Vctrl graph. As my Vctrl increases, my frequency decreases.Anyone have any idea how to explain this case?Thx in advance

 

[saro_k_82]

In the differential ring, if Vctrl increases the bias current, the frequency would increase, otherwise it would decrease giving a negative Kvco.

 

[love_analog]

BTW, you always want a positive KVCO if you are doing a PLL (at least). Otherwise your whole loop will be unstable.

 

[saro_k_82]

Nope. You just have to change the polarity somewhere else in the loop. Negative KVCO is perfectly acceptable. What is not acceptable is the change of sign during operation!