How to explain negative gain of VCO(Kvco)

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B_rok

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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
 

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

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

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!
 
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