How does differential VCO Oscillate ?

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Hi Edaboarders,

I know that ring VCO can function only if the ring is constructed using an impair number of delay element. Could someone help me understand why a differential VCO constructed with a pair number of differential delay cell oscillate ?
 

Oscillator is nothing but a badly designed amplifier!!!
You can move pole to the RHS (e.g) ... etc

Pls refer Razavi: page 515
Differential stage with variable -ve resistance load
 

For a ring oscillator to function, all it needs is 180deg phase difference between the start and end of the chain (so that the oscillation frequency is 1/(2*the total delay)) and enough gain to carry the signal from start till end. While using differential delay blocks, we have complementary outputs., this provides us the luxury of working with any number of delay elements above 1. In single ended ring, we have only one output phase available and this necessitates odd number of stages to be used.
 

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