VCO problem what's problem?????

[EHY]

I made differential 4-stages Ring VCO to make CDR circuit.

When i connect the 4-stages VCO to PD, the 4-stages VCO does not oscillate.

,but when the 4-stages VCO stand alone, it oscillate.

also, When i connect the 3-stages VCO to PD, 3-stages VCO oscillates.


To summarize,

When i use even-stages VCO to connect to PD , the VCO does not oscillate.

but When i use odd-stages VCO to connect to PD, the VCO oscillates.

what's difference?? what's problem??

i have to use 4-stage VCO to utilize 0,90 degree phases clock.

 

[BigBoss]

The VCO is probably heavily loaded by PD.You should use a buffer to decrease this loading effect.Because Oscillators are-more or less-impacted its loads.
But-I think- your VCO is not very well designed and it's pretty sensitive to loading conditions.It shouldn't be too much sensitive.At least, it has to work under moderate load impedances.
I don't talk about extreme cases but it must provide this flexibility..