VCO start up with big decap on top tail resistor

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h237356

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I have a CMOS LCVCO with tail resistor only on top to provide bias current. Between this resistor and sources of cross coupled PMOS transistors, a big decap is added (~70pF). The VCO can start oscillating at TT corner, but couldn't at SS corner. If the big decap is removed, SS corner can work. What could be the reason?

Thanks in advance!!
 

i think i didn't get you right... But as far as i know , decap is used to reduce supply variations...


it shoulb be connected between VDD and GND rails...

And decap must be isolated from the VCo , i think....

Let me know if you have solution... Thanks...
 

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