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Design >> Mixed-Signal Design >> two stage ring vco question?
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Message started by email_gz on Dec 19^th, 2006, 8:57pm

Title: two stage ring vco question?
Post by email_gz on Dec 19^th, 2006, 8:57pm

Hi everybody
I got puzzled by two stage ring vco oscillation . As we kown in frequency domain , Barkhausen criteria tell us , a two stage ring vco can oscillation at a special w, only if each delay cell transfer function satisfy:
∠Av(jw)≥90 Degree ,and |Av(jw)| ≥1.
I simulated a pmos cross-coupling positive feedback delay cell .
Hspice ac analysis resault : its ac phase shift is only about 54 degree when its gain drops to unit.
So theoretically, a two stage ring vco builded by this delay cell should not oscillate at time domain .
But Hspice tran analysis gives a contrary result . But Why???
Any suggestion is welcome!!!

Title: Re: two stage ring vco question?
Post by email_gz on Dec 19^th, 2006, 8:58pm

tran

Title: Re: two stage ring vco question?
Post by email_gz on Dec 19^th, 2006, 9:07pm

schematic vco 2 stage

Title: Re: two stage ring vco question?
Post by jeffyan on Dec 20^th, 2006, 2:36am

hi,
i think, you should make it clear to us that what is your transfer function related in your plot ?
if it is Vop/Vip, then i think, the phase is right, because it >180 degree, which is needed for stable oscillation.
if not, i think, you should check your ckt simulation, if the ckt work in its right operation, because it might be latched to two rail supplies.
good luck
jeff

Title: Re: two stage ring vco question?
Post by email_gz on Dec 20^th, 2006, 6:16pm

jeff ,thanks for your reply!
My ac analysis tool is hspice and input output configuration as following:
******input
vctrn ctrn gnd! 1.8v
vdm dm gnd! dc 0v ac 1v
vcm cm gnd! 0.9v
e1 vip cm dm 0 1
e2 vin cm dm 0 -1
******output
.op
.ac dec 20 0.1 10000Meg
.pz v(vop,von) vdm
.print vdb(vop)
.print vp(vop)
.print vdb(von)
.print vp(von)
**********************************
Hope these can give you some helpul information

Best Regards,
email_gz