Designer’s Guide Community

email_gz

Junior Member

Offline

Posts: 31
gaoz.mail@gmail.com

two stage ring vco question?
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!!!

Back to top

vco_ac.JPG

IP Logged

email_gz Junior Member Offline Posts: 31 gaoz.mail@gmail.com	Re: two stage ring vco question? Reply #1 - Dec 19^th, 2006, 8:58pm tran
Back to top	vco_tran.JPG IP Logged

email_gz Junior Member Offline Posts: 31 gaoz.mail@gmail.com	Re: two stage ring vco question? Reply #2 - Dec 19^th, 2006, 9:07pm schematic vco 2 stage
Back to top	vcos.JPG IP Logged

jeffyan

Community Member

Offline

Posts: 47

Re: two stage ring vco question?
Reply #3 - 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

Back to top

IP Logged

email_gz

Junior Member

Offline

Posts: 31
gaoz.mail@gmail.com

Re: two stage ring vco question?
Reply #4 - 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

Back to top

IP Logged