PLL Phase margin at low frequency

[Y.C Park]

pll phase margin

Hi experts !!!

When check the phase margin in Negative feedback system, Normally we could simulate the phase at unit gain frequency in Open loop transfer-function
Is it right?

But the attached PLL phase margin simulation result give me confusing about low frequency phase margin is vicinity of Zero.
Please let me know the reason why this PLL stable.

Thanks in advance
Y.C

 

[eda4you]

phase margin pll

hi,

its o.k. the pahse margin is here 90 degrees. at moderate frequencies the zero forces a increase in phase before the lowest low pass pole decrease again the .
It seems that you used a normal 2nd order pll filter. the vco of the pll causes a 90 degree phase shift, and one capacitor of the filter also --> resulting in a 180 degree phase shift at low frequencies. in plls the phase margin is defined as the difference between this 180 degree and the phase at the open loop cut off frequency. btw it is always smaller 90 degree!

hope i could help you

 

[xuel]

filter phase margin

1 right
2 your phase margin is about 60 degree, so stable.
For low frequencies, phase is near 180, but loop gain is more than 1, so from close loop transfer function Hclose(s)=Hopen(s)/(1+Hopen(s)), 1+Hopen(s) != 0.

 

[edajason]

phase margin in pll

1+Hopen(s)!=0, so your closed loop is okay?

1 right
2 your phase margin is about 60 degree, so stable.
For low frequencies, phase is near 180, but loop gain is more than 1, so from close loop transfer function Hclose(s)=Hopen(s)/(1+Hopen(s)), 1+Hopen(s) != 0.

 

[shaikhsarfraz]

pll phase margin formula

Hi,

It seems Your loop filter is a 2nd order passive loop filter.
As far as I can see your phase margin is nearly 60 degree.
(And there is never a phase change of 180 so system is unconditonally stable)

thanks
sarfraz

 

[Y.C Park]

pll low frequency

EveryBody thanks!!!
Good luck to you !!!

 

[surianova]

low frequency pll

hi,

its o.k. the pahse margin is here 90 degrees. at moderate frequencies the zero forces a increase in phase before the lowest low pass pole decrease again the .
It seems that you used a normal 2nd order pll filter. the vco of the pll causes a 90 degree phase shift, and one capacitor of the filter also --> resulting in a 180 degree phase shift at low frequencies. in plls the phase margin is defined as the difference between this 180 degree and the phase at the open loop cut off frequency. btw it is always smaller 90 degree!

hope i could help you

i think the PM shd be 120 degree, not 90 or 60?

 

[edajason]

open-loop transfer-function pll spectre

Should be 60 degree.

hi,

its o.k. the pahse margin is here 90 degrees. at moderate frequencies the zero forces a increase in phase before the lowest low pass pole decrease again the .
It seems that you used a normal 2nd order pll filter. the vco of the pll causes a 90 degree phase shift, and one capacitor of the filter also --> resulting in a 180 degree phase shift at low frequencies. in plls the phase margin is defined as the difference between this 180 degree and the phase at the open loop cut off frequency. btw it is always smaller 90 degree!

hope i could help you

i think the PM shd be 120 degree, not 90 or 60?

 

[wjlzhx]

pll phase margin

yes!!
i get the same project.
and i give the charge pumu current is 16uA.
the function is the same as you .
and i got 60 degree .but when i simulate it in cadence use spectre's tran.
it has some different.what's the problem.