needforspeed
Member level 1
hi all,
how can we define gain margin of a 3 order PLL.
how can we define gain margin of a 3 order PLL.
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needforspeed said:hi all,
how can we define gain margin of a 3 order PLL.
LvW said:I doubt if the diagram shows really the loop gain of your linear PLL model of third order. The phase response should start at 90 deg due to the VCO integrating function.
Please give some more information.
LvW said:I doubt if the diagram shows really the loop gain of your linear PLL model of third order. The phase response should start at 90 deg due to the VCO integrating function.
Please give some more information.
jecyhale said:needforspeed said:hi all,
how can we define gain margin of a 3 order PLL.
What is the gain margin?
Why we care the gain margin?
Thanks.
Added after 4 minutes:
LvW said:I doubt if the diagram shows really the loop gain of your linear PLL model of third order. The phase response should start at 90 deg due to the VCO integrating function.
Please give some more information.
I think it is the open loop response of PLL.
needforspeed said:It's a type two PLL. Since the VCO contributes a pole at the origin and loop filter introduces the other one, phase curve approximates minus 180 degree.
LvW said:needforspeed said:It's a type two PLL. Since the VCO contributes a pole at the origin and loop filter introduces the other one, phase curve approximates minus 180 degree.
Yes - 180 deg for very high frequencies. But what about low frequencies ?
Does your phase response aproaches 90 deg ? (Cannot be seen in the diagram).
Gain margin: As your PLL is of second order, it makes no sense resp. it is even not possible to give a gain margin, since the phase never reaches 180 deg.
needforspeed said:LvW said:needforspeed said:It's a type two PLL. Since the VCO contributes a pole at the origin and loop filter introduces the other one, phase curve approximates minus 180 degree.
Yes - 180 deg for very high frequencies. But what about low frequencies ?
Does your phase response aproaches 90 deg ? (Cannot be seen in the diagram).
Gain margin: As your PLL is of second order, it makes no sense resp. it is even not possible to give a gain margin, since the phase never reaches 180 deg.
as shown in fig, phase response aproaches -180deg at low frequency. That is different from conventional control system.
i am confused.
jecyhale said:needforspeed said:LvW said:needforspeed said:It's a type two PLL. Since the VCO contributes a pole at the origin and loop filter introduces the other one, phase curve approximates minus 180 degree.
Yes - 180 deg for very high frequencies. But what about low frequencies ?
Does your phase response aproaches 90 deg ? (Cannot be seen in the diagram).
Gain margin: As your PLL is of second order, it makes no sense resp. it is even not possible to give a gain margin, since the phase never reaches 180 deg.
as shown in fig, phase response aproaches -180deg at low frequency. That is different from conventional control system.
i am confused.
Don't worry, It should be correct.
But I am confused too.
Added after 2 minutes:
As shown in the figure, it should be the open loop result of PLL.
If it is the close loop result, I am sure the loop gain is below 0dB.
LvW said:Quote:
as shown in fig, phase response aproaches -180deg at low frequency. That is different from conventional control system.
i am confused.
Yes, something is wrong.
Question: Did you model the VCO as an integrator (Kvco/s) ? In this case, the open loop response has no other choice than to approach -90 deg for very low frequencies, as the low pass starts at a somewhat higher frequency.
needforspeed said:I still in confused.
In fact the first posted png is a typical open loop response of a 3 order charge pump pll. and i have tried to explain why the phase response start from -180deg at low frequency.
why do you think it's not a 3 order system?
LvW said:Looks good - however, it is really 3rd order ?
I can see a gain slope of only -40dB/dec for high frequencies.