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Basic question about PLL phase margin

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bageduke

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pll phase margin

Hi, guys,

I have a question about PLL phase margin.

Everytime when we talk about negative feedback system, we need to make sure that before the open loop gain drop to zero, phase shift has to be smaller than 180 degree to ensure stability, like opamp.

However, for the PLL, we only think about when open loop gain drop to zero, we need to keep enough phase margin. But when open loop gain is larger than zero, phase shift could be very close to 180 degree, for example, charge pump PLL, since the whole open loop system has two origin poles and can definitely be close to 180 degree shift. Why is that? Why don't we care about phase shift at this region? Or is there anything I missed?

Thanks a lot,
 

pll phase margin formula

In zero frequency, it will not oscillate because frequency is zero.

Therefore, we don't need to consider this case even its phase is 180 degree.

Yibin.
 

acceptable phase margin pll

it needs to be considered
 

calculate phase margin of pll

Phase margin theory lose it's effect in this condition. But you can always use Nyquist plot to judge the stablity of a feedback system.
 

calculate pm of pll

the worst case Feedback is unity gain, ie when whole of the output is fed back to the input. So when gain is unity and you have negative feecback, since negative FB introduces 180 phase shift, you need to take care that the amplifier by itself does not introduce a phase shift of 180.So in effect we want the cumulative phase shift of the amplifier and the feedback network to be less than 360, so that system does not oscillate due to overall positive FB. So even for a pll when you calculate the loop filter parameters you set these parameters so that PM=60.


amarnath
 

what is phase margin in pll

amarnath said:
the worst case Feedback is unity gain, ie when whole of the output is fed back to the input. So when gain is unity and you have negative feecback, since negative FB introduces 180 phase shift, you need to take care that the amplifier by itself does not introduce a phase shift of 180.So in effect we want the cumulative phase shift of the amplifier and the feedback network to be less than 360, so that system does not oscillate due to overall positive FB. So even for a pll when you calculate the loop filter parameters you set these parameters so that PM=60.


amarnath
Click to expand...

Thanks for your answer.

But when we talked about unity gain worst case, we assumed that phase shift is rolling down monotonically so that any feedback factor smaller than unit will fall in the region which phase shift is less than 180-PM. But pll is different case which has phase rolling up first then rolling down, then at certain region which is lower than loop bandwidth, phase shift could be close to 180.
 

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