question about the gain margin of TYPE-II PLL

[bladewade]

According to the bode plot of the loop gain, at low frequency, the phase shift is 180 degree, while its gain is well above 1(0dB).
It is unstable when we refer to Barkhausen criterion.
Will the low frequency component oscillate?
I have seen a similar question in here but the answers in that thread is really chaotic. So I ask this question again.

 

[enthu_elec]

According to the bode plot of the loop gain, at low frequency, the phase shift is 180 degree, while its gain is well above 1(0dB).
It is unstable when we refer to Barkhausen criterion.
Will the low frequency component oscillate?
I have seen a similar question in here but the answers in that thread is really chaotic. So I ask this question again.

Hello,

Yes, the low frequency component will oscillate and if the gain is anything above 0db, the oscillation and low frequency component power would increase.

A neat explaination of the phase margin and gain margin is explained in the following site :
**broken link removed**

Rohit

 

[zorro]

There is another condition for unstability for closed loop sistems in general (PLL is a particular case): the phase characteristic must cross the 180 degrees phase characteristic with negative slope and |G|>=1.
This was discussed rather extensively in this board some time ago. You can search the appropriate thread (I hope it's not the same one you mentioned in you post!).
Regards

Z

 

[zorro]

I'm here again.
This is the thread to which I referred, where conditions for stability/instability were discussed:

https://www.edaboard.com/threads/232066/

Please see my posts (#21, 23 and 25).
Regards

Z

 

[LvW]

According to the bode plot of the loop gain, at low frequency, the phase shift is 180 degree, while its gain is well above 1(0dB).
It is unstable when we refer to Barkhausen criterion.
Will the low frequency component oscillate?
I have seen a similar question in here but the answers in that thread is really chaotic. So I ask this question again.

Be cautious when using the term "loop gain". A stable feedback loop needs a phase shift of -180 deg for very low frequencies (including dc)
and must not reach -360 deg for loop gain values above 0 dB.
With other words: The definition of "loop gain" includes the sgnal inversion for negative feedback.
In many cases, this signal inversion is NOT included in the BODE plot.
Therefore: If your BODE plot starts at -180 deg and does NOT reach -360 deg for loop gains > 0 dB the loop is stable!