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ywguo

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Re: The oscillation of one high-pass filter and its loop gain
Reply #15 - Apr 24^th, 2013, 8:08pm

Hi Raj,

I am sorry that I make this thread confused you. I tried to analyzie the stability of that high-pass filter with Nyquist plot. I am not familiar with this method. So Nyquist plot of the regulator is plotted for comparison. It would be clear if I had started a new topic about the Nyquist plot of the regulator.

Best Regards,
Yawei

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LazyDesigner

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Re: The oscillation of one high-pass filter and its loop gain
Reply #16 - May 27^th, 2014, 4:43pm

Hi
1. I just want to understand in conclusion, was the ckt. with above bode plot and nyquist plot is unstable? I am having the similar non-monotonic loopgain. My ckt. is second order MFB biquad LPF.
2. Does the placement of spectre iprobe at any point matter in loop.
3. Can we have different stability conclusion if loop is low pass and not high pass as we discussed in the case presented.
Regards,
Pravesh

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ywguo

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Re: The oscillation of one high-pass filter and its loop gain
Reply #17 - May 27^th, 2014, 7:05pm

Hi Pravesh,

I need to clarify that the first and second post in this thread is the simulation result of one high-pass filter. The other plots are results of one regulator. The regulator is stable.

Of course the placement of spectre iprobe matters. It must in a point that shared by all loops in the circuit.

Best Regards,
Yawei

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LazyDesigner

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Re: The oscillation of one high-pass filter and its loop gain
Reply #18 - May 27^th, 2014, 9:29pm

Hi Yawei,
Thankyou very much for your prompt reply.
Now It seems that I understand your post and issue that you are raising in Nyquist plot for a stable regulator. In my MFB biquad filter I do have the nyquist plot encircling the (-1,0) point once in clockwise. But my bode plot which is having non-monotonic phase response in which phase rises before 0dB shows ~90degree PM at 0dB point and I verified the same phase margin using step change on supply voltage. But I worried about stability because of my nyquist plot and non-monotonic phase response, what's your opinion here.

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ywguo

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Re: The oscillation of one high-pass filter and its loop gain
Reply #19 - May 28^th, 2014, 7:41pm

Hi Pravesh,

I did try to study Nyquist plot, but I have never analyze stability of my circuits successfully with Nyquist plot. :( So it's hard for me to say that is enough for your MFB biquad filter to be stable.

One more thing is that I check the stability with a step change at input, NOT on supply voltage.

Best Regards,
Yawei

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