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Design >> Analog Design >> What should I infer from the Gain and Phase plot
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Message started by bharat on May 12^th, 2009, 1:34pm

Title: What should I infer from the Gain and Phase plot
Post by bharat on May 12^th, 2009, 1:34pm

All,
I am pasting the phase and gain plot but not sure what to infer regarding numbers of poles and zeros and its location.

Looking for help.

Thanks

Title: Re: What should I infer from the Gain and Phase plot
Post by buddypoor on May 12^th, 2009, 11:53pm

bharat wrote on May 12^th, 2009, 1:34pm:

All,
I am pasting the phase and gain plot but not sure what to infer regarding numbers of poles and zeros and its location.
Looking for help.
Thanks

At first, one general remark: When you ask for interpretation of a gain/phase plot you always have to tell us WHAT the CONTENTS of the plot is.
In this case, it seems to be the loop gain of a system with feedback.
The peak of the gain in the high frequency region and the phase jump at this frequency indicate INSTABILITY of the loop gain.

Title: Re: What should I infer from the Gain and Phase plot
Post by raja.cedt on May 13^th, 2009, 1:18am

as buddy poor said this is completely unstable because of the peaking in high frequency range where loop gain is grater than one.I have a question here that how did you measure the loop gain means are you useing .stb method or putting lc filter.I think because of lc filter that phase change at 10^8 is happening,

Thanks,
Rajasekhar.

Title: Re: What should I infer from the Gain and Phase plot
Post by bharat on May 13^th, 2009, 2:14am

Thanks buddy and raja.

Buddy,
I agree, this is Gain and Phase plot of feedback system. The reply I was expecting to infer the phase and gain plot in a way that;
the first pole occurs at few hundreds of KHz. Once it crosses the 0dB X axis, it sees 3 zeros at almost neighboring frequency.
1st zero will make -20dB slope to straight line, 2nd zero will make it +20dB and third zero as +40dB because there is sharp inflexion in the plot. This is occuring at few hundreds of MHz.
After these zeros, again a pole has occured at around 1GHz freq.
Is this correct inference.
Though I am not quite sure on seeing the phase response.

Thanks

Title: Re: What should I infer from the Gain and Phase plot
Post by raja.cedt on May 13^th, 2009, 4:30am

hi bharat,
there are only two zero's not three and those are complex zero's. One pole is at 2k and another is 1meg.Your magnitude plot is falt at high frequency means poles and zeros should be same...

Hope this helps,
Thanks,
rajasekhar.