---
+ Post New Thread
Results 1 to 13 of 13
  1. #1
    Member level 2
    Points: 1,107, Level: 7

    Join Date
    Nov 2008
    Location
    USA
    Posts
    50
    Helped
    0 / 0
    Points
    1,107
    Level
    7

    phase margin

    To calculate phase margin do I always use
    PM=180+phase(t.f) where, t.f. is the closed loop transfer fn.
    So for example say the phase at 0db is +120, then what is the PM,
    180+120=300 OR 180-120=60

    Thanks

  2. #2
    Member level 2
    Points: 2,163, Level: 10
    patweadock's Avatar
    Join Date
    Feb 2009
    Location
    Near Burlington, VT
    Posts
    42
    Helped
    11 / 11
    Points
    2,163
    Level
    10

    how to calculate phase margin

    I believe the transfer function will determine the sign or direction of the margin



    •   Alt27th February 2009, 04:23

      advertising

        
       

  3. #3
    Advanced Member level 1
    Points: 4,634, Level: 16
    xulfee's Avatar
    Join Date
    May 2008
    Location
    Pakistan
    Posts
    403
    Helped
    52 / 52
    Points
    4,634
    Level
    16

    phase margin basic theory

    transfer function will tell u,but if u r getting 120,then it is to be 120



  4. #4
    LvW
    LvW is offline
    Advanced Member level 5
    Points: 37,670, Level: 47
    LvW's Avatar
    Join Date
    May 2008
    Location
    Germany
    Posts
    5,119
    Helped
    1518 / 1518
    Points
    37,670
    Level
    47

    calculating phase margin

    Quote Originally Posted by analog_match
    To calculate phase margin do I always use
    PM=180+phase(t.f) where, t.f. is the closed loop transfer fn.
    So for example say the phase at 0db is +120, then what is the PM,
    180+120=300 OR 180-120=60
    Thanks
    Hi analog_match !

    Here is a recommendation from my side:
    Before calculating phase margin you should find a definition for that.
    Here it is:
    Phase margin is related to the LOOP GAIN (thatīs the gain of the OPEN LOOP) and it is the difference between 180 deg and the actual phase at the frequency where the LOOP GAIN is 0 dB.



    •   Alt27th February 2009, 08:20

      advertising

        
       

  5. #5
    VVV
    VVV is offline
    Advanced Member level 5
    Points: 20,960, Level: 35
    VVV's Avatar
    Join Date
    Nov 2004
    Posts
    1,582
    Helped
    362 / 362
    Points
    20,960
    Level
    35

    what is phase margin

    In your example the phase margin is 60deg.
    It's really the difference between 180 deg and the actual phase of the system at 0dB.



  6. #6
    LvW
    LvW is offline
    Advanced Member level 5
    Points: 37,670, Level: 47
    LvW's Avatar
    Join Date
    May 2008
    Location
    Germany
    Posts
    5,119
    Helped
    1518 / 1518
    Points
    37,670
    Level
    47

    bode plot, phase correction

    Quote Originally Posted by VVV
    In your example the phase margin is 60deg.
    It's really the difference between 180 deg and the actual phase of the system at 0dB.
    No, thatīs wrong as you are referring to the CLOSED loop phase (transfer function).
    But you have to calculate with the loop gain phase instead (loop open).


    1 members found this post helpful.

    •   Alt2nd March 2009, 17:47

      advertising

        
       

  7. #7
    Member level 2
    Points: 1,107, Level: 7

    Join Date
    Nov 2008
    Location
    USA
    Posts
    50
    Helped
    0 / 0
    Points
    1,107
    Level
    7

    cadence open loop phase margin

    Thanks all for your reply to this question.

    LvW, I have a comment and then a further question to this thread:

    So basically what you are saying is if we have a t.f. G=A/(1+A*f)
    where:
    A->open loop gain.
    f->gain of feedback network.

    Then phase margin is calculated from magnitude and phase plot of the loop gain A*f, correct??

    Now my question which stems from my confusion of how do I plot the magnitude and phase of loog gain, A(s)*f(s) to calculate the phase margin.

    Generally when we measure a gain in a simulation tool like Pspice or cadence spectre etc. we have some input src and we put a signal of magnitude 1 in the "A.C. magnitude" field to linearize the circuit and then simply do an a.c. analysis on the output port. But this gives us the bode plot of G(s).

    If I open the loop then I can get bode plots of A(s) and f(s) individually but how do I get the bode plot of A(s)*f(s).

    Thanks



  8. #8
    LvW
    LvW is offline
    Advanced Member level 5
    Points: 37,670, Level: 47
    LvW's Avatar
    Join Date
    May 2008
    Location
    Germany
    Posts
    5,119
    Helped
    1518 / 1518
    Points
    37,670
    Level
    47

    pspice phasemargin measurement

    Quote Originally Posted by analog_match
    ....................
    LvW, I have a comment and then a further question to this thread:
    .......................
    Generally when we measure a gain in a simulation tool like Pspice or cadence spectre etc. we have some input src and we put a signal of magnitude 1 in the "A.C. magnitude" field to linearize the circuit and then simply do an a.c. analysis on the output port. But this gives us the bode plot of G(s).
    If I open the loop then I can get bode plots of A(s) and f(s) individually but how do I get the bode plot of A(s)*f(s).
    Thanks
    The answer is quite logical:
    1.) Ground the "normal" input
    2.) Open the loop and inject an ac test signal of 1 volt at one node of the cut and watch/draw the voltage at the other node of the cut.
    3.) Thatīs the PRINCIPLE. However, in most cases the bias point is lost or changed by openeung the loop. Thatīs the reason several articles and papers have been written to solve this. There are several "tricky" methods.
    The simplest method is to place the test voltage source BETWEEN both nodes of the cut. This works only if the cut is done at a low impedance point (opamp output)

    Any further questions ?

    Added after 1 hours 4 minutes:

    I forgot to mention, that in case of a test voltage in series with the cut (between) you always have to calculate the ratio output/input - even when the test voltage is 1 volt.


    1 members found this post helpful.

  9. #9
    Member level 2
    Points: 1,107, Level: 7

    Join Date
    Nov 2008
    Location
    USA
    Posts
    50
    Helped
    0 / 0
    Points
    1,107
    Level
    7

    how to calculate phase from transfer function

    Thanks LvW. Your insight has been very helpful.



  10. #10
    Newbie level 1
    Points: 721, Level: 5

    Join Date
    Dec 2008
    Posts
    1
    Helped
    0 / 0
    Points
    721
    Level
    5

    calculate phase margin

    Hello All

    If you decide to inject an AC source directly in your circuit to break the loop, how would you calculate the phase margin?

    Would it be difference of the phase between out and in?



  11. #11
    LvW
    LvW is offline
    Advanced Member level 5
    Points: 37,670, Level: 47
    LvW's Avatar
    Join Date
    May 2008
    Location
    Germany
    Posts
    5,119
    Helped
    1518 / 1518
    Points
    37,670
    Level
    47

    example for phase margin

    Quote Originally Posted by parkl
    Hello All

    If you decide to inject an AC source directly in your circuit to break the loop, how would you calculate the phase margin?

    Would it be difference of the phase between out and in?
    No, of course not. Look at the DEFINITION of the margin. It is the difference between the actual phase shift (at the cross-over frequency) and -180 deg.



  12. #12
    Newbie level 5
    Points: 733, Level: 5

    Join Date
    Mar 2009
    Posts
    10
    Helped
    0 / 0
    Points
    733
    Level
    5

    calculation of gain and phase margin in cadence

    hi,

    Using bode plot u can find the phase margin for both open loop and closed loop systems.. the magnitude plot and the phase plot are drawn and then phase angle at gain cross over frequency is found out and it is added with 180 degree..



  13. #13
    LvW
    LvW is offline
    Advanced Member level 5
    Points: 37,670, Level: 47
    LvW's Avatar
    Join Date
    May 2008
    Location
    Germany
    Posts
    5,119
    Helped
    1518 / 1518
    Points
    37,670
    Level
    47

    open loop closed loop phase margin

    Quote Originally Posted by gaya_pink
    hi,
    Using bode plot u can find the phase margin for both open loop and closed loop systems.. .
    Correction: The phase margin is defined only for the open loop system.



+ Post New Thread
Please login