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Bode Plot

  • It is used to draw the frequency response of an open loop & a closed loop system. It is also called corner plot & asymptotic plot.
  • The representation of logarithm of 688.png & phase angle of 693.png, both plotted against frequency in logarithmic scale & these plots are called Bode plots.
  • Here we determine gain margin & phase margin.
  • The gain margin is determined by:
    698.png

where703.png is the phase cross frequency where708.png line crosses the frequency axis.

- When 713.png < 1, the G..M. in decibel comes out to be positive indicating a stable system.

- When 718.png = 1, the G..M. in decibel comes out to be zero indicating a marginally stable system.

- When 723.png > 1, the G..M. in decibel comes out to be negative indicating an unstable system.

  • The phase margin is given by:
    728.png
    where 733.pngis measured clockwise & 739.pngis the gain crossover frequency at which 744.png line crosses the frequency axis. 
  • For a stable system, phase margin should be positive and the gain cross over frequency should be less than the phase cross over frequency.
  • For unstable systems: the gain cross over frequency > phase cross over frequency.
  • For marginally stable systems: the gain cross over frequency should be equal to the phase cross over frequency.





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