16. For a stable system :

(a) gain margin must be positive but phase margin can be positive or negative

(b) phase margin must be positive but gain margin can be positive or negative

(c) both gain margin and phase margin must be positive

(d) one of them must be zero

17. At the phase crossover frequency, ω =10 rad/s,│G(j ω)H(j ω) │= 15 dB. Its gain margin :

(a) 15 dB

(b) 0 dB

(c) -15 dB

(d) cannot be predicted

18. At the gain crossover frequency :

(a) │G G(jω)H(jω) │ = 0 dB

(b) │ G(jω)H(jω) │ = 1 dB

(c) │ G(jω)H(jω) │ = -20 dB

(d) │ G(jω)H(jω) │ = 20 dB

19. For the pole factor 1/(s+5), the corner frequency is :

(a) ω = 1/5

(b) ω = 5

(c) ω = -5

(d) ω = -1/5

20. For a second – order system, ω_{n} = 8 and ξ= 0.5. The value of ω_{r} in rad/s is :

(a) 4.657

(b) 5.657

(c) 6.657

(d) none of these

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