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# Frequency Domain Analysis – 1 – 4

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

Explanation
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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

Explanation
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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

Explanation
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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

Explanation
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20. For a second – order system, ωn = 8 and ξ= 0.5. The value of ωr in rad/s is :

(a) 4.657
(b) 5.657
(c) 6.657
(d) none of these