Exercise: Communication Systems

Questions for: Communication Systems

The F.T. of {e-1 ∪(t)} is equal to . Therefore
is
A:
ef ∪ (f)
B:
e-f ∪ (f)
C:
ef ∪ (-f)
D:
e-f ∪ (-f)
Answer: C

Apply duality property.

j F[g(t)] G(f)

F[G(t)] G(-f).

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The total number of channels allotted for cellular telephone is
A:
666
B:
333
C:
221
D:
111
Answer: A

Cellular telephones have been assigned 666 channels (826 to 845 MHz and 870 to 890 MHz). Each channel has width of 30 kHz.

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An FM signal with deviation δ is passed through mixer. Its frequency is reduced five fold. The deviation in output of mixer is
A:
0.28
B:
δ
C:
5 δ
D:
very very high
Answer: B

Deviation is not affected by mixer.

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The equation v(t) = A cos [ωct + φ(t)] where A and ωc are constant and φ(t) is a function of base signal represents
  1. Phase modulation
  2. Amplitude modulation
  3. Angle modulation
  4. Frequency modulation
Which of the above are true?
A:
3 only
B:
1, 3 and 4
C:
1, 2, 3, 4
D:
3 and 4 only
Answer: B

This equation can represent frequency, angle and phase modulation.

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An AM signal and a narrowband FM signal with identical carriers, modulating signals and modulation index of 0.1 are added together. The resultant signal can be closely approximated by
A:
Broadband FM
B:
SSB with carrier
C:
DSB-SC
D:
SSB without carrier
Answer: B

s(t)AM + s(t)NBFM = A1 sin ωct + A2 cos ωct + (A - B) m

m(t) sin ωct

it will not be NBFM because μ = 0.1, and SSB with carrier.

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