Communication miscellaneous

Communication miscellaneous

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Communication

Communication miscellaneous
  1. For a bit-rate of 8 kbps the best possible values of the transmitted frequencies in a coherent binary FSK system are








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    Since, bandwidth of signal FSK
    = 4fb = 4 × 8 = 32 kHz

    Correct Option: D

    Since, bandwidth of signal FSK
    = 4fb = 4 × 8 = 32 kHz

  1. Consider a sampled signal
    y(t) = 5 × 10– 6 x(t) ∑∞n = – ∞ δ (t – nTs)
    where x(t) = 10 cos (8π × 103)t and Ts = 100μ sec. When y(t) is passed through an ideal low pass filter with a cut-off frequency of 5 kHz, the output of the filter is








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    Given sampled signal,
    y(t) = 5 × 10– 6 x(t) ∑∞n = – ∞ δ(t – nTs)
    Ts = 100 μ sec
    x(t) = 10 cos (8π × 103t)

    fs =
    1
    		 = 10kHz
    100 ×10−6

    and
    fm= 4 kHz(given)
    Since, fs > 2fm and cut-off frequency fc > fm. Hence the original signal can be recovered in frequency domain by passing its sampled version through a low pass filter (LPF).
    The output of the filter = 5 × 10–6 × 10 cos (8π × 103t)
    = 5 × 10–5 cos (8π × 103t)

    Correct Option: B

    Given sampled signal,
    y(t) = 5 × 10– 6 x(t) ∑∞n = – ∞ δ(t – nTs)
    Ts = 100 μ sec
    x(t) = 10 cos (8π × 103t)

    fs =
    1
    		 = 10kHz
    100 ×10−6

    and
    fm= 4 kHz(given)
    Since, fs > 2fm and cut-off frequency fc > fm. Hence the original signal can be recovered in frequency domain by passing its sampled version through a low pass filter (LPF).
    The output of the filter = 5 × 10–6 × 10 cos (8π × 103t)
    = 5 × 10–5 cos (8π × 103t)

  1. A 1 MHz sinusoidal carrier is amplitude modulated by a symmetrical square wave of period 100 μ sec. Which of the following frequencies will not be present in the modulated signal?








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    Given square wave, this carrier, square wave can be represented as From this equation we see that only odd components are present. Hence from the given option frequency component 1020 kHz is not present in the modulated signal. Hence alternative (C) is the correct choice.


    Correct Option: C

    Given square wave, this carrier, square wave can be represented as From this equation we see that only odd components are present. Hence from the given option frequency component 1020 kHz is not present in the modulated signal. Hence alternative (C) is the correct choice.


  1. A linear phase channel with phase delay τp and group delay τg must have—








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    For a linear phase channel both phase delay τp and group delay τg should be constant.

    Correct Option: A

    For a linear phase channel both phase delay τp and group delay τg should be constant.

  1. In an AM signal the received signal power is 10– 10 W with a maximum modulation signal of 5 kHz. The noise spectral density at the receiver input is 10–18 W/Hz. If the noise power is restricted to the message signal bandwidth only, the signal-to-noise ratio at the input to the receiver is—








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    Message signal BW fm = 5kHz
    Noise power density = 10–18 W/Hz
    Total noise power = 10–18 × 5k = 5 × 10–15W
    Input signal-to-noise ratio

    (SNR)i=
    10−18
    		 = 2 × 10−4
    5 ×10−15

    or (SNR)i = 10log10 2 × 104 = 43dB
    Hence alternative (A) is the correct choice.

    Correct Option: A

    Message signal BW fm = 5kHz
    Noise power density = 10–18 W/Hz
    Total noise power = 10–18 × 5k = 5 × 10–15W
    Input signal-to-noise ratio

    (SNR)i=
    10−18
    		 = 2 × 10−4
    5 ×10−15

    or (SNR)i = 10log10 2 × 104 = 43dB
    Hence alternative (A) is the correct choice.