# Digital Communication MCQ for ESE (Multiple Choice Questions)

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## Digital Communication MCQ for ESE

Q.1 The bandwidth of a ‘N’ bit binary coded PCM signal for modulating a signal having a bandwidth of ‘f’ Hz is

• (a) \frac{f}{N}Hz
• (b) \frac{f}{N^2}Hz
• (c) Nf\;Hz
• (d) N^2f\;Hz

Explanation: Bandwidth = \frac{R_b}{2} = \frac{Wf_s}{2}

where, Rb = Bit rate (bits/sec), fs = sampling frequency, fs = 2f, where f = signal bandwidth

\therefore B.W. = \frac{N 2 f}{2} = Nf\;Hz

Q.2 Time division multiplexing requires

• (a) constant data transmission
• (b) transmission of data samples
• (c) transmission of data at random
• (d) transmission of data of only one measured

Answer: (b) transmission of data samples

• (a) same as FDM
• (b) same as TDM
• (c) a combination of FDM and TDM
• (d) the scheme where the same carrier frequency is used for two different signals

Answer: (d) the scheme where the same carrier frequency is used for two different signals

Q.4 The ramp signal m(t) = at is applied to a delta modulator with sampling period Ts and step size \delta. Slope overload distortion would occur if

• (a) \delta < a
• (b) \delta > a
• (c) \delta < aT_s
• (d) \delta > aT_s

Q.5 In a PCM system each quantization level is encoded into 8 bits. The signal-to-quantization noise ratio is equal to

• (a) 24 dB
• (b) 48 dB
• (c) 64 dB
• (d) 256 dB

Explanation: \frac{S}{N_q}(dB)\approx 6n, where n -> number of bits

\frac{S}{N_q}(dB)\approx 6 \times 8 \approx 48\; dB

Q.6 Generally, a transversal type equalizer with 5 taps can take care of distorted signal due to intersymbol interference in the received signal at

• (a) 4 sampling instants
• (b) 5 sampling instants
• (c) 9 sampling instants
• (d) 10 sampling instants

Q.7 For a 10-bit PCM system, the signal-to-quantization noise ratio is 62 dB. if the number of bits is increased by 2, then the signal-to-quantization noise ratio will

• (a) increased by 6 dB
• (b) increased by 12 dB
• (c) decreased by 6 dB
• (d) decreased by 12 dB

Answer: (b) increased by 12 dB

Explanation: \frac{S}{N_q}(dB)\approx 6n, where n -> number of bits

If the number of bits increased by 2, then the S/Nq will be increased by 6 x 2 = 12 dB

Q.8 For a given data rate, the bandwidth \beta_p of a BPSK signal and the bandwidth \beta_0 of the OQPSK signal are related as

• (a) \beta_p = \beta_0/2
• (b) \beta_p = 2\beta_0
• (c) \beta_p = \beta_0/4
• (d) \beta_p = \beta_0

Q.9 In a certain ’12 channel TDM’ system, it is found that channel number 3 and channel number 8 are connected to the same input signal. The technique

• (a) wastes the channel capacity
• (b) takes care of different sampling rates
• (C) is required when different bandwidth signals are to be transmitted
• (d) reduces noise

Answer: (b) takes care of different sampling rates

Q.10 Which one of the following pulse modulating is analog?

• (a) PCM
• (b) Diffrential PCM
• (c) PWM
• (d) Delta

Q.11 In an asynchronous TDM, for n signal sources, each frame contains m slots, where m is usually

• (a) less than n
• (b) 2n
• (c) n
• (d) greater than 2n

Q.12 The number of signaling bits per channel per frame in T1 multiplexer following CCITT hierarchy is

• (a) 64000
• (b) 128
• (c) 4
• (d) 400

Q.13 In a BPSK signal detector, the local oscillator has a fixed phase error of 200. By what factor does this phase error deteriorate in the SNR at the output?

• (a) cos2 20o
• (b) cos 20o
• (c) cos 40o
• (d) cos 70o

Q.14 What is the SNR improvement with FSK over ASK in most types of noise environments?

• (a) 200 to 300 dB
• (b) 3 to 4 dB
• (c) 10 to 12 dB
• (d) 0 dB

Answer: (b) 3 to 4 dB

Q.15 WHich one of the following statements is correct? Digital modulation techniques are used in satellite communication systems since

• (a) they are easier to handle
• (b) large bandwidth utilization is possible
• (c) they have a higher spectral efficiency
• (d) they are less prone to interference

Answer: (d) they are less prone to interference

Q.16 A continuous signal has a voltage range of -2v to +2v. if this is quantized to 8 bits, what does the resulting signal have?

• (a) 255 level of step size 4/255
• (b) 25 levels of step size 4/8
• (c) 8 bits plus 2 levels at -2 and +2 volts
• (d) 8 bits per sample if properly sampled

Answer: (d) 8 bits per sample if properly sampled

Q.17 Signal x(t) = 3 \sin 2\pi 10^3 t + s \sin 2\pi 660t. At what sampling frequency should this signal be sampled to avoid aliasing?

• (a) 2 \times 660\; Hz
• (b) 2 \times 1000\; Hz
• (c) 2 [1000+600] \; Hz
• (d) 2 [1000-600] \; Hz

Answer: (b) 2 \times 1000\; Hz

Q.18 An analog signal has significant spectral components from 1 kHz to 5 kHz. What is the Nyquist sampling rate for this signal?

• (a) 5 k samples/s
• (b) 4 k samples/s
• (c) 8 k samples/s
• (d) 10 k samples/s

Q.19 In delta modulation, when is the slope overloading noise absent? (h is the pulse height and sampled every Ts seconds, f(t) is the input signal, h and Ts are step-size of integration and sampling period, respectively:)

• (a) \frac{df(t)}{dt} \leq (h/T_S)
• (b) \frac{df(t)}{dt} > (h/T_S)
• (c) \frac{df(t)}{dt} \leq (h.T_S)
• (d) \frac{df(t)}{dt} > (h.T_S)

Q.20 which one of the following statements is correct?

• (a) Equalisation is required in telephone lines but not in wireless communication
• (b) Equalisation is required both in telephone lines and wireless communication
• (c) Equalisation is required in wireless communication whereas this is not required in telephone lines
• (d) Equalisation is not required in wireless communication since phase equalization is required only when lines are used

Answer: (b) Equalisation is required both in telephone lines and wireless communication

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