Bits Pilani KK Birla Goa Campus 2023-24 (I sem)

MM: 60
Communications System EEE/ECE F311
Midsem Examination Time: 1.5 hrs
Note: 1. All
. questions
Answers
are
the compulsory.
3. Use
4.
questions
appropriate sequentially,
date wherever solve all parts together.
Highlight the final answer for required.
each question.
Q.1 Answef the following short
answer
puise Code modulation (PCM)type questions in brief. 10
encoder ie uced twice to encode a
sinusoidal signal. ny
ete OT oits per sample for each use of PCM and n2 are the
SONR in dB is S+a encoder. and, n, = n, + 1. The ratio of resepective
S Determine a. average
(b) A
phase-modulated signal, modulated by animpulsetrain wil! have infinite power
this statement true or false?
Give an answer with proper because of impuises. IS
(c) The Bessel functions of a
wide band frequency justification.
modulation
J-1(2)=-0.5767, and Io(2) =0.2239. Determine I (2) and (WBFM) signal are J-2(2) =0.3sZ0,
justification. I,(2). Give the answer with proper
(d) The bandwidth of a narrow band frequency
statemnent true or false? Give an answer with
modulation (NBFM) is close to bandwidth of AM. Is this
(e) The rate of convergence to zero of Nyquist
proper justification.
pulse is quite faster as compared to raised cosine
RC pulse converges to Nyquist pulse if roll of (RC) pulse, and
factor is zero. ls this statement true or false? Give
with proper justification. ananswer

Q. 2 (a) An AM signal has the form s(t) = (20+ 2

cos3000nt + 10 cos6000nt] cos2rfet, where fc = 10°Hz. 10
i Sketch the spectrum of s(t).
Determine the power in each frequency component present in the signal.
(iii) Determine the modulation index.
(b)For a message signal m(t) =cos (2rf: t) and carriers signal c(t) = 10cos
(2nf-t)
ADetermine the SSB-USB modulated signal frequency in Hz for fi = 1 kHz, and f= 10kHz.
(iiYif SSB-USB modulated signal acts as a modulating signal to
create a narrowband FM signal. Determine
the bandwidth of the narrowband FM signal.

Q. 3 (a) In the figure shown below, m(t) = 2sin2t

t s(t) = cos200rt. and n() Sin199t
Determine the output y(t)

m(t) Low Pas Filler

Cut off FrequeICy 1llz

n(t) s(t) Pass Band Gaiu -1
s)

(b) The system shown in figure below is used to generate an AM signal. The modulating sienal m/t) has zore
men 2nd its maximum (absolute) value Is Am= max|m(t)|. The nonlinear device has the
input-outrnut
characteristic v/t) = ax(t) + bx(t). Determine the output expression u(t) of the linear filter (BPE) and
modulation index. 5
 m() () Nonlinear ) Linear ut)
memot yless filter
system AM signal

dt) cos (2 nfo!)

input to a FM modulato The modulated signal from the modulator is

a. 4 Amessage signal m(t) is given1
X 1500t) +sin(2n X 750t)] as shown in the following block
1

SE (t) = 10cos[2n X 10t+sin(2t

10
only triplers. The signal obtained at the
diagram. The block M1 consists of only doublers, and M2 consists of
oscillator with frequency fo = 3MHz.
output of the M1 block is multiplied with signal produced by alocal
MHz. The signal obtained at the output
The cut-off frequencies of the band-pass filter (BPF) are 5 MHz and 7
frequency deviation at the output of the
of the M2 has a frequency deviation 4fo = a Af, where 4f is the
index (B) of the FM modulator, 4f, and the
FM modulator, and 280 < as 290. Determine modulation
10
carrier frequency at the output of the M2 biock.

FM SFM()
m(t) M1
BPE M2
Modulator

Q. 5 (a) Asinusoidal signal x(t) with peak amplitude V, is tracked by a delta modulator (DM) insuch a way that the
slope overload error is avoided. The step size A is used in DM. Determine the expression of average signal
6
to-quantization noise ratio.
(b)Asignal x(t) = 12cos(2nt) is given as an input to a non-uniform quantizer (NQ). The input (x) and output
()relation of the quantizer is as follows:
1, 0<|x| s1
y
2, 1< |x| s5
3, 5 <|x|<14
0 otherwise
Determinea ifMQE = a Va, where MOE is the maximum guantization error induced by NQ and V s peak
inputvoltage.
(a) A leased telephone line of bandwidth 4 kHz is used to transmit binary data. Calculate the data rate (in bits
6
per second)that can be transmitted if we use
(i) Polar signal with half width pulses (iü)) Polar signal with rectangular full width pulses
(ii) Nyquist pulse (iv) Raised cosine pulse with a= 1.
with bandwidth= 100 kHz. The bit rate
(b) M-ary PAM is used to transmit binarv digits over abaseband channel
Determine the theoretical minimum value
IS 200kbps for araised cosine pulse with 50% excess bandwidth.
of M.