Communications - Online Test
Q1. A message signal m(t) = cos 200πt + 4cosπt modulates the carrier
c(t) = coe 2πfct where fc = 1MHz to produce an AM signal. For demodulating
the generated AM signal using an envelope detector, the time constant RC of
the detector circuit should satisfy
Answer : Option B
Explaination / Solution:
Highest frequency component in m(t) is 
Carrier frequency fc = 1 MHz
For Envelope detector condition
Q2. In the circuit shown below, the network N is described by the following Y matrix: 
the voltage gain (V2/V1)is
the voltage gain (V2/V1)is
Answer : Option D
Explaination / Solution:
From given admittance matrix we get
Q3. A four-phase and an eight-phase signal constellation are shown in the figure
below.
For the constraint that the minimum distance between pairs of signal points be
d for both constellations, the radii r1, and r2 of the circles are
Answer : Option D
Explaination / Solution:
Four phase signal constellation is shown below
d2 = r12 + r12
d2 = 2r12
r1 = d/√2 = 0.707d
Q4. A four-phase and an eight-phase signal constellation are shown in the figure
below.
Assuming high SNR and that all signals are equally probable, the additional
average transmitted signal energy required by the 8-PSK signal to achieve the
same error probability as the 4-PSK signal is
Answer : Option D
Explaination / Solution:
Here Pe for 4 PSK and 8 PSK is same because Pe depends on d . Since Pe is same,
d is same for 4 PSK and 8 PSK.
Additional Power SNR
Q5. The block diagram of a frequency synthesizer consisting of a Phase Locked Loop (PLL) and a
divide-by-𝑁 counter (comprising ÷2, ÷4, ÷8, ÷16 outputs) is sketched below. The synthesizer is
excited with a 5 kHz signal (Input 1). The free-running frequency of the PLL is set to20 kHz.
Assume that the commutator switch makes contacts repeatedly in the order 1-2-3-4.
The corresponding frequencies synthesized are:
Answer : Option A
Explaination / Solution:
No Explaination.
Q6. The propagation constant of a lossy transmission line is (2 + 𝑗5) m-1 and its characteristic impedance is (50 + j0)𝜔 at 𝜔 = 106 rad s-1. The values of the line constants L, C, R, G are, respectively,
Answer : Option B
Explaination / Solution:
Q7. An antenna pointing in a certain direction has a noise temperature of 50K. The ambient
temperature is 290 K. The antenna is connected to a pre-amplifier that has a noise figure of 2
dB and an available gain of 40 dB over an effective bandwidth of 12 MHz. The effective input
noise temperature Te for the amplifier and the noise power Pao at the output of the preamplifier,
respectively, are
Answer : Option A
Explaination / Solution:
No Explaination.
Q8. The electric field of a uniform plane wave travelling along the negative z direction is given by
the following equation:
This wave is incident upon a receiving antenna placed at the origin and whose radiated electric
field towards the incident wave is given by the following equation:
The polarization of the incident wave, the polarization of the antenna and losses due to the
polarization mismatch are, respectively,
Answer : Option C
Explaination / Solution:
No Explaination.
Q9. The far-zone power density radiated by a helical antenna is approximated as:
The radiated power density is symmetrical with respect to ϕ and exists only in the upper
hemisphere: C0 is a constant. The power radiated by the antenna (in
watts) and the maximum directivity of the antenna, respectively, are
Answer : Option B
Explaination / Solution:
No Explaination.
Q10. Two magnetically uncoupled inductive coils have Q factors q1 and q2 at the
chosen operating frequency. Their respective resistances are R1 and R2. When
connected in series, their effective Q factor at the same operating frequency is
Answer : Option C
Explaination / Solution:
The quality factor of the inductances are given by
So, in series circuit, the effective quality factor is given by
