Analog Communication Systems · Communications · GATE ECE
Marks 1
1
An amplitude modulator has output (in Volts)
$$s(t) = A \cos(400 \pi t) + B \cos(360 \pi t) + B \cos(440 \pi t)$$.
The carrier power normalized to $1\Omega$ resistance is 50 Watts. The ratio of the total sideband power to the total power is 1/9. The value of $B$ (in Volts, rounded off to two decimal places) is _______.
GATE ECE 2024
2
A super heterodyne receiver operates in the frequency range of 58 MHz – 68 MHz. The
intermediate frequency f1F and local oscillator frequency fL0 are chosen such that
f1F $$\leq$$ fL0.It is
required that the image frequencies fall outside the 58 MHz – 68 MHz band. The minimum
required
f1F (in MHz) is ___________.
GATE ECE 2016 Set 1
3
For a superheterodyne receiver, the intermediate frequency is 15 MHz and the local oscillator frequency
is 3.5 GHz. If the frequency of the received signal is greater than the local oscillator frequency, then
the image frequency (in MHz) is _________.
GATE ECE 2016 Set 3
4
Consider the signal
$$s\left(t\right)\;=\;m\left(t\right)\;\cos\left(2{\mathrm{πf}}_\mathrm c\mathrm t\right)\;+\widehat m\left(t\right)\;\sin\left(2\mathrm\pi\;{\mathrm f}_\mathrm c\mathrm t\right)$$ where $$\widehat m\left(t\right)\;$$
denotes the Hilbert
transform of m(t) and the bandwidth of m(t) is very small compared to fc. The signal s(t) is a
GATE ECE 2015 Set 1
5
Consider an FM signal $$f\left(t\right)\;=\;\cos\left[2{\mathrm{πf}}_\mathrm c\mathrm t\;+\;{\mathrm\beta}_1\sin\;2{\mathrm{πf}}_1\mathrm t\;+\;{\mathrm\beta}_2\sin\;2{\mathrm{πf}}_2\mathrm t\right]$$ . The maximum
deviation of the instantaneous frequency from the carrier frequency fc
is
GATE ECE 2014 Set 3
6
Consider an angle modulated signal x(t) = 6cos[2$$\mathrm\pi$$x106
t+2sin(8000$$\mathrm\pi$$t) +
4cos(8000$$\mathrm\pi$$t)] V. The average power of x(t) is.
GATE ECE 2010
7
Consider the amplitude modulated (AM) signal $$A_c\cos\omega_ct\;+\;2\cos\omega_mt\;\cos\omega_ct$$. For
demodulating the signal using envelope detector, the minimum value of Ac should
be
GATE ECE 2008
8
Which of the following analog modulation scheme requires the minimum
transmitted power and minimum channel bandwidth?
GATE ECE 2005
9
An AM signal and a narrow-band FM signal with identical carriers, modulating
signals and modulation indices of 0.1 are added together. The resultant signal
can be closely approximated by
GATE ECE 2004
10
An AM signal is detected using an envelope detector. The carrier frequency and
modulating signal frequency are 1 MHz and 2 KHz respectively. An appropriate
value for the time constant of the envelope detector is
GATE ECE 2004
11
A 1 MHz sinusoidal carrier is amplitude modulated by a symmetrical square wave
of period 100 µsec. Which of the following frequencies will NOT be present in the
modulated signal?
GATE ECE 2002
12
A modulated signal is given by, $$s\left(t\right)=m_1\left(t\right)\cos\left(2{\mathrm{πf}}_\mathrm c\mathrm t\right)\;+m_2\left(t\right)\sin\left(2{\mathrm{πf}}_\mathrm c\mathrm t\right)$$, where
the baseband signal m1(t) and m2(t) have bandwidths of 10 kHz and 15 kHz,
respectively. The bandwidth of the modulated signal, in kHz, is
GATE ECE 1999
13
The image channel selectivity of super heterodyne receiver depends upon
GATE ECE 1998
14
A DSB-SC signal is generated using the carrier $$\cos\left(\omega_ct\;+\;\theta\right)$$ and modulating singal
x(t). The envelope of the DSB-SC signal is
GATE ECE 1998
15
$$v\left(t\right)\;=5\left[\cos\left(10^6\mathrm{πt}\right)\;-\;\sin\left(10^3\mathrm{πt}\right)\times\sin\left(10^6\mathrm{πt}\right)\right]$$ represents
GATE ECE 1994
Marks 2
1
Let a frequency modulated (FM) signal $$x(t) = A\cos ({\omega _c}t + {k_f}\int_{ - \infty }^t {m(\lambda )d\lambda )} $$, where $$m(t)$$ is a message signal of bandwidth W. It is passed through a non-linear system with output $$y(t) = 2x(t) + 5{(x(t))^2}$$. Let $${B^T}$$ denote the FM bandwidth. The minimum value of $${\omega _c}$$ required to recover $$x(t)$$ from $$y(t)$$ is
GATE ECE 2023
2
Let x$$_1$$(t) and x$$_2$$(t) be two band-limited signals having bandwidth $$B=4\pi\times10^3$$ rad/s each. In the figure below, the Nyquist sampling frequency, in rad/s, required to sample y(t), is
GATE ECE 2023
3
Consider an FM broadcast that employs the pre-emphasis filter with frequency response
$${H_{pe}}(\omega ) = 1 + {{j\omega } \over {{\omega _0}}}$$,
where $$\omega$$0 = 104 rad/sec.For the network shown in the figure to act as a corresponding de-emphasis filter, the appropriate pairs of (R, C) values is/are ____________.
GATE ECE 2022
4
The un-modulated carrier power in an AM transmitter is 5kW. This carrier is modulated by a
sinusoidal modulating signal. The maximum percentage of modulation is 50%. If it is reduced
to 40%, then the maximum un-modulated carrier power (in kW) that can be used without
overloading the transmitter is ___________
GATE ECE 2017 Set 2
5
A modulating signal given By $$x\left(t\right)=5\sin\left(4\mathrm\pi10^3\mathrm t\;-\;10\mathrm{πcos}2\mathrm\pi10^3\mathrm t\right)$$ V
is fed to a phase modulator
with phase deviation constant kp=0.5rad/V. If the carrier frequency is 20 kHz, the
instantaneous frequency (in kHz) at t = 0.5 ms is __________
GATE ECE 2017 Set 2
6
In the system shown in figure (a), m(t) is a low-pass signal with bandwidth W Hz. The
frequency response of the band-pass filter H(f) is shown in figure (b). If it is desired that the
output signal z(t) = 10x(t), the maximum value of W (in Hz) should be strictly less than
____________________.
GATE ECE 2015 Set 1
7
The signal m(t) as shown is applied both to a phase modulator (with
kp
as the phase constant) and
a frequency modulator (with
kf
as the frequency constant) having the same carrier frequency.
The ratio kp/kf (in rad/Hz) for the same maximum phase deviation is
GATE ECE 2012
8
The signal $$\cos\;\omega_ct\;+\;0.5\;\cos\;\omega_mt\;s\mathrm{in}\;\omega_ct$$ is
GATE ECE 2008
9
A Hilbert transformer is a
GATE ECE 2007
10
The diagonal clipping in Amplitude Demodulation (using envelope detector) can
be avoided if RC time-constant of the envelope detector satisfies the following
condition, (here W is message bandwidth and ωc
is carrier frequency both in
rad/sec)
GATE ECE 2006
11
A carrier is phase modulated (PM) with frequency deviation of 10 kHz by a single
tone frequency of 1 kHz. If the single tone frequency is increased to 2 kHz,
assuming that phase deviation remains unchanged, the bandwidth of the PM
signal is
GATE ECE 2005
12
A device with input x(t) and output y(t) is characterized by: y(t) = x2(t). An FM signal with frequency deviation of 90 kHz and modulating signal bandwidth of 5 kHz is applied to this device. The bandwidth of the output signal is
GATE ECE 2005
13
Two sinusoidal signals of same amplitude and frequencies 10 kHz and 10.1 kHz
are added together. The combined signal is given to an ideal frequency detector.
The output of the detector is
GATE ECE 2004
14
A DSB-SC signal is to be generated with a carrier frequency fc = 1MHz using a
nonlinear device with the input-output characteristic $$v_0=a_0v_i\;+\;a_1v_i^3$$.where a0 and a1 are constants. The output of the nonlinear device can be filtered
by an appropriate band-pass filter. Let $$v_0=A_c'\;\cos\left(2{\mathrm{πf}}_\mathrm c'\mathrm t\right)\;+\;m\left(t\right)$$ where m(t) is the message signal. Then the value of
$$f_c'$$ (in MHz) is
GATE ECE 2003
15
Let $$m\left(t\right)\;=\cos\left[\left(4\mathrm\pi\times10^3\right)t\right]$$ be the message signal and
$$c\left(t\right)\;=5\cos\left[2\mathrm\pi\times10^6t\right]$$ be the carrier.
c(t) and m(t) are used to generate an AM signal. The modulation index of the generated AM signal is 0.5. Then the quantity $$\frac{Total\;sideband\;power}{Carrier\;power}$$ is
GATE ECE 2003
16
Let $$m\left(t\right)\;=\cos\left[\left(4\mathrm\pi\times10^3\right)t\right]$$ be the message signal and
$$c\left(t\right)\;=5\cos\left[2\mathrm\pi\times10^6t\right]$$ be the carrier.
c(t) and m(t) are used to generate an FM signal. If the peak frequency deviation of the generated FM signal is three times the transmission bandwidth of the AM singal, then the coefficient of the term $$\cos\left[2\mathrm\pi\left(1008\times10^3\right)t\right]$$ in the FM signal (in terms of the Bessel coefficients) is
GATE ECE 2003
17
In an FM system, a carrier of 100 MHz is modulated by a sinusoidal signal of 5
KHz. The bandwidth by Carson’s approximation is 1 MHz. If y(t) = (modulated
waveform)3, then by using Carson’s approximation, the bandwidth of y(t) around
300 MHz and the and the spacing of spectral components are, respectively.
GATE ECE 2000
18
A superheterodyne radio receiver with an intermediate frequency of 455 KHz is
tuned to a station operating at 1200 KHz. The associated image frequency is
______ KHz.
GATE ECE 1993
19
Which of the following demodulator(s) can be used for demodulating the signal
$$x(t)\;=\;5\left(1+2\;\cos200\mathrm{πt}\right)\cos\;20000\;\mathrm{πt}$$ .
GATE ECE 1993
20
The maximum power efficiency of an AM modulator is
GATE ECE 1992