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GATE EC : Communications - National Champion Test
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Question 1
What is the maximum possible power efficiency of AM signal?
Question 2
Consider the signal
Where
denotes the Hilbert transform of m(t) and the bandwidth of m(t) is very small compared to 
. The signal s(t) is a
Where
denotes the Hilbert transform of m(t) and the bandwidth of m(t) is very small compared to . The signal s(t) is aQuestion 3
A non linear device characterised by V0 = a Vi + b Vi3 where Vi = m(t) + Ac cos(2πf1t). What is the value of f1 in MHz if only DSB terms consider at output of device (given carrier frequency = 2 MHz)
Question 4
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
Question 5
In this circuit, Vout is:
Question 6
Following constellation diagram represents:
Question 7
A source alphabet consists of N(even) symbols with the probability of all symbols being the same. A source encoder increases the probability of the first half symbol by a small amount e and decrease for other half by a small amount e. After encoding, the entropy of system.
Question 8
The minimum sampling frequency (in samples/sec) required to reconstruct the following signal from its samples without any distortion
Question 9
A sinusoidal signal of amplitude A is quantized by a uniform quantizer Assume that the signal utilizes all the representation levels of the quantizer. If the signal to quantization noise ratio is 31.8 dB the number of levels in the quantizer is _________-
Question 10
Let Q(√Y) be the BER of a BPSK system over an AWGN channel with two-sided noise power spectral density N0/2. The parameter γ is a function of bit energy and noise power spectral density.
A system with two independent and identical AWGN channels with noise power spectral density is shown in the figure. The BPSK demodulator receives the sum of outputs of both the channels.
If the BER of this system is Q(b√γ), then the value of b is _____.
A system with two independent and identical AWGN channels with noise power spectral density is shown in the figure. The BPSK demodulator receives the sum of outputs of both the channels.
If the BER of this system is Q(b√γ), then the value of b is _____.
Question 11
An Ideal power limited Communications channel with additive white Gaussian noise is having 4 KHz band width and Signal to Noise ratio of 255. The channel capacity is:
Question 12
A message m(t) band-limited to the frequency fmhas a power of Pm. The power of the output signal in the following figure is
Question 13
Let g(t) = p(t) * p(t), where * denotes convolution and p(t) = u(t) − u(t − 1) with u(t) being the unit step function
Amplitude Modulated signal is given as
In the interval 0 ≤ t ≤1. One set of possible values of the modulating signal and modulation index would be
Amplitude Modulated signal is given as
In the interval 0 ≤ t ≤1. One set of possible values of the modulating signal and modulation index would be
Question 14
Let g(t) = p(t) * p(t), where * denotes convolution and p(t) = u(t) − u(t − 1) with u(t) being the unit step function
The impulse response of matched filter output for the signal s(t) = g(t) – δ(t – 2) * g(t) is given as:
Question 15
Consider the following Amplitude Modulated (AM) signal, where fm < fc :
The average side band power for the AM signal given above is:
Question 16
Consider the following Amplitude Modulated (AM) signal, where fm < B:
The AM signal gets added to a noise with Power Spectral Density Sn(f) given in the figure below. The ratio of average sideband power to mean noise power would be:
Question 17
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 earner frequency.
The ratio kp/kf (in rad/Hz) for the same maximum phase deviation is
The ratio kp/kf (in rad/Hz) for the same maximum phase deviation is
Question 18
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
Question 19
A message signal with bandwidth 10 kHz is Lower-Side Band SSB modulated with carrier frequency 
. The resulting signal is then passed through a Narrow-Band Frequency Modulator with carrier frequency 
. The bandwidth of the output would be:
. The resulting signal is then passed through a Narrow-Band Frequency Modulator with carrier frequency . The bandwidth of the output would be: Question 20
A carrier signal with fc and message signal with 5HZ is given to both AM and FM transmitter. It is founded that maximum frequency deviation of FM is twice the bandwidth of AM. Find modulation index of AM such that the strength of frequency component ‘fc + 5KHz’ is same in both AM and FM spectrum [given J1 (0) = 1, J1 (2) = 0.57, J1 (4) = 0.37]
Question 21
Consider the frequency modulated signal
With carrier frequency of 105 Hz. The modulation index is
With carrier frequency of 105 Hz. The modulation index is
Question 22
In the following scheme, it the spectrum M(f) of m(t) is as shown, then the spectrum Y(f) of y(t) will be:
Question 23
Consider the frequency modulated signal 
with carrier frequency of 105 Hz. The modulation index is
with carrier frequency of 105 Hz. The modulation index is Question 24
A superheterodyne receiver is to operate in the frequency range 550 kHz - 1650 kHz, with the intermediate frequency for 450 kHz. Let 
denotes the required capacitance ratio of the local oscillator and 1 denote the image frequency (in kHz) of the incoming signal. If the receiver is tuned to 700 kHz, then
denotes the required capacitance ratio of the local oscillator and 1 denote the image frequency (in kHz) of the incoming signal. If the receiver is tuned to 700 kHz, then Question 25
For a parallel RC circuit, as shown in the figure, R = 20 Q and C = 0.9 pF, which is to be matched to 50 Q over a bandwidth,
The circuit has to be matched over 6 GHz to 18 GHz. What is the very best τ that can be achieved:
The circuit has to be matched over 6 GHz to 18 GHz. What is the very best τ that can be achieved:
Question 26
For this RF Circuit, the correct statement is:
Question 27
The amplitude of a random signal is uniformly distributed between -5 V and 5 V.
If the positivevalues of the signal are uniformly quantized with a step size of 0.05 V, and the negative values are uniformly quantized with a step size of 0.1 V, the resulting signal to quantization noise ratio is approximately
Question 28
In a Direct Sequence CDMA system the chip rate is 1.2288 ×106 chips per second. If the processing gain is desired to be AT LEAST 100, the data rate
Question 29
Four messages band limited to W, W, 2W and 3W respectively are to be multiplexed using Time Division Multiplexing (TDM). The minimum bandwidth required for transmission of this TDM signal is
Question 30
In a GSM system, 8 channels can co-exist in 200 KHz bandwidth using TDMA. A GSM based cellular operator is allocated 5 MHz bandwidth. Assuming a frequency reuse factor of 1/5 i.e. a five cell repeat pattern, the maximum number of simultaneous channels that can exist in one cell is:
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Jan 20ESE & GATE EC
