11. The main advantage of frequency modulation over amplitude modulation is
That there will be no distortion
That the complete information is contained in the sidebands
That is uses a wider band of frequencies
The elimination of noises
Answer.4. The elimination of noises
Explanation:-
The Noise affects the amplitude of the modulated signal
In the Amplitude Modulated system, the message is contained in the amplitude modulations of the carrier; hence the introduction of noise distorts the amplitude and hence the message contained in the signal
In a frequency modulated system the message is contained in the frequency variations of the carrier, therefore, the introduction of noise does not affect the message contained in the message signal
Since the amplitude of FM remains constant, the noise can be eliminated using an amplitude limiter at the demodulator.
12. A 5-bit encoder is used. Each level represents 1 V. On account of modulation, voltage involved is 27.39 V. The quantization error is
0.39 V
0.6 V
0.09 V
0 V
Answer.1. 0.39 V
Explanation:-
Number of levels = 25 = 32
Each level represents 1 V, so for 32 levels = 32 V
∴ The digital voltage levels of the quantizer will have values 0, 1, 2,…, 32 Volts
Now, the given analog value is 27.39
The closest digital value for the quantizer will be 27 V.
∴ The quantization error (Qe) will be:
Qe = 27.39 – 27 = 0.39 V
13. The bandwidth required in DPCM is less than that of PCM because
The number of bits per code is reduced resulting in a reduced bit rate
The difference signal is larger in amplitude than actual signal
More quantization levels are needed
The successive samples of signal often differ in amplitude
Answer.1. The number of bits per code is reduced resulting in a reduced bit rate
Explanation:-
PCM:
PCM stands for Pulse Code Modulation.
With PCM, the amplitude of the analog signal is sampled at regular intervals and translated into a binary number.
The difference between the original signal and the translated digital signal is called the quantizing error.
DPCM:
DPCM stands for Differential Pulse Code modulation.
It is a signal encoder that uses PCM but adds some functionalities based on the prediction of the samples of the signal.
The input can be an analog signal or a digital signal.
The bandwidth required in DPCM is less than that of PCM because the number of bits per code is reduced resulting in a reduced bit rate.
The number of encoded bits is smaller than the PCM system. This results in fewer bandwidth requirements by the DPCM system.
14. Consider the following statements comparing delta modulation (DM) with PCM system: Digital Modulation requires
1. A lower sampling rate
2. A higher sampling rate
3. A lower bandwidth
4. Simple hardware
Which one of the above statements are correct?
1 and 3 only
2 and 4 only
1, 3 and 4
2, 3 and 4
Answer.4. 2, 3 and 4
Explanation:-
In PCM an analog signal is sampled and encoded into different levels before transmission
The bandwidth of PCM depends on the number of levels If each sample is encoded into n bits, then the bandwidth of PCM is nfs
However, in the case of Delta modulation, each sample is sent using only 1 bit which is +Δ or -Δ Hence there is a bandwidth saving in Delta modulation
DM has a simple hardware requirement in comparison to PCM.
15. In PCM, if the number of quantization levels is increased from 4 to 64, then the bandwidth requirement will approximately be:
4 times
16 times
8 times
3 times
Answer.4. 3 times
Explanation:-
The number of levels for an n-bit PCM system is given by:
L = 2n
We can also state that the number of bits for a given quantization level will be:
n = log2 L
Also, the bandwidth of PCM is given by:
BW = nfs
n = number of bits to encode
fs = sampling frequency
Calculation:
For L = 4 quantization levels, the number of bits n = log2 4 = 2 bits. The bandwidth is, therefore:
B.W. = 2 fs
Similarly, For L = 64 quantization levels, the number of bits n = log2 64 = 6 bits. The bandwidth is, therefore:
B.W. = 6 fs
Clearly, the Bandwidth is increased by 3 times.
16. The plot of modulation index versus carrier amplitude for an amplitude modulated system yields
Horizontal line
Vertical line
Parabola
Hyperbola
Answer.4. Hyperbola
Explanation:-
For an amplitude modulated system, the modulation index is defined as:
µ = Am/Ac
Am = Message signal amplitude
Ac = Carrier Amplitude
Observation: Since the modulation index is related to the carrier amplitude through an inverse relationship, we conclude that the plot of modulation index versus carrier amplitude will result in a hyperbola.
17. In TDM systems, channel separation is done with the use of
AND gates
Bandpass filter
Differentiator circuit
Integrator circuit
Answer.1. AND gates
Explanation:-
Time Division Multiplexing:
Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing.
In Time Division Multiplexing (TDM), the time frame is divided into slots. This technique is used to transmit a signal over a single communication channel, by allotting one slot for each message.
For separating channels in TDM, it is necessary to use Time slots. For this purpose AND gates are used.
Time Division Multiplexing (TDM) can be classified into Synchronous TDM and Asynchronous TDM.
18. Limiter circuit is not needed in the following detector:
Foster – Seeley discriminator
Balanced slope
Ratio detector
None
Answer.3. Ratio detector
Explanation:-
Frequency Demodulators are often sensitive to amplitude variations. Therefore a limiter amplifier stage must be used before the detector, to remove amplitude variations in the signal which would be detected as noise.
The limiter acts as a Class-A amplifier at lower amplitudes; at higher amplitudes, it becomes a saturated amplifier that clips off the peaks and limits the amplitude.
The FM ratio detector was more common because it offered a better level of amplitude modulation rejection of amplitude modulation.
This enabled the circuit to provide a greater level of noise immunity as most noise is amplitude noise.
It also enables the FM detector to operate more effectively even with lower levels of limiting in the preceding IF stages of the receiver.
19. The bandwidth requirement of a telephone channel is
3 kHz
5 kHz
10 kHz
15 kHz
Answer.1. 3 kHz
Explanation:-
The internationally accepted frequency range of the standard telephone channel is 300 – 3400 Hz.
The range of audible human frequency varies from 20 Hz to about 20 kHz which is much higher than the bandwidth of the telephone channel.
Clearly, the telephone channel’s bandwidth is much lower than the human ear’s audible bandwidth. The main reasons for it are:
Acceptable Voice Quality when reproduced at the receiving end.
Low Cost.
Savings in Bandwidth.
Hence, if we provide about 4 kHz of BW (including a guard band) per telephone channel, it would be sufficient for voice quality.
Ques.20. A high PRF would
Increase the maximum range
Decreases the maximum range
Affect range ambiguity
Increase the efficiency of radar
Answer.3. Affect range ambiguity
Explanation:-
Pulse Repetition Frequency (PRF) of the radar system is defined as the number of pulses that are transmitted per second.
It is normally measured in pulses per second.
Radar systems radiate each pulse at the carrier frequency during transmit time (or Pulse Width PW), wait for returning echoes during listening or rest time and then radiate the next pulse.
The time between the beginning of one pulse and the start of the next pulse is called pulse-repetition time (PRT) and is equal to the reciprocal of PRF.
High PRF:
Systems using high PRF i.e. above 30 kHz function better known as interrupted continuous-wave (ICW) radar because direct velocity can be measured up to 4.5 km/s at L band, but range resolution becomes more difficult.
High PRF is limited to systems that require close-in performance, like proximity fuses and law enforcement radar.
Higher PRFs produce shorter maximum ranges but broadcast more pulses.
High PRF pulse-Doppler radar has no ambiguity in Doppler frequency but suffers from range ambiguities.
A high PRF pulse-Doppler radar provides an accurate measurement of the target’s radial velocity.
In high PRF, It becomes increasingly difficult to take multiple samples between transmit pulses at these pulse frequencies, so range measurements are limited to short distances.
