DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING
ANALOG DIGITAL AND DATA COMMUNICATION ( EC351 )
UNIT – I
(Two Marks Q&A)
1.What do you mean by Frequency Multiplexing?
Transmision of multiple signals simultaneously over a single channel can be
achieved by translating each signal to a different position in the frequency
spectrum. Such a multiplexing is called Frequency Multiplexing.
2.Define AM.
Amplitude modulation(AM) is the process by which amplitude of the carrier
signal is varied in accordance with the instantaneous value of the modulating signal
(baseband signal).
3.Define FM.
Frequency modulation(FM) is the process by which frequency of the carrier
signal is varied in accordance with the instantaneous value of the modulating signal.
4.Define PM.
Phase modulation(PM) is the process by which phase angle of the carrier
signal is varied in accordance with the instantaneous value of the modulating signal.
5.What is upper-sideband and lower-sideband signal?
In the Translated signal, the part of the signal which consists of spectral
components above the auxiliary signal (in the range fc to fc+fm) is called uppersideband signal.
In the Translated signal, the part of the signal which consists of spectral
components below the auxiliary signal (in the range fc - fm to fc ) is called lowersideband signal.
6.What is demodulation?
The process of recovering baseband signal(message signal) form the
modulated signal is called demodulation or detection.
7.Define Pulse Modulation.
Dividing the message signal(which is continuous) into number of
sample(pulses) (which is discrete)on regular interval and interleaving samples of
different signals to achieve multiplexing is called pulse modulation. It is some times
called as Analog - to - Digital conversion.
8.Define PAM.
If the amplitude of the pulse is altered in accordance with the amplitude of
the modulating signal to accommodate the information signal, that process is called
pulse amplitude modulation (PAM).
9.Define PWM.
If the duration(width) of the pulse is altered in accordance with the
amplitude of the modulating signal to accommodate the information signal, that
process is called pulse width modulation (PWM) or pulse duration modulation
(PDM).
10.Define PPM.
If the pulse position is altered in accordance with the amplitude of the
modulating signal to accommodate the information signal, that process is called
pulse position modulation (PPM).
11.Draw the Frequency spectrum of AM .
V(t)
Ac
MaAc/2 LSB
fc-fm
USB MaAc/2
fc
fc+fm
where, Ma = Am/Ac = modulation index (depth of modulation)
12.What are the types of pulse modulation.
1. Analog Pulse modulation
a. Pulse Amplitude Modulation
b. Pulse Duration Modulation.
c. Pulse Position Modulation.
2. Digital Pulse Modulation
a. Pulse Code Modulation
b. Delta Modulation.
13.State sampling theorem.
Let m(t) be a signal, its highest frequency component is fm. Let the values of
m(t) is divided at regular intervals Ts<=1/2fm. Then these samples m(nTs) uniquely
determine the signal, and the signal may be reconstructed from these samples with
no distortion.
f
14.Differentiate FM and PM.
Frequency modulation(FM) is the process by which frequency of the carrier
signal is varied in accordance with the instantaneous value of the modulating signal.
The amplitude of the carrier remains constant in this process.
Phase modulation(PM) is the process by which phase or angle of the carrier
signal is varied in accordance with the instantaneous value of the modulating signal.
The amplitude and frequency remains constant after the modulation process.
(The frequency and phase modulation are combinely called as Angle Modulation.)
15.Define modulation index of FM.
The modulation index of FM can be defined as the ratio of maximum
frequency deviation to the modulating frequency.
Mf = kAm / wm
Where,
Mf - Modulation index.
Am - Maximum amplitude of message signal.
wm - Angular Frequency of message signal.
k – proportionality constant.
wd = kAm =frequency deviation.
16.Distingush between Direct and Indirect method of FM generation.
Direct Method
1.In this method, the Modulating signal
directly varies the instantaneous
frequency of carrier signal by a device
known as voltage controller.
Indirect Method
1.In this method, first the modulating
signal is integrated and then phase
modulated with carrier signal to form
FM signal.
2.The direct modulator cannot employ
crystal oscillator to obtain high
frequency stability
2.It uses a stable crystal oscillator to
generate PM from which FM is
generated.
17.Compare narrow band FM and wide band FM.
Depending on modulation index FM can be divided into two types:
1. Narrow band FM for which modulation index is small compared to one
radian.
2. Wide band FM for which modulation index is large compared to one radian.
18.How FM wave can be converted to PM wave?
PM wave can be obtained from FM by differentiating the modulating signal
before applying it to frequency modulator.
Message signal
Differentiator
FM
PM signal
Carrier
19.A transmitter supplies 8 KW to the antenna when unmodulated. Determine total
power radiated when modulated to 30%.
Given
Pc = 8 KW.
ma = 30/100 = 0.3
[
Total radiated power Pt = Pc 1 + (ma )2 / 2
]
[
= 8 * 103 1 + (0.3)2 / 2
= 8360 W
= 8.36 KW
]
20.Write the advantages of SSB-SC-AM .
1.Band width of SSB is half that of DSB-SC-AM. Thus twice the number of
channels can be accommodated at a given frequency spectrum.
2.No carrier is transmitted, hence possibility of interference with other
channels are avoided.
3.There is the improvement in signal to noise ratio at the receiver output over
DSB-SC-AM
21.Draw the phasor diagram of FM signal.
USB
Carrier
Vc
LSB
Resultant FM
VFM
22.Draw the phasor diagram of DSB-SC-AM and SSB-SC-AM.
DSB-SC-AM
USB
VC
Resultant
VAM
LSB
SSB-SC-AM
USB
VC
Resultant
VAM
LSB
23.Draw the graphical representation of PAM, PDM and PPM
M(t)
t
PAM
t
PDM
t
PPM
t
24.What is Quantization error?
The difference between quantized signal and the orignal signal is called
Quantization error{m(t) – mq(t)}.This can be viewed as noise or error due to
Quantization process.
25.What is the use of commutator in Transmission?
Commutator is used to divide the message signal into number of samples and
combines the samples then transmit through the channel.
(Big Questions)
1.Explain Amplitude Modulation and the operation of Balanced Modulator for
generating AM signal.
2.Explain the operation of Envelope detector and Square-Law demodulator.
3.Explain Any two methods of generating FM signal.
4.State and prove sampling theorem for low pass signal.
5.Explain the principle of operation of Quantization.