Question: Q. 2. (i) Given a block diagram of a generalized communication system.
| Information Source |
$\frac{\text { Message }}{\text { Signal }}$ | $X$ | $\frac{\text { Transmitted }}{\text { Signal }}$ | Y | Recieve | $\frac{\text { Message }}{\text { Signal }}$ | User |
|---|
Identify the boxes ’ $\mathrm{X}$ ’ and ’ $\mathrm{Y}$ ’ and write their functions.
(ii) Mention three different modes of propagation used in communication system. Distinguish between “Point to Point” and “Broadcast” modes of communication.
U] [Delhi I, II, III 2015]
Show Answer
Solution:
Ans. (i) $X$ : Transmitter
$Y$ : Channel
Their functions :
Transmitter : To convert the message signal into suitable form for transmission through channel.
Channel : It sends the signal to the receiver.
(ii) (a) Ground wave or surface wave propagation.
(b) Sky wave propagation or ionospheric propagation.
(c) Space wave propagation / Line of sight propagation
In point to point mode, communication takes place between a single fransmitter and receiver. In broadcast mode, large number of receivers are connected to a single transmitter.
Revision Notes
Low frequency signal could not travel large distance because of following reasons :
$>$ Low frequency means low power, henceitgets áttenuated i.e., loss of signal strength.
$>$ Minimum size of antenna $=\frac{\lambda}{4}$ and low frequency means large wavelength so size of antenna becomes impractical.
Overlapping of signals Difficult to incorporate multiple transmitting stations.
Hence the signal should be transmitted at high frequency.
Combining low frequency message signal with high frequency carrier wave is modulation.
A high frequency wave has certain features like amplitude, frequency and phase.
$$ y=a \cos (\omega t+\phi) $$
So, variable parameters are amplitude (a), frequency $(v)$ and phase $(\phi)$.
Depending upon the parameter which we are varying in carrier wave with our signal, there are three main types of modulation techniques.
- Amplitude modulation
- Frequency modulation
- Phase modulation
Amplitude Modulation : The amplitude of the carrier wave changes according to the intensity of the signal. The amplitude variation of the carrier wave is at the signal frequency $f_{s}$.
Modulating Sine Wave Signal
Amplitute Modulated Signal
| If message signal | $m(t)$ | $=A_{m} \sin \omega_{m} t$ | |
|---|---|---|---|
| carrier wave | $c(t)$ | $=A_{c} \sin \omega_{c} t$ |
are combined together then bandwidth of modulated wave is $\left(\omega_{c}-\omega_{m}\right)$ to $\left(\omega_{c}+\omega_{m}\right)$
$\left(\omega_{c}-\omega_{m}\right)$ and $\left(\omega_{c}+\omega_{m}\right)$ are known as lower and upper sideband frequency respectively. Signal is in these side band frequencies.
Amplitude
Modulation Index: The ratio of change of amplitude of modulated wave to the amplitude of normal carrier wave is called modulation index $(\mu)$.
$\mu=\frac{A_{m}}{A_{c}}$ To prevent distortion $\mu \leq 1$.
We can derive that
Hence,
$$ \begin{aligned} A_{m} & =\frac{A_{\max }-A_{\min }}{2} \text { and } A=\frac{A_{\max }+A_{\min }}{2} \ \mu & =\frac{A_{\max }-A_{\min }}{A_{\max }+A_{\min }} \end{aligned} $$
Effect of Noise on AM wave : AM signal is more noisy than $F M$ because in $A M$ message is transmitted through modulating the amplitude of carrier signal. A low frequencynoise can alter the amplitude of carrier message. In frequency modulation message is transmitted through frequency changes and hence amplitude of noise signal will not effected.
Detection of amplitude modulated wave :
Demodulation : Demodulation is the process of Yecovering the signal frequency from a modulated carrier wave.
- The detected signal may not be strong enough to be made use of and hence is required to be amplified.
Below is the block diagram of typical receiver circuit.
Block diagram of a receiver
Other communicating modes
Internet
Facsimile (FAX)
Very Short Answer Type Questions
(1 mark each)
