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(a) Infrared rays

(d) A. B. C Code and Bentley Code

There is a facility provided by the post and telegraph department to send the messages through telephone. This method of sending a message is called ‘Phonogram’. This arrangement of the phonogram is made in the town having telephone. The telephone subscriber can convey the message of telegram to the telegraph office. The department sends the message to the addressee by telegram and the charge is added to the telephone bill of the subscriber.

1. Inland Telegram 

2. Foreign Telegram.

Foreign Letters.

Correct option is (a) Cypher

International Subscriber Dialing.

Correct answer is 1986.

Correct answer is Tally

Computer is the most important part in a business office. 

Following are uses of computers in a business office: 

1. Help in planning: By the help of computer we can collect data which is very helpful in planning. 

2. Help in Finance: Most companies world wide are rapidly switching to a computerised accounting system. Long time consuming calculations and other mathematics-intensive problems can now be solved at the press of a button. 

3. Help in Management: The personal computer has revolutionised management functions. Computer reduces paper-work, speeds up communication and improves the quality of work. 

4. Help in Accounting: Accounting work in a business office becomes early with the help of a computer. A computer has made accounting easier with the help of many softwares. 

5. Others: 

• It saves time and money. 
• It seams to have become the backbone of all activities. 
• The computer helps to prepare different reports. 
• It helps to take decisions earlier.

Radio Paging: This is a one-way communication system. Messages are transmitted to subscribers by means of aerial signals in the form of tone or vibrations through pocket-size receiver called pager. It was introduced in Delhi in 1985. It is now available in all over the country.

Internal Telephone Exchange: In big business organizations and offices there are many departments like purchases, sales, accounts, administration etc. Every department has its own officer and each officer has a different telephone connection so that they can talk to each other on telephone which saves the time of going and coming from one department to another. Under this system, an Internal Telephone Exchange is established within the premises. An operator is appointed who helps in connecting the calls.

(a) Subscribers Trunk Dialing

(b) Phonogram

It is the frequency range with in which a transmission is made. 

(a) Transmitter 

(b) Transmission channel 

(c) Receiver.

Correct option: (2) rectifier and envelope detector

Explanation: 

At the receiving end, the AM intent wave is first rectified by using a rectifier. The rectified output is then processed by an envelope detector which removes its RF component and gives its envelope (the original message signal) as its output.

(a) A low frequency signal is modulation for the following purposes : 

(i) It reduces the wavelength of transmitted signal  and the minimum height of antenna for effective communication is   λ / 4. Therefore height of antenna becomes practically achievable.

(ii) Power radiated into the space by an antenna is inversely proportional to 1.2. Therefore, the power radiated into the space increases and signal can  travel larger distance.

(Give full credit of parts for any other correct answer)

(b) (i) High efficiency

(ii) Less noise 

(iii) Maximum use of transmitted power

Two factors justifying the need of modulation for transmission of a signal are : 

(a) Managable size of the antenna: Audio signals when converted into emwaves have low frequency and large wavelength λ, so an antenna of large length L =  λ/4 ≈ 3.75 km is required, which is unpractical.

However for modulating audio signals with carrier waves of large frequencies, antenna of small managable size is required.

(b) More effective power radiated by antenna: Power radiated by antenna is p ∝ l²/λ²

This shows that there is a need of higher frequency conversion for effective power transmission by the antenna.

Correct option: (4) Total internal reflection,of light rays

Explanation: 

Ionospheric reflection, of radio waves, may be regarded as similar to the phenomenon of ‘total internal reflection’ in optics.

(i) Here P = Intermediate Frequency Stage Q = Detector

(ii) IF Stage is used to change the carrier frequency to lower frequency. 

Detector is used to recover the modulating signal from the modulated carrier wave

The method of transmitting information from one place to another in terms of light pulses through an optical fiber is called fiber optic communication. It is in the process of replacing wire transmission in communication systems. Light has very high frequency (400 THz – 790 THz) than microwave radio systems. The fibers are made up of silica glass or silicon dioxide which is highly abundant on Earth.

Now it has been replaced with materials such as chalcogenide glasses, fluoroaluminate crystalline materials because they provide larger infrared wavelength and better transmission capability. As fibers are not electrically conductive, it is preferred in places where multiple channels are to be laid and isolation is required from electrical and electromagnetic interference.

Applications:

Optical fiber system has a number of applications namely, international communication, inter-city communication, data links, plant and traffic control and defense applications.

Merits: 

1. Fiber cables are very thin and weight lesser than copper cables. 

2. This system has much larger bandwidth. This means that its information canying capacity is larger. 

3. Fiber optic system is immune to electrical interferences. 

4. Fiber optic cables are cheaper than copper cables.

Demerits:

1. Fiber optic cables are more fragile when compared to copper wires. 

2. It is an expensive technology.

Correct option is (b) 1985

10 MHz frequency/frequencies will be suitable for beyond the horizon communication using sky wave

Correct option: (1) 10 MHz

Explanation: 

10 KHz can not be used for long distance communication and 1 GHz and 1000 GHz will penetrate through the ionosphere. It is only 10 MHz that can get reflected back from the ionosphere.

At frequencies above 40 MHz, the communication is limited to line of sight paths. Hence, the options (a) and (b) are correct.

(a), (b), (c)

(a) (i) followed by (ii).

(b) (ii) followed by (i).

(c) (iii).

(b), (d)

(b) m > 1 will result in overlapping of both side bands resulting into loss of information. 

(d) m > 1 indicates amplitude of message signal greater than amplitude of carrier signal resulting into distortion.

Similar to message signals, different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space and fiber optic cable. 

(a) Bandwidth of cable wire : Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz. 

(b) Bandwidth in free space : Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. 

(c) Bandwidth of optical fiber : Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz. 

1. Telephone

2. Internet.

Advantages of telegrams are as follows:

1. If the message is long, then telephone charges would be much more than expenses incurred on telegram. 

2. Telegram is also sent when the message is important and prompt action is required.

1. Yes. Because wavelength of radio wave is larger than microwave.

2. To have large coverage, increase the height of antenna or stations. Microwaves are not suitable for this kind of transmission.

(c) because decimal system implies continuous set of values.

1. Space wave

2. d_m = \(\sqrt{2Rh}\)

d_m = \(\sqrt{2\times6.4\times10^6\times200}m\)

3. Space waves can not be reflected back by iono sphere. Space wave can be reflected back to earth by making use of artificial satellite.

Answer is No.

Amplitude modulation. 

(b) 5.00

 m_f = \(\frac{Δf}{f}\)= \(\frac{10 kHz}{2 kHz}\) = 5

Frequency modulation. 

When the frequency of the baseband signal is zero (no input signal), there is no change in the frequency of the carrier wave. It is at its normal frequency and is called as centre frequency or resting frequency

(a) 2 MHz 

Critical frequency, 

f_c = 9 (N_max)^1/2 

= 9(10¹¹)^1/2 = 2.8 x 10⁶ Hz = 2.8 MHz

∴ The wave of frequency 2 MHz will be reflected by the ionosphere.

In standard AM broadcast, ground wave is used as mode of propagation.This mode is limited up to a few MHz, because the attenuation of surface waves increases very rapidly with increase in frequency.

(d) space waves. 

Frequency in the UHF (0.3 to 3 GHz) range normally propagate by means of space waves. Their sky wave reflection from ionosphere is not possible.

Correct option: (2) Ground waves, sky waves, space waves

Explanation: 

In the ground wave mode of propagation the wave glides over the surface of the earth. This mode can be used only for frequencies less than a few MHz. 

The sky wave uses the ionospheric reflection of radio waves; the ionosphere does not reflect back waves of frequencies more than (30–40) MHz. 

The space wave mode of propagation is used for line sight communication and satellite communication is done primarily through this mode only.

Need of modulation : 

(i) audio/video signals do not have sufficiently high energy to travel upto long distances, because of their lower frequency

(ii) For effective transmission, the size of the antenna should be at least of the size λ/4,where λ is wavelength of signal to be sent. For an e.m. wave of the frequency of the order of audio signal, we need an antenna of size 3.75 km, which is practically impossible. Hence these low frequency base band signals are first converted into high frequencies, through modulation

Modulation index, μ = A_m/A_c

So peak voltage of modulating signal, Am = μ A_c

∴ A_m = 0.75 × 12 = 9 V

Range : It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

Bandwidth : Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal. 

In a communication system, the message signal can be voice, music, picture or computer data. Each of these signals has different ranges of frequencies. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process. 

(a) Bandwidth of speech : For speech signals, frequency range 300 Hz to 3100 Hz is considered adequate. Therefore speech signal requires a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for commercial telephonic communication. 

(b) Bandwidth of Music : To transmit music, an approximate bandwidth of 20 kHz is required because of the high frequencies produced by the musical instruments. 

(c) Bandwidth of Audio signal : The audible range of frequencies extends from 20 Hz to 20 kHz. 

(d) Bandwidth of video signal : Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission. 

(e) Bandwidth of digital signal : Digital signals can be ideally reconstructed if we have superimpose fundamental frequency and all the harmonics and hence implies an infinite bandwidth. However, in practice, the contribution from higher harmonics can be neglected, thus limiting the bandwidth. As a result, received waves are a distorted version of the transmitted one. If the bandwidth is large enough to accommodate a few harmonics, the information is not lost and the rectangular signal is more or less recovered. This is so because the higher the harmonic, less is its contribution to the wave form. 

In AM, the carrier waves instantaneous voltage is varied by modulating waves voltage. On transmission, noise signals can also be added and receiver assumes noise a part of the modulating signal. 

However, in FM, the carriers frequency is changed as per modulating waves instantaneous voltage. This can only be done at the mixing/ modulating stage and not while signal is transmitting in channel. Hence, noise doesn’t effect FM signal.