Question

Write notes on the following:
A) Fax
B) Optical detector
C) Diode Laser
D) Difference between amplitude modulation and frequency modulation

Answer 1

There are various sources used for communication. The communication system is a system which describes the information exchange between two points. The process of transmission and reception of information is called communication. The major elements of communication are the Transmitter of information, Channel or medium of communication and the Receiver of information.

Complete step by step solution:
(A) Fax:
The electronic reproduction of a document at a distant place is known as facsimile telegraphy or fax. The original written document is converted into transmittable codes at the sending code.
These codes are converted back into a copy of the original document at the receiving end. To send the document through fax, the following functions are performed.
(i) Optical scanning.
(ii) Conversion of data for transmission and reception.
(iii) Printing a copy of the image of the original document at the receiving end.

(B) Optical Detector:
An optical detector is a device which separates the information signal and the optical carrier waves from the optical modulated signal reaching at the receiver. A photodiode is used for this purpose.
A photo diode is a P−N junction diode made up of a light sensitive semiconductor. It is kept in reverse bias. The saturated current obtained in the reverse bias is called OFF current.
Now light of frequency v is made incident on it, such that the energy of a photon of light must be greater than the forbidden energy between the valence band and conduction band of the semiconductor. As a result, the electron moves from the valence band to the conduction band and hence a hole is created in the valence band. Thus due to production of electron-hole pairs, the value of saturated current flowing through the photodiode increases. It is called ON current.
Since the reverse saturation current is directly proportional to the intensity of incident light, so it separates the information signal.

(C) Diode Laser:
These are very convenient devices. These lasers have p−n junctions made of special semiconducting material known as gallium arsenide (Ga−As).
It has the following main parts:
(i) Working material: It is a P−N junction semiconductor made by doping the impurities of In−as, In−P or In−Sb.
(ii) Resonant cavity: The two opposite faces of semiconductor crystals behave like the resonant cavity.
(iii) Pumping arrangement: For this, P−N junction semiconductor is kept in forward bias by means of a battery.

(D) Difference between Amplitude modulation and Frequency modulation are as follows:
1. Amplitude modulation amplitude of carrier wave is changed in accordance with basic voice frequency of modulating signal while Frequency of carrier wave is changed in accordance with basic voice frequency of modulating signal.
2. The process of amplitude modulation produces two side band frequencies corresponding to original carrier frequency while FM wave consists of a carrier and an infinite number of pair of side bands
3. Amplitude modulation is more likely to suffer from noise while FM is free from effects of noise.
4. Apparatus used in AM are simple and cheaper while Apparatus used in FM are complex and costly.

Note:
Frequency modulation and phase modulation are the two complementary principal methods of angle modulation. Phase modulation is often used as an intermediate step to achieve frequency modulation. These methods contrast with amplitude modulation, in which the amplitude of the carrier wave varies, while the frequency and phase remain constant.