By going through these CBSE Class 12 Physics Notes Chapter 15 Communication Systems, students can recall all the concepts quickly.

## Communication Systems Notes Class 12 Physics Chapter 15

→ A basic communication system consists of information source, transmitter, receiver, and the link between transmitter and receiver.

→ The setup used for exchanging information between a sender and a receiver is called a communication system.

→ The transmitter is a part of the communication system which sends out the information.

→ The receiver is a part of the communication system that picks up the information sent by the transmitter.

→ The communication channel is a medium or link which transfers the message from the transmitter to the receiver of a communication system.

→ Low frequencies cannot be transmitted to long distances.

→ Pulse modulation is of four types:

1. Pulse amplitude modulation (PAM)
2. Pulse duration modulation (PDM)
3. Pulse position modulation (PPM)

→ Transducer: It is a device that converts energy in one form to another.

→ Bandwidth: It is defined as the frequency range of a signal. The information-bearing signal is called a baseband signal.

→ Sampling converts an analog signal into a digital signal.

→ The number of samples of the analog signal taken per second is called the sampling rate.

→ Sampling rate = $$\frac{1}{T}$$, where T = time period of sampling of the analog signal.

→ The discrete signals having only two levels are called digital signals.

→ The signals which vary continuously with time are called analog signals.

→ Modulation: It is defined as the process of superposing an audio signal on a high-frequency carrier wave.

→ Demodulation: It is the process of separating the audio wave from a modulated wave.

→ The degree to which a carrier wave is modulated is measured in terms of modulation index.

→ Quantization: It is the process of dividing the maximum amplitude of the voltage signal into a fixed number of levels.

→ The electronic transmission of a document to a distant place via telephone line is called FAX (Facsimile). It scans the contents of a document to create electronic signals.

→ The Internet permits the communication and sharing of all types of information between two or more computers connected through a large and complex network.

→ E.mail: It allows the exchange of text/graphic material using e¬mail software.

→ File transfer is done through a file transfer program (FTP). It allows the transfer of files or software from one computer to another connected through the internet.

→ Hypertext: It is a powerful feature of the web which automatically ‘ links relevant information from one page on the web to another
using HTML.

→ E-commerce: It is the process of using the internet to promote business using electronic means such as credit cards etc.

→ Chat: It is a real-time conversation among people with common interests through typed messages. Everyone belonging to the chat group gets the message instantaneously and can respond rapidly.

→ FAX provides images of a static document unlike the image provided by television of objects that might be dynamic.

→ Mobile telephones operate typically in the UHF (Ultra High Frequencies) range of frequencies about 800 – 950 MHz.

→ The central part of the mobile telephony system is to divide the service area into a suitable number of cells centered on an office called MTSO (Mobile Telephone Switching Office).

→ Base Station: It is a low-power transmitter contained in each cell. It caters to a large number of mobile receivers.

→ Pulse modulation: It is the process of producing a train of the pulse; of the carrier, some characteristics of which vary as a function of the instantaneous value of the message signal.

→ Pulse amplitude modulation (PAM): It is the process in which the amplitude of the pulses of carrier pulse train varies in accordance with the instantaneous value of the message signal.

→ Pulse width Modulation (PWM): The process of pulse modulation in which the width of the pulses of the carrier pulse train varies in accordance with the instantaneous value of the message signal.

→ Pulse code modulation (PCM): It is the process of converting an analog signal into a digital signal by sampling it in time, then quantizing and coding it.

→ Atmosphere: It is defined as the gaseous envelope surrounding the earth.

→ The radio waves from the transmitting antenna to the receiving. antenna propagate either by ground waves (i.e., space wave or surface wave) or sky waves.

→ The T.V. signals are frequency modulated.

→ The maximum distance up to which TV signals can be received is given by d = $$\sqrt{2hR}$$, where h is the height of the TV antenna and R is the radius of the earth.

→ The modulation index (mf) of a frequency modulated wave is defined as the ratio of maximum frequency deviation to the modulating frequency.
i.e., mf = $$\frac{\delta_{\max }}{f_{m}}=\frac{f_{\max }-f_{c}}{f_{m}}=\pm \frac{k V_{m} f_{c}}{f_{m}}$$
where fm = modulating frequency, Em = amplitude of modulating wave.

→ A Hertz antenna is a straight conductor of length equal to half the wavelength of radio signals to be transmitted or received i.e., l = $$\frac{λ}{2}$$

→ A Marconi antenna is a straight conductor of length equal to a quarter of the wavelength of radio signals to be transmitted or received i.e., l = $$\frac{λ}{4}$$

→ Amplitude modulated signal contains frequencies (Wc – Wm), Wc, and (Wc + Wm).

→ Attenuation: It is the loss of strength of a signal while propagating through a medium.

→ Noise: It is defined as the unwanted signal that tends to disturb the transmission and processing of message signals in a communication system.

→ Amplification is the process of increasing the amplitude of a signal using an electronic circuit.

→ For demodulation i.e. detection of a signal, $$\frac{1}{\mathrm{f}_{\mathrm{c}}}$$ < < τ where fc frequency of classier wave,
τ = time constant of the circuit.