CATHODE RAY OSCILLOSCOPE (CRO)

CRO is a very essential and versatile lab instrument used for display , measurement and analysis of waveforms having electrical nature. This is a X-Y plotter which display input signal versus another input signal or versus time. The stylus of this plotter is a luminous spot which moves in accordance to input voltage. The luminous spot  is produced by electron beam striking the flourescent screen.
The normal form of CRO uses a horizontal input voltage which is an internally generated ramp voltage. This horizontal voltage move the luminous spot in horizontal direction on the screen. The vertical input voltage is the voltage under investigation. This vertical voltage moves the spot up - down in accordance with its instantaneous value. Thus a waveform of input voltage is traced on screen wrt time.

CRO basically consist of CRT ( cathode ray tube) which has some additional circuitry. The main parts of CRT are electron gun assembly , deflection plate assembly , flourescent screen , glass envelope and base. Electron gun is used to produce focussed and accelerated beam of electrons. The deflection plate assembly is used to move beam in vertical as well as horizontal direction.
There are numerous applications of CRO. They can accept multiple input signals and waveforms can be traced and analysed easily. These oscilloscopes employ time sampling and are able to measure singals of 20GHz.The complete waveform can be stored and can be displayed as stationary signal.       

LIGHT EMITTING DIODES

LEDs are p-n junction diodes which can emit radiation in the visible range (370nm to 770nm) under forward biased condition.

In a p-n junction electrons from n side diffuses to p side and holes from p side moves towards n side forming a depletion region. So no mobile charges present in the depletion region.When sufficient voltage is applied movement of electrons across junction takeplace. These electrons when recombined with holes , a quantum of electromagnetic energy is emitted in the form of photon of light with frequency charateristic of semiconductor material ( generally GaAsP , GaP). 

Characteristics:
1. These give adequate power.
2. These give low noise.
3. LEDs are compatible with optical fibre.
4. The light from LED can be varied or modulated by altering applied voltage or current which is helpful in transmitting information.

Applications:
1. They can be used as cheap source of light.
2. In optical fibre applications.
3. They can be used as sensor as in case of optical mouse of computer.

COMMUNICATION SYSTEM

Communication is the process of exchanging information. It is the process of stablishing link between two points for information exchange.
The electronic equipment used for communication purpose are known as communication equipment. These communication equipment are assembled togather to form a communication system.
Examples: line telephony , line telegraphy , mobile communication , radio broadcast ,tv broadcast etc.

Communication System Elements :
1. Information Source:The source which generate useful information is termed as information source.
2. Transducer: It is a device which is used to convert one form of energy to other form. We can say that it converts information signal into electrical pulses. For example microphone converts sound waves into electrical signals.
3. Transmitter : In this signal processing is done so as the signal can be transmitted easily and safely. In this amplification and modulation of signal takes place.
4. Channel: It is the medium through which information travels from transmitter to reciever. It can be wirelines , microwave links , optical fibre cables etc.
5. Reciever: It is used to reproduce the signal in original form. The reproduction of signal is done with the help of demodulation.