Radio waves : transmission and reception

Sound waves are produced by the source, a microphone turns the sound waves into an electrical signal. the sound waves cause an electric current that runs through the microphone to vary, these variation form audio frequency waves. The audio frequency waves are combined with waves at a much higher frequency, called radio frequency, produced by an oscillator. These are known as carrier waves because they carry the program signal. The radio frequency waves are modulated either by amplitude modulation, in which their amplitude varies to match changes in the program signal, or by frequency modulation (FM) in which their frequency varies. The modulated signal is amplified to increase its power. The amplified modulated signal is passed to an antenna from which it is transmitted each station broadcasts on a different channel (its assigned frequency measured in units called hertz.

The receiver aerial picks up the radio waves carrying the signal, the signal, which has become very weak following its transmission, is amplified, the rectifier removes half the  voltage swing in the amplified signal, the signal is separated from the carrier waves, the signal is again amplified in order to drive the loudspeaker, the loudspeaker converts the electrical signal back into sound waves which can be heard.

Inside the radio

An aerials receives radio waves many station on many different frequencies at same time . in order to hear a particular station a listener must tune the radio by adjusting the radio frequency filter, select a particular frequency  the selected frequency is sent to a mixer where it is combined with oscillator. The mixer and oscillator together  form a frequency changer which changes the selected RF signal  that is intermediate frequency  signal. The IF is fed into the IF amplifier and later in the demodulator. This circuit detects the slight variation in the strengthen of the signal produced by the audio frequency , which represent the original  sound. Some of the audio is fed back to the IF amplifier to provide automatic control gain  this ensures  that when tuning from a weak station to a strong one the loudness from the loudspeaker stays the same. The audio signal voltage is increased by an voltage amplifier, later the signal is fed into amplifier power  then increases the power level sufficiently to drive the loudspeaker which translate the electric signal into sound  

Analog to digital conversion

The basis of high fidelity telecommunication is the binary signal created by a process called analog to digital conversion. The binary signal is a reliable carrier because it consist of only two possible values, making it much less susceptible to the effects of noise and distortion. Analog  to digital conversion begins with sampling, it consist measuring the amplitude of the analogue waveform at equally spaced intervals of time. The difference between  the highest  and lowest frequency of the waves making up the signal is known as bandwidth. In order for the sampled signal to be then stored or transmitted in digital form, each sampled amplitude must be quantized, converted to one of finite number of possible values or levels. For ease in conversion to binary form, the number of levels is usually power of 2 ,and so on depending on degree of precision required. The degree of inaccuracy depends on the number of output levels used by the quantizer. More quantization levels  increase the accuracy of the representation but they also increase the storage capacity or transmission speed required.in the next step in the digitization process the output of the quantizer is encoded into a binary sequence. The three bit code shown in the can represent eight levels .16 levels would require four bits and 256 levels would require eight bits. When a digital signal hasa to be changed back to an analogue one, the decoding produces a stepped waveform if a high sampling frequency has been used the steps are very small  and the decoded signal is a good copy of the original analogue signal 

Modulation

Modulation is common technique for incorporating informating information, onto a radio -frequency carrier wave  by varing one or more characteristics  of the wave in accordance  with the information signal.
 the most commonly  altered characteristics  are amplitude and frequency.
in amplitude modulation the information is impressed on a carrier  wave by varing the amplitude
The AM is the oldest and simplest method for radio trasmission ,
 the advantage the signal is easily rcovered in the receiver , the audio  signal can be extracted simply  by rectifying  the receiver's  output  and then removing the carrier using a capacitor,
in this trassion (AM) uis used for radio broadcasting low and middle frequencies
In frequency modulation (FM) The amplitude  of the carrier is costant but the frequency varying in accordance  with the variation of the audio signal,
the FM is less suspeptible to certain kind of interference such as thundrestorm
in addition the FM give better qualitysignal than AM in the end FM (frequency modulation) is  used for radio and television broadcasting

Aerials

An aerials or antenna is a device used to trasmit or receive radio waves. most antennae for radio or television consist of metal wires or rods connected to trasmitter or receiver .
When the antennae is used for trasmission,electric currents are made to oscillate along  the wires by the trasmitter .
Energy  from this oscillating charge is emitted into the space
When the antenna is used for reception  these waves induce a weak electric current in the antenna wire. this current is then employed by the radio receiver 
the radio wavelength determines  the dimension of the antenna  
For the antenna to resonate electrically at the desired  wavelength, its should  be  one half or one quarter of the wavelength of the radio waves
-The simplest aerial called a monopole consist of a single wire suspended horizontally, or of a straight metallic rod.
A single-wire aerial is used in many short-wave receivers,
the extendible rod aerials are used  in portable devices, such as Fm radios or television .
-The most widely used TV aeriales dipole, which consist of a straight metal rod is split  in the centre .
-An other type is multiple element antenna,it consist more than one dipole that gives a better performance 
-Dish aerials are bowl-shaped. the antenna is section of a parabola and a small dipole is placed in its focus ,a dish aerial is highly directional and can trasmit or receive radio waves  in narrow beam these aerials  are used to trasmit signal to satellites and signal overland.
The location  of trasmitting aerials is also imprtant. Fm and television aerials  must be placed  as high as possible  to increase  their range.

Electromagnetic waves

Electromagnetic waves are patterns of the electric and magnetic energy generated by an oscillation .
the visible light is one form of electromagnetic radiation and the other types of electromagnetic wave share many properties of the light. they travel through space at the speed of lightthey can be reflected diffracted and refracted.
The Strength of the waves varies , with alternating crests and trough.
The distance from trough to crest is called the wavelenght .
The number of waves produced per second is called frequency.
Electromagnetic radiation forms a spectrum that extends from waves of extremely  high frequency and short wavelenght  to extremely low frequency and long wavelenght
Visible radiation is one form of electromagnetic radiation and the other types of electromagnetic wave share many of the properties of light.
visible light is only a small part of the electromagnetic spectrum.
in order of increasing wavelenghtthe spectrum consist:gamma rays, x rays, ultraviolet rays,, visible light,infrared radiation microwaves radio waves
The wavelenght and frequency of electromagnetic waves are important in determing , their heating effect visibility and other characteristics.
Radio waves are used in radio and television broadcasting. in addition  the microwaves use for cooking.
infrared  rays given off  by hot objects are used  in medical treatments  .
ultraviolet ray are used in sun lamps and as disinfectant
X ray are used to help to locate and  diagnose internal disorders in the body
in the end gamma rays given off by radium, are used to treat cancer.

Microprocessor

Microprocessor

Microprocessor is chip with a memory which can store lists of coded instructions.

They are classified according to:  
  1) clock speed which determines in megahertz

  2) How many instruction per second they can execute

  3) The number of bits processed in a single instruction.

Furthermore they can be classified according to  architecture that is :

Risc (reduce instruction set computer )                          
Cisc (complex  instruction  set computer )

A microprocessor consists of the ALU which can do an arithmetic calculations and makes logical decisions and CU which provides timing instructions and synchronization signals for all other units. Every data and instruction is an electric signals carried by the bus , the electronic circuits uses only two type of signals  that is analogue and digital 

Analogue signal vary continuously in voltage or current corresponding to the input information, these fluctuating  can represent changes in light ,sound , pressure and an analogue circuits work by measuring these changes

By contrast ….

Digital Signal can have two values 0 and the code used by the digital signal is called binary code , and  a digital circuits process information by simply counting or comparing signals.Every data is a particular combination of numbers ,every single number is called bit 

Logic Circuits in the CPU manipulate data according to instructions. The bits go through a sequence of switches that changes them in some way The switches can also be  used to compare  two bits in order to generate a particular output based on Boolean algebra in this case the digit 0 represent false and the digit 1 represent true .

Control circuit directs and coordinates  the work of all other parts of the system according to instructions stored in the memory , they organize the movement of the bits through  the system , the bits are moved by an oscillator called clock which generate pulses

Memory circuits store bits temporarily or permanently

These memory are:
Rom (which stands for read only memory) .                                                                                                                     in this memory can be neither erased nor added to .it is a type of  non volatile memory,                            that is the data is stored when the power is off ,and normally the rom contains                                    instructions  and programs for the operating system.

                                      
 Ram (which stands for Random access memory ). In this memory can be either erased .  .                                   or added to. This memory is used to store information for short periods, because  it is                 .                                   volatile that is the data is stored only when the  power  is on because when the power .                .                                  is off  the data is lost

Program counter (PC) is one the main registers of the cpu. The cpu must have some  means of determing which instruction is the next one to be executed . The PC fulfills this role before a program can be executed the starting address, must be loaded into the PC the continents of the PC are then used as an address to fetch the first instructions from memory. The PC is automatically updated so that it always contains the address of the next instruction to be executed, and this process is usually a simple increment of the PC by one. 

The accumulator (AC) functions as a working area for all computations performed by the ALU. For example, before an addition is performed, one of the operands is temporarily placed in the AC. The second operand is then retrieved from memory and is added to the contents of the AC the resulting sum appears in the AC destroying the original contents of the AC. Some instructions, such as "Clear Accumulator" or "Complement Accumulator", operate directly on the contents of the AC. Some central processors may have more than one accumulator. In other processors the functions normally provided by the AC are handled by general purpose registers (GPRs).