Transcript Basicx
Communication Electronics By Saneeju M Salu What is Communication ? The communication system basically deals with the transmission of information from one point to another using the well defined steps which are carried out in sequential manner. Communication is the basic process of exchanging information. Elements of communication system Transmitting Signal Received Signal Input Signal Transmitter Transmission Channel Noise Destination Receiver Information source The information or message signal is originated from information source. Source of information generates message signal examples of which are human voice, telephone pictures, teletype data, atmospheric temperature and pressure in the above example. In short, we can say that the function of information source is to produce required message signal which has to be transmitted. Input Transducer A transducer is a device which converts one form of energy into another form. The message from the information source may or may not electrical in nature. In a case when the message signal produced by information source is not electrical in nature, an input transducer is used to convert it into a time varying electrical signal. For example, in case of radio-broadcasting, a microphone converts the information or message which is in the form of sound waves into corresponding electrical signal. Transmitter The function of transmitter is to process the electrical signal from different aspects. The signal received from the information source after converting it into electrical signal is not suitable for transmission over the channel. The message signal requires same processing like filtering and modulation etc, so that it is suitable for the transmission over the channel. Inside the transmitter, signal processing such as restriction of range of audio frequencies, amplification and modulation are achieved. All these processing of the message signal are done just to ease the transmission of the signal through the channel. Channel The physical connection between transmitter output and receiver input is provided by the channel. There are mainly two types of channels. A) Point to point channel B) Broad cast channel Point to point channel : The point to point channels are wire lines, microwave links, optical fibers. The wire lines are operated by guided electromagnetic waves used in local telephone transmission. In microwave links, the transmitted signal is radiated as an electromagnetic wave in free space and or used in long distance communication. An optical fiber is lossless well controlled, guided optical medium used in optical fiber communication system. Broadcast channel : Broadcast channels provides a capability where several receiving stations can be reached simultaneously from a single transmitter. An examples of Broadcast channels is a satellite in geostationary orbit, which covers one third of earth’s surface. Noise Noise is an unwanted signal which tend to interface with the required with the required signal. Noise signal is always random in character. Noise may interfere with signal at any point in a communication system. Receiver A receiver extracts the desired message signals from the received signals at the channel output. It consists of a pickup antenna to pick up signal, demodulator, an amplifier and the transducer. The receiver reconstructs a recognizable form of the original message signal for delivering it to the user of information. Destination Destination is the final stage which is used to convert an electrical message signal into its original form. For example in radio broadcasting, the destination is a loudspeaker which works as a transducer i.e. it converts the electrical signal form of original sound signal. Basic terms in communication Transducer : Any device that converts one form of energy into another can be termed as transducer. An electrical transducer can be defined as a device which converts some physical variable (pressure, displacement, temperature, force etc) into corresponding variations in the electrical signal at its output. Attenuation : The loss of strength of a signal while propagating through a medium is known as attenuation. Amplification : Amplification is the process of increasing the amplitude (and also strength) of a signal using an electronic circuit called the amplifier. It is necessary to compensate for the attenuation of the signal in communication systems. Cont…. Range : The maximum (largest) distance between a source and a destination upto which the signal is received with sufficient strength is termed as range. Bandwidth : The frequency range over which an equipment operates or the' potion of the spectrum occupied by the sig Modulation : The process of superimposing a low frequency signal on a high frequency wave, which acts as a carrier wave for long distance transmission is known as modulation. Cont.. Demodulation: The process of regaining (retrieval) of information from carrier wave at the receiver is termed as demodulation. (This is the reverse process of modulation). Repeater: A repeater is a combination of a receiver and transmitter. Repeaters are used to extend the range of a communication. Information Source Encoding Modulation Transmitter Channel Destination Decoding Demodulator Receiver Analog Vz Digital Communication The difference between analog and digital technologies is that in analog technology, information is translated into electric pulses of varying amplitude. In digital technology, translation of information is into binary format (zero or one) where each bit is representative of two distinct amplitudes. Analog Vz Digital Communication Analog Digital Analog signal is a continuous Digital signals are discrete signal which represents physical measurements. Affected by noise. Analog hardware is not flexible Less Bandwidth Low cost Analog instruments draws large power time signals generated by digi Less affected by noise, because noise are analog in nature Digital hardware is flexible in implementation High Bandwidth Cost is High Digital instruments draws only negligible power Noise Noise Internal Noise External Noise Due to random movement of Electrons in a electronic circuit Electronic components, Resistor Diodes, Transistor Man made noise and natural resources Industrial resources, Generators, motor , vehicle Atmospheric sources Signal to Noise power (SNR)= 10log (S/N) dB S – Signal Power N- Noise Power Need for Modulation Modulation Modulation is an important step of communication system. Modulation is defined as the process whereby some characteristic (line amplitude, frequency, phase of a high frequency signal wave (carrier wave) is varied in accordance with instantaneous value intensity of low frequency signal wave (modulating wave.) (i) To separate signal from different transmitters :- Audio frequencies are within the range of 20 Hz to 20 kHz. Without modulation all signals at same frequencies from different transmitters would be mixed up. There by giving impossible situation to tune to any one of them. In order to separate the various signals, radio stations must broadcast at different frequencies. (ii) Size of the antenna : – For efficient transmission the transmitting antennas should have length at least equal to a quarter of the wavelength of the signal to be transmitted. For an electromagnetic wave of frequency 15 kHz, the wavelength λ is 20 km and one-quarter of this will be equal to 5 km. Obviously, a vertical antenna of this size is impracticable. On the other hand, for a frequency of 1 MHz, this height is reduced to 75m. λ=v/f & l= λ/4 (iii) Signal Power :- The power radiated by an antenna of length l is proportional to (l/λ)2. This shows that for the same antenna length, power radiated is large for shorter wavelength. Thus, our signal which is of low frequency must be translated to the high frequency spectrum of the electromagnetic wave. This is achieved by the process of modulation. (iv) Noise : Most of the signal are low frequency range, so possibility to mix with information is high. Bandwidth requirements, The channel bandwidths needed to transmit various types of signals, u sing various processing schemes.Every signal observed in practice can be expressed as a sum (discrete or over a frequency continuum) of s inusoidal components of various frequencies. The plot of the amplitude versus frequency constitutes one feature of the frequency sp ectrum (the other being the phase versus frequency). The difference between the highest and the lowest frequencies of the frequency com ponents of significant amplitudes in the spectrum is called the bandwidth of the signal, expressed in the unit of frequency, hertz. Ever y communication medium (also called channel) is capable of transmitting a frequency band (spectrum of frequencies) with reasona ble fidelity. Qualitatively speaking, the difference between the highest and the lowest frequencies of components in the band over which the c hannel gain remains reasonably constant (or within a specified variation) is called the channel bandwidth. Non-sinusoidal waveforms Non-sinusoidal waveforms are waveforms that are not pure sine waves. They are usually derived from simple math functions. While a pure sine consists of a single frequency, non-sinusoidal waveforms can be described as containing multiple sine waves of different frequencies. These "component" sine waves will be whole number multiples of a fundamental or "lowest" frequency. The frequency and amplitude of each component can be found using a mathematical technique known as Fourier analysis sinusoidal waveforms Any periodic waveform may be expressed as Sum of a series of sinusoidal waveforms at different frequencies and amplitudes Any periodic waveform Expressed as an infinite series of sinusoidal waveforms A periodic waveform can be written as: f(t) = a0 + a1cos t + a2cos 2t + ∙∙∙ + an cos nt + ∙∙∙ + b1sin t + b2 sin 2t + ∙∙∙ + bn sin nt + ∙∙∙ Coefficients of terms of Fourier series Found by integrating original function over one complete period 1 t1T a0 f (t ) dt T t1 2 t1T an f (t ) cos (nt ) dt T t1 2 t1T bn f (t ) sin (nt ) dt t 1 T Individual components combined to give a single sinusoidal expression as: an cos nx bn sin nx an sin ( nx 90) bn sin nx cn sin ( nx ) where cn an bn 2 2 and a n tan 1 b n Fourier equivalent of any periodic waveform may be simplified to f(t) = a0 + c1sin(t + 1) + c2sin(2t + 2) + ∙∙∙ a0 term is a constant that corresponds to average value cn coefficients are amplitudes of sinusoidal terms Sinusoidal term with n = 1 Same frequency as original waveform First term Called fundamental frequency All other frequencies are integer multiples of fundamental frequency These frequencies are harmonic frequencies or simply harmonics Frequency Spectrum Waveforms may be shown as a function of frequency Amplitude of each harmonic is indicated at that frequency