Content:- Fundamentals of Communication Engineering : Elements of a Communication System, Need of modulation, electromagnetic spectrum and typical applications, Unit V (Communication terminologies in communication System) systems, Basics of signal representation and analysis, Fundamentals of amplitude and angle modulation, modulation and demodulation techniques. Communication System:- A Communication is a set up used in the transmission & reception of information from one place to another place.in communication system, transmitter is located at one place & receiver is located at another place & the communication channel connects the transmitter & receiver. the channel is physical mediums. Elements of Communication System Channel Fig: 1: Block Diagram of Communication System Elements 1- Information Source 2-Transmitter 3=Communication channel 4- Noise 5- Receiver 6-Destination Description 1. Information Source- The information source provides a message signal being non- electrical signal like voice signal. The transducer converts the non-electrical voice signal into into electrical form. 2. Transmitter- The transmitter is a collection of electronic circuit designed to convert the information into a signal suitable for transmission over a given communication medium. 3. Communication Channel- The communication channel is the medium by which the electronic signal is transmitted from one place to another. 4. Noise- Noise is random, undesirable electrical energy that enters the communication system via medium & interferes with transmitted message. Some noise is also produced in the receiver. 5. Receiver- A receiver is a collection of electronic circuits designed to convert the signal back to the original information. Terminology in Communication System 1. Signal- It is a signal valued function of time that carries the information. It is in the electrical form suitable for transmission. Signals may be analog or digital signal. 2. Transducer- Transducer is a device, which is used to convert non electrical signal to the electrical signal. 3. Noise- It refers to the disturbance or distortion in the transmission & processing of message signals in communication system. 4. Transmitter- The transmitter is a collection of electronic circuits designed to convert the information into a signal suitable for transmission over a given communication medium. 5. Receiver- A receiver is a collection of electronic circuits designed to convert the signal back to the original information. 6. Spectrum- The spectrum is the entire range of wavelengths of electromagnetic radiations. The spectrum of frequency component is the frequency domain representation of the signal. 7. Amplification- In this process, The strength of the transmitted signal is increased by using some electronic circuit. B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 1
8. Attenuation- It refers to the loss of signal strength during its propagation through the communication channel. 9. Baseband Signal- The information/ message signal in its original frequency range is known as baseband signal. 10. Modulation- Modulation is the process of super imposing the low frequency message signal (modulating signal) on a high frequency wave (called the carrier signal). The Resulting wave is the modulated wave which is transmitted over the channel. 11. Demodulation- A reverse process of retrieve the information from the modulated wave at receiver is known as demodulation. 12. Bandwidth- Bandwidth refers to the range of frequencies over which communication system works. 13. Broadband Signal- This information/ Message signal in its modulated frequency range is known as broadband signal. 14. Broadband Transmission- The Transmission of information/ Message signal in the modulated frequency range is known as broadband transmission. Electromagnetic Spectrum & Typical Applications Radio Frequency- Radio Frequency or RF refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna.the radio frequency range is divided by the ITU (International Telecommunication Unit) into a number of bands of frequencies, where all frequencies in the band exhibit similar properties. The bands are ELF, VF, VLF, LF, MF, HF, VHF, UHF, SHF & EHF. S.No. Frequency Band Frequency Applications 1 Very Low Frequency (VLF) 3 KHz - 30 KHz Long distance point to point communication 2 Low Frequency (LF) 30 KHz 300 KHz Marine & navigational purpose 3 Medium frequency (MF) 300 KHz 3 MHz Marine & broadcasting purpose 4 High Frequency (HF) 3 MHz 30 MHz Communication of all types landline telephony 5 Very High frequency (VHF) 30 MHz 300 MHz FM, TV, radar & Air navigation 6 Ultra High Frequency (UHF) 300 MHz 3000 MHz Radar & microwave communication 7 Super High Frequency (SHF) 3 GHz 30 GHz Wireless LAN, Radar, Radar relays & navigation purpose 8 Extremely High Frequency (EHF) 30 GHz 300 GHz Optical fiber communication Modulation & Demodulation In electronic communication system, a high frequency signal called the carrier signal is used to transmit original signal to the destination. Modulation- The process by which the baseband signal modifies the carrier signal is called Modulation & the resulting signal is called modulated signal. Demodulation- At the receiving end, the original signal & carrier signal are separated from modulated signal by the process of demodulation. Need of Modulation The original signal either in analog or digital form (baseband signals) are incompatible for direct transmission over the medium. They cannot travel longer distances in air. For ex. Voice signal. Therefore, we have to use modulation technique for the communication of baseband signal. To decrease the length of transmitting & the receiving antenna. Long distance communication is possible. To obtain higher value of signal of signal to noise ratio for modulation. To have frequency division multiplexing where large number of signals is transmitted at different frequencies. Avoids mixing of signals. Allows multiplexing of signals. Allows Adjustments in the bandwidth. Improves Quality of reception. B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 2
Height of Antenna The height of antenna required for transmission & reception of radio waves in radio transmission is a function of wavelength of the frequency used. The minimum height of the antenna is given as λ/4. The wavelength λ is given as λ= where c velocity of light f frequency Modulation It is defined as the process by which some characteristics of signal called CARRIER is varied in accordance with a modulating signal or base-band signal or message signal. The result of the modulation process is referred as modulated wave. At the receiving end of the system we reconstruct the original base-band signal & this is accomplished by using a process called DEMODULATION. Demodulation is the reverse process of Modulation. Continuous Wave Modulation Amplitude Modulation (AM) Angle Modulation Amplitude Modulation (AM) In AM, the maximum amplitude of the high frequency carrier wave is varied in accordance to the instantaneous value (amplitude) of the modulating or base-band signal. Frequency Modulation (FM) Phase Modulation (PM) Equation for AM: Let C(t) = A c Cos ω c t.is a carrier wave having Where A c = carrier amplitude ω c = carrier frequency & m(t)=modulating signal band limited to maximum frequency ω m. Hence ω c =2πf c >> ω m Now the AM wave is described as X Where, A=A c AM (t)= A Cos ω c t +m(t) So, X AM (t)= [A c +m(t)] Cos ω c t = A c [1+K a m(t)] Cos ω c t Where K a = constant called amplitude sensitivity of modulation Also, we can write X AM (t)= A c Cos ω c t + m(t) Cos ω c t Sinusoidal AM: Let C(t) = A c Cos ω c t. Carrier Wave m(t) = A m Cos ω m t. Modulating Wave Then after modulation, we get X AM (t)= A Cos ω c t Where A= Ac + Am Cos ω m t So we get, X AM (t)= [A c + A m Cos ω m t] Cos ω c t X AM (t)= A c [1+ Cos ω m t] Cos ω c t so, X AM (t)= A c [1+ m a Cos ω m t] Cos ω c t Where m a = = modulation index Frequency Spectrum: The Modulated carrier has new signals at different frequencies, called side frequencies or B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 3
sidebands, which occur in the frequency spectrum directly above & below the carrier frequency. We know that X AM (t)= A c [1+ m a Cos ω m t] Cos ω c t = A c Cos ω c t + m a A c Cos ω m t Cos ω c t X AM (t)= A c Cos ω c t + m a A c Cos (ω c + ω m )t + m a A c Cos (ω c - ω m )t Full Carrier Upper Lower Side band Side band Modulation Index: In Amplitude modulation, the modulation index is defined as the ratio of maximum amplitude of modulating signal to the maximum amplitude of the carrier signal. % modulation = m a x 100 Modulation index gives the depth to which the carrier signal is modulated. 0 m a 1 for A m A c M a = = Power Relation in AM wave: Where P T = Total Power P C = Carrier Power Transmission Efficiency (η): η= Current Relation in AM wave: Let R be the resistance into which the current flow. where I t = Total transmitted current I c = Carrier current m = Modulating index Effective Modulation Index: If a carrier is modulated by more than a single sine wave, the effective modulation index is given by: Note that the total modulation index must not exceed 1 or distortion will occur. m eff can be used in all previously developed equations using m. Bandwidth of AM wave: BW= 2f m Bandwidth required for the amplitude modulation is twice the frequency of modulating signal. B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 4
Total phase angle of high frequency carrier wave is varied in accordance with the modulating or message signal. Let C(t) = A Cos(ω c t + θ a ) = A Cos ф(t) Where ф(t)= total phase angle of carrier ф(t)= ω c t + θ a so, = ω c The time dependent angular frequency= instantaneous angular frequency. ω i = Angle Modulation so, ф(t)= ω i dt If angle ф is varied according to instantaneous value of m(t). the carrier signal C(t) is then said to be angle modulated. Frequency Modulation(FM) In FM, the frequency of the high frequency carrier wave is varied in accordance to the instantaneous frequency of modulating or base-band signal. In frequency modulation ω i is varied linearly with the modulating signal m(t) about an unmodulated carrier frequency ω c. he derivative of phase angle is varied in accordance with the m(t). We have, ω I (t)= ω c + k f m(t) So, Instantaneous value of frequency of modulated FM wave. K f = frequency sensitivity of the modulator = rad/sec/volt = Hz/volt Since, C(t)= A c Cos ф(t) where ф(t)= ω i t + θ a ф(t)= ω c dt for instantaneous phase angle So, ф i (t)= ω c dt After FM, the carrier signal is as C(t)= A c Cos ф i (t) Now, if we take limit of integration as t 0 to t 1, then, ф i (t) = *ω c t + k F m(t)] dt ф i (t)= ω c t + k F m(t)] dt Now the general expression for FM wave is given as X FM (t)=a c Cos [ω c t + k F m(t)] dt] Sinusoidal FM Let c(t)= A c Cos ω c t A carrier Signal m(t)= A m Cos ω m t.. A message signal So, ω i (t)= ω c + K f A m Cos ω m t Δω = maximum frequency deviation = K f A m B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 5
In terms of modulation index X FM (t) = A c Cos ф i (t) Bandwidth of FM Where β modulation index www.uptunotes.com β= = = BW= 2 (β+1)ω m BW= 2 (β+1)f m Δf= = (Hz) X FM (t) = A c Cos *ω c t + βsin ω m t] Phase Modulation(PM) In PM, the phase of the high frequency carrier wave is varied in accordance to the instantaneous phase of modulating or base-band signal The phase modulating signal m(t) about an unmodulated phase angle ω c t. Where β= =, β= modulation index or deviation ratio ф i (t)= ω c t + K p m(t) Instantaneous value of phase angle of modulated PM wave So, Where K p = phase sensitivity of modulator in rad/volt Let C(t)= A c Cos ф(t), is an unmodulated carrier. Now after PM, we may write X PM (t) = A c Cos ф i (t) So the general expression for PM wave is given as X PM (t)= A c Cos [ω c t + K p m(t)] AM Modulator is one which is used for superimposing a low frequency signal on a high frequency carrier signal. In this modulator the amplitude of a carrier is varied in accordance with instantaneous value of message signal. Linear Modulators- This is achieved by multiplying message signal m(t) by carrier signal A c Cos ω c t using analog multipliers whose output is proportional to the product of two input signals. A.M Modulators/Demodulators Fig:-Multiplier based AM modulator B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 6
Non-Linear Modulators- In non-linear modulators, non- linear devices like diode and transistors are used. Fig: Block Diagram for non linear modulators AM Demodulator Envelop Detector- This is essentially just a half wave rectifier which charges a capacitor to a voltage to the peak voltage of the incoming AM waveform S(t). When the input wave's amplitude increases, the capacitor voltage is increased via the rectifying diode. When the input's amplitude falls, the capacitor voltage is reduced by being discharged by a bleed resistor, R. The main advantage of this form of AM Demodulator is that it is very simple and cheap! Just one diode, one capacitor, and one resistor. That's why it is used so often. However, it does suffer from some practical problems. Fig: Envelop Detector Question-1 Solved Examples Question-2 B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 7
Question-3 Question-4 Question-5 A 400kHz sinusoidal carrier of amplitude 5V is frequency modulated by a 3kHz sinusoidal information signal of amplitude 3V. The behaviour of the carrier is governed by the frequency deviation per volt and for this system is 25kHz per volt. Describe how the resulting FM signal changes with time. Solution: The FM carrier will change in frequency from 400 khz to 475 khz to 400 khz to 325 khz and back to 400 khz, 3000 times per second. This is because the frequency deviation f c = 3 x 25 khz = 75 khz. The amplitude of the carrier will remain fixed at 5 V. Question-6 In national radio broadcasts using FM, the frequency deviation of the carrier f c, is chosen to be 75 khz, and the information baseband is the high fidelity range 20 Hz to 15 khz. Solution: The modulation index, is 5 (i.e. 75 khz 15 khz), and such a broadcast requires an FM signal bandwidth given by: Bandwidth FM Radio 2 ( fc fi(max) ) 2(75 15) 180kHz Question-7 A 10 MHz carrier is frequency modulated by a pure signal tone of frequency 8 khz. The frequency deviation is 32 khz. Calculate the bandwidth of the resulting FM waveform. Bandwidth 2( f f (max)) c 2( 32 8) 80kHz i B y : - N a v n e e t P a l E m a i l : - e r. n a v n e e t p a l @ g m a i l. c o m Page 8