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Transcript
Principles of Communication EEC-502 Unit-I • Introduction of Communication System • Amplitude Modulation Elements of a Communication System Source Transmitter Receiver Recipient Description of Communication Elements • Source: Analogue or digital like video, text or speech. • Input Transducer: e.g. microphone that converts audio signal into electrical signal. • Transmitter: Transducer, amplifier, modulator, oscillator, power amp., antenna. It modulates and transmits source data. • Channel: Coaxial cable, optical fiber, free space etc. • Receiver: Antenna, amplifier, demodulator, oscillator, power amplifier, transducer. It demodulates the original signal from modulated signal. • Output Transducer: e.g. loudspeaker that converts electrical signal into sound signal. • Recipient: Person, speaker, computer etc. Communication Channels • Telephone channels • Optical Fiber • Mobile radio channel Communication Channels • Satellite channel Modulation • Modulation is the process of impressing information onto a high-frequency carrier for transmission. • Reasons for modulation: – – – – to prevent mutual interference between stations to reduce the size of the antenna required Common processing Narrowbanding Frequency Bands Baseband Signals • The message signal generated from the information source is known as baseband signal. Analog baseband Signal Digital baseband Signal Band pass Signals • Baseband signal is impressed upon a carrier, modulated signal is produced. This signal has fixed band of frequencies around carrier frequency so known as bandpass type. Analog band pass Signal Digital band pass Signal Information and Bandwidth Bandwidth required by a modulated signal depends on the baseband frequency range (or data rate) and the modulation scheme. Hartley’s Law: I = k t B where I = amount of information; k = system constant; t = time available; B = channel bandwidth Shannon’s Formula: I = B log2 (1+ S/N) in bps where S/N = signal-to-noise power ratio Transmission Modes Simplex (SX) – One direction only, e.g. TV Half Duplex (HDX) – Both directions but not at the same time, e.g. CB radio Full Duplex (FDX) – Transmit and receive simultaneously between two stations, e.g. standard telephone system Full/Full Duplex (F/FDX) - Transmit and receive simultaneously but not necessarily just between two stations, e.g. data communications circuits Basic Analog Communications System Baseband signal (electrical signal) Input transducer Transmitter EM waves (modulated signal) Transmission Channel Modulator EM waves (modulated signal) Carrier Baseband signal (electrical signal) Output transducer Receiver Demodulator Time and Frequency Domains • Time domain: An oscilloscope displays the amplitude versus time. • Frequency domain: A spectrum analyzer displays the amplitude or power versus frequency. • Frequency-domain display provides information on bandwidth and harmonic components of a signal. Non-sinusoidal Waveform • Any well-behaved periodic waveform can be represented as a series of sine and/or cosine waves plus (sometimes) a dc offset: e(t)=Co+SAn cos nw t + SBn sin nw t (Fourier series) Amplitude Modulation Waveforms of Amplitude Modulation Analysis of Amplitude Modulated Carrier Wave Let vc = Vc Sin wct vm = Vm Sin wmt The amplitude modulated wave is given by the equation A = Vc + vm = Vc + Vm Sin wmt = Vc [1+ (Vm/Vc Sin wmt)] = Vc (1 + mSin wmt) m – Modulation Index. The ratio is Vm/Vc. v = A Sin wct = Vc (1 + m Sin wmt) Sin wct = Vc Sin wct + mVc (Sin wmt Sin wct) v = Vc Sin wct + [mVc/2 Cos (wc-wm)t – mVc/2 Cos (wc + wm)t] Frequency Spectrum of AM Lower side frequency – (wc – wm)/2 Upper side frequency – (wc +wm)/2 Modulation Index The ratio between the amplitude change of carrier wave to the amplitude of the normal carrier wave is called modulation index. Modulation Index m Em Em ax Em in or Ec Em ax Em in where, Emax = Ec + Em; Emin = Ec - Em (all peak values) When Em = Ec , m =1 or 100% modulation. Over-modulation, i.e. Em>Ec , should be avoided because it will create distortions and splatter. Effects of Modulation Index Effect of Over Modulation AM in Frequency Domain • The expression for the AM signal: es = (Ec + em) sin wct es = Ec sin wct + ½ mEc[cos (wc-wm)t-cos (wc+wm)t] • The expanded expression shows that the AM signal consists of the original carrier, a lower side frequency, flsf = fc - fm, and an upper side frequency, fusf = fc + fm. AM Spectrum Ec mEc/2 mEc/2 fm flsf fm fc fusf f fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2 Bandwidth, B = 2fm AM Power Ptotal = Pcarrier + PLSB + PUSB Considering additional resistance like antenna resistance R. Pcarrier = [(Vc/√2)/R]2 = V2C/2R Each side band has a value of m/2 Vc and r.m.s value of mVc/2√2. Hence power in LSB and USB can be written as PLSB = PUSB = (mVc/2√2)2/R = m2/4*V2C/2R = m2/4 Pcarrier Ptotal = V2C/2R + [m2/4*V2C/2R] + [m2/4*V2C/2R] = V2C/2R (1 + m2/2) = Pcarrier (1 + m2/2) PT Pc (1 m2 ) 2 AM Current • The modulation index for an AM station can be measured by using an RF ammeter and the following equation: I Io m2 1 2 where I is the current with modulation and Io is the current without modulation. Complex AM Waveforms • For complex AM signals with many frequency components, all the formulas encountered before remain the same, except that m is replaced by mT. For example: 2 PT mT PC (1 ); I 2 mT Io 1 2 2 Limitations of Amplitude Modulation • • • • Low Efficiency Limited Operating Range Noisy Reception Poor Audio Quality Block Diagram of AM TX Transmitter Stages • Crystal oscillator generates a very stable sinewave carrier. Where variable frequency operation is required, a frequency synthesizer is used. • Buffer isolates the crystal oscillator from any load changes in the modulator stage. • Frequency multiplier is required only if HF or higher frequencies is required. Transmitter Stages (cont’d) • RF voltage amplifier boosts the voltage level of the carrier. It could double as a modulator if low-level modulation is used. • RF driver supplies input power to later RF stages. • RF Power amplifier is where modulation is applied for most high power AM TX. This is known as high-level modulation. Transmitter Stages (cont’d) • High-level modulation is efficient since all previous RF stages can be operated class C. • Microphone is where the modulating signal is being applied. • AF amplifier boosts the weak input modulating signal. • AF driver and power amplifier would not be required for low-level modulation. Linear AM Modulator Circuits Square Law Diode Modulator Trapezoidal Pattern • Instead of using the envelope display to look at AM signals, an alternative is to use the trapezoidal pattern display. This is obtained by connecting the modulating signal to the x input of the ‘scope and the modulated AM signal to the y input. • Any distortion, over modulation, or non-linearity is easier to observe with this method. Trapezoidal Pattern (cont’d) m<1 m m=1 Vm ax Vm in Vm ax Vm in m>1 Improper -Vp>+Vp phase Amplitude Demodulators • Demodulators, or detectors, are circuits that accept modulated signals and recover the original modulating information. Diode Detector Waveforms of Diode Detector Double Side Band –Suppressed Carrier(DSB-SC) In this information is contained in two sidebands only and carrier is suppressed. Double Side Band –Suppressed Carrier(DSB-SC) Balanced Modulator Balanced Modulator • A balanced modulator is a circuit that generates a DSB signal, suppressing the carrier and leaving only the sum and difference frequencies at the output. • The output of a balanced modulator can be further processed by filters or phase-shifting circuitry to eliminate one of the sidebands, resulting in a SSB signal. Waveforms for Balanced Modulator V2, fm Vo V1, fc fc-fm fc+fm f Mathematical Analysis of Balanced Modulator • V1 = A1sin ct; V2 = A2sin mt • Vo = V1V2 = A1A2sin ct sin mt = ½A1A2{cos( c- m)t – cos( c+ m)t} • The equation above shows that the output of the balanced modulator consists of a lower side-frequency ( c - m) and an upper side-frequency ( c+ m) Suppressed-Carrier AM Systems • Full-carrier AM is simple but not efficient in terms of transmitted power, bandwidth, and SNR. • Using single-sideband suppressed-carrier (SSBSC or SSB) signals, since Psf = m2Pc/4, and Pt=Pc(1+m2/2 ), then at m=1, Pt= 6 Psf . • SSB also has a bandwidth reduction of half, which in turn reduces noise by half. Generating SSB - Filtering Method • The simplest method of generating an SSB signal is to generate a double-sideband suppressed-carrier (DSB-SC) signal first and then removing one of the sidebands. Balanced Modulator DSB-SC USB BPF AF Input Carrier Oscillator or LSB Filter for SSB • Filters with high Q are needed for suppressing the unwanted sideband. fa = f c - f2 fb = fc - f1 fd = fc + f1 fe = f c + f 2 Q f c anti log( X dB / 20) where X = attenuation of 4 f sideband, and f = fd - fb Typical SSB TX using Filter Method SSB Waveform Generating SSB - Phasing Method Phasing vs Filtering Method • Advantages of phasing method : No high Q filters are required. Therefore, lower fm can be used. SSB at any carrier frequency can be generated in a single step. Disadvantage: Difficult to achieve accurate 90o phase shift across the whole audio range. Peak Envelope Power • SSB transmitters are usually rated by the peak envelope power (PEP) rather than the carrier power. With voice modulation, the PEP is about 3 to 4 times the average or rms power. PEP Vp 2 2 RL where Vp = peak signal voltage and RL = load resistance Non-coherent SSB BFO RX Coherent SSB BFO Receiver RF SSBRC RF input signal RF amplifier and preselector IF SSBRC IF amp. & RF mixer bandpass filter RF LO Carrier recovery and frequency synthesizer IF mixer BFO Demod. info Explanation of SSB Receivers • The input SSB signal is first mixed with the LO signal (low-side injection is used here). • The filter removes the sum frequency components and the IF signal is amplified. • Mixing the IF signal with a reinserted carrier from a beat frequency oscillator (BFO) and low-pass filtering recovers the audio information. SSB Receivers (cont’d) • The product detector is often just a balanced modulator operated in reverse. • Frequency accuracy and stability of the BFO is critical. An error of a little more than 100 Hz could render the received signal unintelligible. • In coherent or synchronous detection, a pilot carrier is transmitted with the SSB signal to synchronize the RF local oscillator and BFO. Vestigial Side Band Vestigial Side Band Quadrature Amplitude Modulator AM Receivers • Basic requirements for receivers: Ability to tune to a specific signal Amplify the signal that is picked up Extract the information by demodulation Amplify the demodulated signal Two important receiver specifications: sensitivity and selectivity Tuned-Radio-Frequency (TRF) Receiver • The TRF receiver is the simplest receiver that meets all the basic requirements. Drawbacks of TRF Receivers • Difficulty in tuning all the stages to exactly the same frequency simultaneously. • Very high Q for the tuning coils are required for good selectivity BW=fo/Q. • Selectivity is not constant for a wide range of frequencies due to skin effect which causes the BW to vary with fo. Superheterodyne Receiver Block diagram of basic superhetrodyne receiver: Antenna and Front End • The antenna consists of an inductor in the form of a large number of turns of wire around a ferrite rod. The inductance forms part of the input tuning circuit. • Low-cost receivers sometimes omit the RF amplifier. • Main advantages of having RF amplifier: improves sensitivity and image frequency rejection. Mixers • A mixer is a nonlinear circuit that combines two signals in such a way as to produce the sum and difference of the two input frequencies at the output. • A square-law mixer is the simplest type of mixer and is easily approximated by using a diode, or a transistor (bipolar, JFET, or MOSFET). Dual-Gate MOSFET Mixer Good dynamic range and fewer unwanted o/p frequencies. Balanced Mixers • A balanced mixer is one in which the input frequencies do not appear at the output. Ideally, the only frequencies that are produced are the sum and difference of the input frequencies. Circuit symbol: f1 f1+ f2 f2 Equations for Balanced Mixer Let the inputs be v1 = sin w1t and v2 = sin w2t. A balanced mixer acts like a multiplier. Thus its output, vo = Av1v2 = A sin w1t sin w2t. Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)] Therefore, vo = A/2[cos(w1-w2)t-cos(w1+w2)t]. The last equation shows that the output of the balanced mixer consists of the sum and difference of the input frequencies. Balanced Ring Diode Mixer Balanced mixers are also called balanced modulators. Waveforms for Frequency Multipliers Mixer and Local Oscillator • The mixer and LO frequency convert the input frequency, fc, to a fixed fIF: High-side injection: fLO = fc + fIF Autodyne Converter • Sometimes called a self-excited mixer, the autodyne converter combines the mixer and LO into a single circuit: IF Amplifier, Detector, & AGC IF Amplifier and AGC • Most receivers have two or more IF stages to provide the bulk of their gain (i.e. sensitivity) and their selectivity. • Automatic gain control (AGC) is obtained from the detector stage to adjusts the gain of the IF (and sometimes the RF) stages inversely to the input signal level. This enables the receiver to cope with large variations in input signal. Sensitivity and Selectivity • Sensitivity is expressed as the minimum input signal required to produce a specified output level for a given (S+N)/N ratio. • Selectivity is the ability of the receiver to reject unwanted or interfering signals. It may be defined by the shape factor of the IF filter or by the amount of adjacent channel rejection. Shape Factor SF B 60 dB B 6 dB Image Frequency • One of the problems with the superhet receiver is that an image frequency signal could interfere with the reception of the desired signal. The image frequency is given by: fimage = fsig + 2fIF where fsig = desired signal • An image signal must be rejected by tuning circuits prior to mixing. Image-Frequency Rejection Ratio • For a tuned circuit with a quality factor of Q, its imagefrequency rejection ratio is: IFRR x 2 1 Q x 2 f image f sig f sig f image where, In dB, IFRR(dB) = 20 log IFRR IF Transformers • The transformers used in the IF stages can be either single-tuned or double-tuned. Single-tuned Double-tuned Loose and Tight Couplings • For single-tuned transformers, tighter coupling means more gain but broader bandwidth: Under, Over, & Critical Coupling • Double-tuned transformers can be over, under, critically, or optimally coupled: Coupling Factors • Critical coupling factor kc is given by: kc 1 Q pQs where Qp, Qs = prim. & sec. Q, respectively. IF transformers often use the optimum coupling factor, kopt = 1.5kc , to obtain a steep skirt and flat passband. The bandwidth for a double-tuned IF amplifier with k = kopt is given by B = kfo. Overcoupling means k>kc; undercoupling, k< kc