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Transcript

ANALOGUE TELECOMMUNICATIONS 1 MAIN TOPICS (Part I) 1) 2) 3) 4) 5) 6) Introduction to Communication Systems Filter Circuits Signal Generation Amplitude Modulation AM Receivers AM Transmitters 2 MAIN TOPICS (Part II) 7) 8) 9) 10) 11) Single-Sideband Communications Systems Angle Modulation Transmission Angle Modulated Receivers & Systems Introduction To Transmission Lines & Antennas Mobile Telecommunications 3 Elements of a Communication System • Communication involves the transfer of information or intelligence from a source to a recipient via a channel or medium. • Basic block diagram of a communication system: Source Transmitter Receiver Recipient 4 Brief Description • Source: analogue or digital • Transmitter: transducer, amplifier, modulator, oscillator, power amp., antenna • Channel: e.g. cable, optical fibre, free space • Receiver: antenna, amplifier, demodulator, oscillator, power amplifier, transducer • Recipient: e.g. person, speaker, computer 5 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 • Types of analogue modulation: AM, FM, and PM • Types of digital modulation: ASK, FSK, PSK, and QAM 6 Frequency Bands BAND Hz ELF 30 - 300 AF 300 - 3 k VLF 3 k - 30 k LF 30 k - 300 k MF 300 k - 3 M HF 3 M - 30 M BAND Hz VHF 30M-300M UHF 300M - 3 G SHF 3 G - 30 G EHF 30 G - 300G •Wavelength, l = c/f 7 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 8 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 9 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 10 11 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) 12 Effect of Filtering • Theoretically, a non-sinusoidal signal would require an infinite bandwidth; but practical considerations would band-limit the signal. • Channels with too narrow a bandwidth would remove a significant number of frequency components, thus causing distortions in the timedomain. A square-wave has only odd harmonics 13 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). 14 Dual-Gate MOSFET Mixer Good dynamic range and fewer unwanted o/p frequencies. 15 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 16 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. 17 Balanced Ring Diode Mixer Balanced mixers are also called balanced modulators. 18 External Noise • Equipment / Man-made Noise is generated by any equipment that operates with electricity • Atmospheric Noise is often caused by lightning • Space or Extraterrestrial Noise is strongest from the sun and, at a much lesser degree, from other stars 19 Internal Noise • Thermal Noise is produced by the random motion of electrons in a conductor due to heat. Noise power, PN = kTB where T = absolute temperature in oK k = Boltzmann’s constant, 1.38x10-23 J/oK B = noise power bandwidth in Hz Noise voltage, VN 4kTBR 20 Internal Noise (cont’d) • Shot Noise is due to random variations in current flow in active devices. • Partition Noise occurs only in devices where a single current separates into two or more paths, e.g. bipolar transistor. • Excess Noise is believed to be caused by variations in carrier density in components. • Transit-Time Noise occurs only at high f. 21 Noise Spectrum of Electronic Devices Device Noise Transit-Time or High-Frequency Effect Noise Excess or Flicker Noise Shot and Thermal Noises 1 kHz fhc f 22 Signal-to-Noise Ratio • An important measure in communications is the signal-to-noise ratio (SNR or S/N). It is often expressed in dB: PS VS S (dB) 10 log 20 log N PN VN In FM receivers, SINAD = (S+N+D)/(N+D) is usually used instead of SNR. 23 Noise Figure • Noise Factor is a figure of merit that indicates how much a component, or a stage degrades the SNR of a system: F = (S/N)i / (S/N)o where (S/N)i = input SNR (not in dB) and (S/N)o = output SNR (not in dB) • Noise Figure is the Noise Factor in dB: NF(dB)=10 log F = (S/N)i (dB) - (S/N)o (dB) 24 Equivalent Noise Temperature and Cascaded Stages • The equivalent noise temperature is very useful in microwave and satellite receivers. Teq = (F - 1)To where To is a ref. temperature (often 290 oK) • When two or more stages are cascaded, the total noise factor is: F2 1 F3 1 FT F1 + + + ... A1 A1A 2 25 High-Frequency Effects • Stray reactances of components (including the traces on a circuit board) can result in parasitic oscillations / self resonance and other unexpected effects in RF circuits. • Care must be given to the layout of components, wiring, ground plane, shielding and the use of bypassing or decoupling circuits. 26 Radio-Frequency Amplifiers 27 Narrow-band RF Amplifiers • Many RF amplifiers use resonant circuits to limit their bandwidth. This is to filter off noise and interference and to increase the amplifier’s gain. • The resonant frequency (fo) , bandwidth (B), and quality factor (Q), of a parallel resonant circuit are: fo RL fo ; B ; Q Q XL 2 LC 1 28 Narrowband Amplifier (cont’d) • In the CE amplifier, both the input and output sections are transformer-coupled to reduce the Miller effect. They are tapped for impedance matching purpose. RC and C2 decouple the RF from the dc supply. • The CB amplifier is quite commonly used at RF because it provides high voltage gain and also avoids the Miller effect by turning the collector-tobase junction capacitance into a part of the output tuning capacitance. 29 Wideband RF Amplifiers • Wideband / broadband amplifiers are frequently used for amplifying baseband or intermediate frequency (IF) signals. • The circuits are similar to those for narrowband amplifiers except no tuning circuits are employed. • Another method of designing wideband amplifiers is by stagger-tuning. 30 Stagger-Tuned IF Amplifiers 31 Amplifier Classes An amplifier is classified as: • Class A if it conducts current throughout the full input cycle (i.e. 360o). It operates linearly but is very inefficient - about 25%. • Class B if it conducts for half the input cycle. It is quite efficient (about 60%) but would create high distortions unless operated in a push-pull configuration. 32 Class B Push-Pull RF Amplifier 33 Class C Amplifier • Class C amplifier operates for less than half of the input cycle. It’s efficiency is about 75% because the active device is biased beyond cutoff. • It is commonly used in RF circuits where a resonant circuit must be placed at the output in order to keep the sine wave going during the non-conducting portion of the input cycle. 34 Class C Amplifier (cont’d) 35 Frequency Multipliers One of the applications of class C amplifiers is in “frequency multiplication”. The basic block diagram of a frequency multiplier: Input fi High Distortion Device + Amplifier Tuning Filter Circuit Output N x fi 36 Principle of Frequency Multipliers • A class C amplifier is used as the high distortion device. Its output is very rich in harmonics. • A filter circuit at the output of the class C amplifier is tuned to the second or higher harmonic of the fundamental component. • Tuning to the 2nd harmonic doubles fi ; tuning to the 3rd harmonic triples fi ; etc. 37 Waveforms for Frequency Multipliers 38 Neutralization • At very high frequencies, the junction capacitance of a transistor could introduce sufficient feedback from output to input to cause unwanted oscillations to take place in an amplifier. • Neutralization is used to cancel the oscillations by feeding back a portion of the output that has the opposite phase but same amplitude as the unwanted feedback. 39 Hazeltine Neutralization 40 Review of Filter Types & Responses • • • • 4 major types of filters: low-pass, high-pass, band pass, and bandreject or band-stop 0 dB attenuation in the passband (usually) 3 dB attenuation at the critical or cutoff frequency, fc (for Butterworth filter) Roll-off at 20 dB/dec (or 6 dB/oct) per pole outside the passband (# of poles = # of reactive elements). Attenuation at any frequency, f, is: f atten. (dB) at f log x atten. (dB) at f dec fc 41 Review of Filters (cont’d) • Bandwidth of a filter: BW = fcu - fcl • Phase shift: 45o/pole at fc; 90o/pole at >> fc • 4 types of filter responses are commonly used: – Butterworth - maximally flat in passband; highly non-linear phase response with frequecny – Bessel - gentle roll-off; linear phase shift with freq. – Chebyshev - steep initial roll-off with ripples in passband – Cauer (or elliptic) - steepest roll-off of the four types but has ripples in the passband and in the stopband 42 Low-Pass Filter Response Gain (dB) BW = fc Vo 0 Ideal -20 1 -40 0.707 Passband BW 0 -60 fc Basic LPF response f fc 10fc 100fc 1000fc f LPF with different roll-off rates 43 High-Pass Filter Response Gain (dB) 0 Vo -20 1 -40 0.707 0 Passband fc Basic HPF response -60 f 0.01fc 0.1fc fc f HPF with different roll-off rates 44 Band-Pass Filter Response Centre frequency: Vout 1 0.707 BW fc1 fo fc2 BW = fc2 - fc1 fo f c1 f c 2 Quality factor: Q f o BW Q is an indication of the selectivity of a BPF. Narrow BPF: Q > 10. Wide-band BPF: Q < 10. f Damping Factor: DF 1 Q 45 Band-Stop Filter Response • Also known as band-reject, or notch filter. • Frequencies within a certain BW are rejected. • Useful for filtering interfering signals. Gain (dB) 0 -3 Pass band Passband fc1 fo fc2 f BW 46 Filter Response Characteristics Av Chebyshev Bessel Butterworth f 47 Damping Factor Vin Frequency selective RC circuit Vout + _ R1 R2 General diagram of active filter The damping factor (DF) of an active filter sets the response characteristic of the filter. R1 DF 2 R2 Its value depends on the order (# of poles) of the filter. (See Table on next slide for DF values.) 48 Values For Butterworth Response Order 1st Stage Poles DF 1 1 optional 2 2 1.414 3 2 4 2 2nd Stage Poles DF 1 1 1 1.848 2 0.765 49 Active Filters • Advantages over passive LC filters: – Op-amp provides gain – high Zin and low Zout mean good isolation from source or load effects – less bulky and less expensive than inductors when dealing with low frequency – easy to adjust over a wide frequency range without altering desired response • Disadvantage: requires dc power supply, and could be limited by frequency response of op-amp. 50 Single-pole Active LPF R Vin C + _ Vout R1 R2 1 fc 2 RC R1 Acl 1 + R2 Roll-off rate for a single-pole filter is -20 dB/decade. Acl is selectable since DF is optional for single-pole LPF 51 Sallen-Key Low-Pass Filter CA RA Selecting RA = RB = R, and CA = CB = C : RB Vin CB + _ Vout R1 Sallen-Key or VCVS (voltage-controlled voltage-source) secondorder low-pass filter R2 1 fc 2 RC The roll-off rate for a two-pole filter is -40 dB/decade. For a Butterworth 2ndorder response, DF = 1.414; therefore, R1/R2 = 0.586. 52 Cascaded Low-Pass Filter CA1 RA1 RB1 Vin CB1 2 poles + _ Roll-off rate: -60 dB/dec RA2 CA2 R1 R2 + _ Vout R3 1 pole R4 Third-order (3-pole) configuration 53 Single-Pole High-Pass Filter C Vin R + _ Vout R1 • Roll-off rate, and formulas for fc , and Acl are similar to those for LPF. • Ideally, a HPF passes all frequencies above fc. However, the op-amp has an upper-frequency limit. R2 54 Sallen-Key High-Pass Filter RA CA CB Vin RB + _ Vout R1 Basic Sallen-Key second-order HPF R2 Again, formulas and roll-off rate are similar to those for 2nd-order LPF. To obtain higher rolloff rates, HPF filters can be cascaded. 55 BPF Using HPF and LPF CA1 Vin RA1 RA2 + _ R1 Av (dB) CA2 + _ Vout R3 R2 R4 0 -3 HP response LP response fc1 fo fc2 f 56 More On Bandpass Filter If BW and fo are given, then: f c1 BW 2 BW 2 + fo ; fc2 4 2 BW 2 BW 2 + fo + 4 2 A 2nd order BPF obtained by combining a LPF and a HPF: BiFET op-amp has FETs at input stage and BJTs at output stage. 57 Notes On Cascading HPF & LPF • Cascading a HPF and a LPF to yield a band-pass filter can be done as long as fc1 and fc2 are sufficiently separated. Hence the resulting bandwidth is relatively wide. • Note that fc1 is the critical frequency for the HPF and fc2 is for the LPF. • Another BPF configuration is the multiple-feedback BPF which has a narrower bandwidth and needing fewer components 58 Multiple-Feedback BPF C1 R1 C2 Making C1 = C2 = C, R2 _ Vin R3 Vout 1 fo 2 C R1 + R3 R1 R2 R3 Q = fo/BW + Max. gain: Q Q ; R2 2 f oCAo f oC R2 Ao Q 2R1 R3 2 2 f oC (2Q Ao ) 2 R1 R1, C1 - LP section R2, C2 - HP section Ao < 2Q 59 Broadband Band-Reject Filter A LPF and a HPF can also be combined to give a broadband BRF: 2-pole band-reject filter 60 Narrow-band Band-Reject Filter Easily obtained by combining the inverting output of a narrow-band BPF and the original signal: The equations for R1, R2, R3, C1, and C2 are the same as for BPF. RI = RF for unity gain and is often chosen to be >> R1. 61 Multiple-Feedback Band-Stop Filter C1 R1 Vin C2 R2 _ Vout + R3 R4 When C1 = C2 =C 1 fo 2 C R1 R2 The multiple-feedback BSF is very similar to its BP counterpart. For frequencies between fc1 and fc2 the op-amp will treat Vin as a pair of common-mode signals thus rejecting them accordingly. 62 Filter Response Measurements • Discrete Point Measurement: Feed a sine wave to the filter input with a varying frequency but a constant voltage and measure the output voltage at each frequency point. • A faster way is to use the swept frequency method: Sweep Generator Filter Spectrum analyzer The sweep generator outputs a sine wave whose frequency increases linearly between two preset limits. 63 Signal Generation - Oscillators • Barkhausen criteria for sustained oscillations: The closed-loop gain, |BAV| = 1. The loop phase shift = 0o or some integer multiple of 360o at the operating frequency. Output AV AV = open-loop gain B = feedback factor/fraction B 64 Basic Wien-Bridge Oscillator Voltage Divider R1 R1 _ R2 R3 + C1 R4 Vout Lead-lag C2 circuit R2 R3 R4 C1 _ + Vout C2 Two forms of the same circuit 65 Notes on Wien-Bridge Oscillator • • • At the resonant frequency the lead-lag circuit provides a positive feedback (purely resistive) with an attenuation of 1/3 when R3=R4=XC1=XC2. In order to oscillate, the non-inverting amplifier must have a closedloop gain of 3, which can be achieved by making R1 = 2R2 When R3 = R4 = R, and C1 = C2 = C, the resonant frequency is: 1 fr 2 RC 66 Phase-Shift Oscillator Rf _ C1 C2 C3 Vout + R1 R2 Each RC section provides 60o of phase shift. Total attenuation of the three-section RC feedback, B = 1/29. R3 Acl Rf R3 29 Choosing R1 = R2 = R3 = R, C1 = C2 = C3 = C, the resonant frequency is: 1 fr 2 6 RC 67 Hartley Oscillators L1 + L2 B L1 1 fo ; LT L1 + L2 2 LT C1 L2 B L1 68 Colpitts Oscillator C1 1 C1C2 B ; fo ; CT C2 C1 + C2 2 LCT 69 Clapp Oscillator C2 1 B ; fo C2 + C3 2 LCT 1 CT 1 1 1 + + C2 C3 C4 The Clapp oscillator is a variation of the Colpitts circuit. C4 is added in series with L in the tank circuit. C2 and C3 are chosen large enough to “swamp” out the transistor’s junction capacitances for greater stability. C4 is often chosen to be << either C2 or C3, thus making C4 the frequency determining element, since CT = C4. 70 Voltage-Controlled Oscillator • VCOs are widely used in electronic circuits for AFC, PLL, frequency tuning, etc. • The basic principle is to vary the capacitance of a varactor diode in a resonant circuit by applying a reverse-biased voltage across the diode whose capacitance is approximately: Co CV 1+ 2Vb 71 72 Crystals • For high frequency stability in oscillators, a crystal (such as quartz) has to be used. • Quartz is a piezoelectric material: deforming it mechanically causes the crystal to generate a voltage, and applying a voltage to the crystal causes it to deform. • Externally, the crystal behaves like an electrical resonant circuit. 73 Packaging, symbol, and characteristic of crystals 74 Crystal-Controlled Oscillators Pierce Colpitts 75 IC Waveform Generation • There are a number of LIC waveform generators from EXAR: – – – – XR2206 monolithic function generator IC XR2207 monolithic VCO IC XR2209 monolithic VCO IC XR8038A precision waveform generator IC • Most of these ICs have sine, square, or triangle wave output. They can also provide AM, FM, or FSK waveforms. 76 Phase-Locked Loop • The PLL is the basis of practically all modern frequency synthesizer design. • The block diagram of a simple PLL: fr Phase Detector Vp LPF Loop Amplifier VCO fo •Examples of a PLL I.C.: XR215, LM565, and CD4046 77 Operation of PLL Initially, the PLL is unlocked, i.e.,the VCO is at the free-running frequency, fo. Since fo is probably not the same as the reference frequency, fr , the phase detector will generate an error/control voltage, Vp. Vp is filtered, amplified, and applied to the VCO to change its frequency so that fo = fr. The PLL will then remain in phase lock. 78 PLL Frequency Specifications There is a limit on how far apart the free-running VCO frequency and the reference frequency can be for lock to be acquired or maintained. Lock Range Capture Range Free-Running Frequency fLL fLC fo fHC fHL f 79 Basic PLL Frequency Synthesizer fr Phase comparator fc = fout/N LPF VCO fout = Nfr N For output frequencies in the VHF range and higher, a prescaler is required. The prescaler is a fixed divider placed ahead of the programmable divide by N counter. 80 Frequency Synthesizer Using Prescaling fr Phase comparator N LPF VCO fout =(NP+M)fr Prescaler P or (P+1) M 2-modulus prescaler divides by P+1 when M counter is non zero; it divides by P when M counter reaches zero. N counter counts down (N-M) times. E.g. of I.C. prescaler: LMX5080 for UHF operation. 81 AM Waveform ec = Ec sin wct em = Em sin wmt AM signal: es = (Ec + em) sin wct 82 Modulation Index • The amount of amplitude modulation in a signal is given by its modulation index: Em Emax Emin m or Ec Emax + Emin where, Emax = Ec + Em; Emin = Ec - Em (all pk 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. 83 Effects of Modulation Index m=1 m>1 In a practical AM system, it usually contains many frequency components. When this is the case, mT m12 + m22 + ... + mn2 84 AM in Frequency Domain • The expression for the AM signal: es = (Ec + em) sin wct can be expanded to: 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. 85 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 86 AM Power • Total average (i.e. rms) power of the AM signal is: PT = Pc + 2Psf , where Pc = carrier power; and Psf = side-frequency power • If the signal is across a load resistor, R, then: Pc = Ec2/(2R); and Psf = m2Pc/4. So, m2 PT Pc (1 + ) 2 87 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. 88 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 mT mT PT PC (1 + ); I I o 1 + 2 2 2 89 Block Diagram of AM TX 90 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. 91 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 highlevel modulation. 92 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. 93 AM Modulator Circuits 94 Impedance Matching Networks • Impedance matching networks at the output of RF circuits are necessary for efficient transfer of power. At the same time, they serve as low-pass filters. Pi network T network 95 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, overmodulation, or non-linearity is easier to observe with this method. 96 Trapezoidal Pattern (cont’d) m<1 m=1 Vmax Vmin m Vmax + Vmin m>1 Improper -Vp>+Vp phase 97 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 98 Tuned-Radio-Frequency (TRF) Receiver • The TRF receiver is the simplest receiver that meets all the basic requirements. 99 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. 100 Superheterodyne Receiver Block diagram of basic superhet receiver: 101 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. 102 Mixer and Local Oscillator • The mixer and LO frequency convert the input frequency, fc, to a fixed fIF: High-side injection: fLO = fc + fIF 103 Autodyne Converter • Sometimes called a self-excited mixer, the autodyne converter combines the mixer and LO into a single circuit: 104 IF Amplifier, Detector, & AGC 105 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. 106 Diode Detector Waveforms 107 Diagonal Clipping Distortion Diagonal clipping distortion is more pronounced at high modulation index or high modulation frequency. 108 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. 109 Shape Factor B60dB SF B6 dB 110 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. 111 Image-Frequency Rejection Ratio • For a tuned circuit with a quality factor of Q, its image-frequency rejection ratio is: IFRR 1 + Q x 2 x f image f sig 2 where, f sig f image In dB, IFRR(dB) = 20 log IFRR 112 IF Transformers • The transformers used in the IF stages can be either single-tuned or double-tuned. Single-tuned Double-tuned 113 Loose and Tight Couplings • For single-tuned transformers, tighter coupling means more gain but broader bandwidth: 114 Under, Over, & Critical Coupling • Double-tuned transformers can be over, under, critically, or optimally coupled: 115 Coupling Factors • Critical coupling factor kc is given by: 1 kc Q p Qs 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 116 Piezoelectric Filters • For narrow bandwidth (e.g. several kHz), excellent shape factor and stability, a crystal lattice is used as bandpass filter. • Ceramic filters, because of their lower Q, are useful for wideband signals (e.g. FM broadcast). • Surface-acoustic-wave (SAW) filters are ideal for high frequency usage requiring a carefully shaped response. 117 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. 118 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 119 Waveforms for Balanced Modulator V2, fm Vo V1, fc fc-fm fc+fm f 120 Mathematical Analysis of Balanced Modulator • V1 = A1sin wct; V2 = A2sin wmt • Vo = V1V2 = A1A2sin wct sin wmt = ½A1A2{cos(wc- wm)t – cos(wc+ wm)t} • The equation above shows that the output of the balanced modulator consists of a lower sidefrequency (wc - wm) and an upper side-frequency (wc+ wm) 121 LIC Balanced Modulator 1496 122 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 f c anti log( X dB / 20) where X = attenuation of Q 4f sideband, and f = fd - fb 123 Typical SSB TX using Filter Method 124 SSB Waveform 125 Generating SSB - Phasing Method • This method is based on the fact that the lsf and the usf are given by the equations: cos {(wc - wm)t} = ½(cos wct cos wmt + sin wct sin wmt) cos {(wc + wm)t} = ½(cos wct cos wmt - sin wct sin wmt) • The RHS of the 1st equation is just the sum of two products: the product of the carrier and the modulating signal, and the product of the same two signals that have been phase shifted by 90o. • The 2nd equation is similar except for the (-) sign. 126 Diagram for Phasing Method Modulating signal Em cos wmt Balanced Modulator 1 Carrier oscillator 90o phase shifter Ec cos wct 90o phase shifter + SSB output Balanced Modulator 2 127 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. 128 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 129 Non-coherent SSB BFO RX 130 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 Demod. info BFO 131 Notes On 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. 132 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. 133 Angle Modulation Angle modulation includes both frequency and phase modulation. FM is used for: radio broadcasting, sound signal in TV, two-way fixed and mobile radio systems, cellular telephone systems, and satellite communications. PM is used extensively in data communications and for indirect FM. 134 Comparison of FM or PM with AM Advantages over AM: 1) 2) 3) better SNR, and more resistant to noise efficient - class C amplifier can be used, and less power is required to angle modulate capture effect reduces mutual interference Disadvantages: 1) 2) much wider bandwidth is required slightly more complex circuitry is needed 135 Frequency Shift Keying (FSK) Carrier Modulating signal FSK signal 136 FSK (cont’d) • The frequency of the FSK signal changes abruptly from one that is higher than that of the carrier to one that is lower. • Note that the amplitude of the FSK signal remains constant. • FSK can be used for transmission of digital data (1’s and 0’s) with slow speed modems. 137 Frequency Modulation Carrier Modulating Signal FM signal 138 Frequency Modulation (cont’d) • Note the continuous change in frequency of the FM wave when the modulating signal is a sine wave. • In particular, the frequency of the FM wave is maximum when the modulating signal is at its positive peak and is minimum when the modulating signal is at its negative peak. 139 Frequency Deviation • The amount by which the frequency of the FM signal varies with respect to its resting value (fc) is known as frequency deviation: f = kf em, where kf is a system constant, and em is the instantaneous value of the modulating signal amplitude. • Thus the frequency of the FM signal is: fs (t) = fc + f = fc + kf em(t) 140 Maximum or Peak Frequency Deviation • If the modulating signal is a sine wave, i.e., em(t) = Emsin wmt, then fs = fc + kfEmsin wmt. • The peak or maximum frequency deviation: d = kf Em • The modulation index of an FM signal is: mf = d / fm • Note that mf can be greater than 1. 141 Relationship between FM and PM • For PM, phase deviation, f = kpem, and the peak phase deviation, fmax = mp = mf. • Since frequency (in rad/s) is given by: d (t ) w (t ) dt or (t ) w (t )dt the above equations suggest that FM can be obtained by first integrating the modulating signal, then applying it to a phase modulator. 142 Equation for FM Signal • If ec = Ec sin wct, and em = Em sin wmt, then the equation for the FM signal is: es = Ec sin (wct + mf sin wmt) • This signal can be expressed as a series of sinusoids: es = Ec{Jo(mf) sin wct - J1(mf)[sin (wc - wm)t - sin (wc + wm)t] + J2(mf)[sin (wc - 2wm)t + sin (wc + 2wm)t] - J3(mf)[sin (wc - 3wm)t + sin (wc + 3wm)t] + … .} 143 Bessel Functions • The J’s in the equation are known as Bessel functions of the first kind: mf J o J1 J2 J3 J4 J5 J6 . . . 0 0.5 1 2.4 5.5 1 .94 .77 0.0 0.0 .24 .44 .52 -.34 .03 .11 .43 -.12 .02 .20 .26 .06 .40 .02 .32 .19 . . . 144 Notes on Bessel Functions • Theoretically, there is an infinite number of side frequencies for any mf other than 0. • However, only significant amplitudes, i.e. those |0.01| are included in the table. • Bessel-zero or carrier-null points occur when mf = 2.4, 5.5, 8.65, etc. These points are useful for determining the deviation and the value of kf of an FM modulator system. 145 Graph of Bessel Functions 146 FM Side-Bands • Each (J) value in the table gives rise to a pair of sidefrequencies. • The higher the value of mf, the more pairs of significant side- frequencies will be generated. 147 Power and Bandwidth of FM Signal • Regardless of mf , the total power of an FM signal remains constant because its amplitude is constant. • The required BW of an FM signal is: BW = 2 x n x fm ,where n is the number of pairs of side-frequencies. • If mf > 6, a good estimate of the BW is given by Carson’s rule: BW = 2(d + fm (max) ) 148 Narrowband & Wideband FM • FM systems with a bandwidth < 15 kHz, are considered to be NBFM. A more restricted definition is that their mf < 0.5. These systems are used for voice communication. • Other FM systems, such as FM broadcasting and satellite TV, with wider BW and/or higher mf are called WBFM. 149 Pre-emphasis • Most common analog signals have high frequency components that are relatively low in amplitude than low frequency ones. Ambient electrical noise is uniformly distributed. Therefore, the SNR for high frequency components is lower. • To correct the problem, em is pre-emphasized before frequency modulating ec. 150 Pre-emphasis circuit • In FM broadcasting, the high frequency components are boosted by passing the modulating signal through a HPF with a 75 ms time constant before modulation. t = R1C = 75 ms. 151 De-emphasis Circuit • At the FM receiver, the signal after demodulation must be de-emphasized by a filter with similar characteristics as the preemphasis filter to restore the relative amplitudes of the modulating signal. 152 FM Stereo Broadcasting: Baseband Spectra • To maintain compatibility with monaural system, FM stereo uses a form of FDM or frequency-division multiplexing to combine the left and right channel information: 19 kHz Pilot Carrier L+R (mono) .05 15 23 L-R L-R 38 SCA (optional) 53 60 67 74 kHz 153 FM Stereo Broadcasting • To enable the L and R channels to be reproduced at the receiver, the L-R and L+R signals are required. These are sent as a DSBSC AM signal with a suppressed subcarrier at 38 kHz. • The purpose of the 19 kHz pilot is for proper detection of the DSBSC AM signal. • The optional Subsidiary Carrier Authorization (SCA) signal is normally used for services such as background music for stores and offices. 154 Block Diagram of FM Transmitter FM Modulator Frequency Multiplier(s) Buffer Pre-emphasis Antenna Driver Power Amp Audio 155 Direct-FM Modulator • A simple method of generating FM is to use a reactance modulator where a varactor is put in the frequency determining circuit. 156 Crosby AFC System • An LC oscillator operated as a VCO with automatic frequency control is known as the Crosby system. 157 Phase-Locked Loop FM Generators • The PLL system is more stable than the Crosby system and can produce wide-band FM without using frequency multipliers. 158 Indirect-FM Modulators • Recall earlier that FM and PM were shown to be closely related. In fact, FM can be produced using a phase modulator if the modulating signal is passed through a suitable LPF (i.e. an integrator) before it reaches the modulator. • One reason for using indirect FM is that it’s easier to change the phase than the frequency of a crystal oscillator. However, the phase shift achievable is small, and frequency multipliers will be needed. 159 Example of Indirect FM Generator Armstrong Modulator 160 Block Diagram of FM Receiver 161 FM Receivers • FM receivers, like AM receivers, utilize the superheterodyne principle, but they operate at much higher frequencies (88 - 108 MHz). • A limiter is often used to ensure the received signal is constant in amplitude before it enters the discriminator or detector. The limiter operates like a class C amplifier when the input exceeds a threshold point. In modern receivers, the limiting function is built into the FM IF integrated circuit. 162 FM Demodulators • The FM demodulators must convert frequency variations of the input signal into amplitude variations at the output. • The Foster-Seeley discriminator and its variant, the ratio detector are commonly found in older receivers. They are based on the principle of slope detection using resonant circuits. 163 S-curve Characteristics of FM Detectors vo Em d fi fIF d 164 PLL FM Detector • PLL and quadrature detectors are commonly found in modern FM receivers. FM IF Signal Phase Detector f LPF Demodulated output VCO 165 Quadrature Detector • Both the quadrature and the PLL detector are conveniently found as IC packages. 166 Types of Transmission Lines • Differential or balanced lines (where neither conductor is grounded): e.g. twin lead, twisted-cable pair, and shielded-cable pair. • Single-ended or unbalanced lines (where one conductor is grounded): e.g. concentric or coaxial cable. • Transmission lines for microwave use: e.g. striplines, microstrips, and waveguides. 167 Transmission Line Equivalent Circuit R Zo C L G R C L G “Lossy” Line R + jwL Zo G + jwC L Zo C L C Lossless Line L Zo C 168 Notes on Transmission Line • Characteristics of a line is determined by its primary electrical constants or distributed parameters: R (/m), L (H/m), C (F/m), and G (S/m). • Characteristic impedance, Zo, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, ZL = Zo. 169 Formulas for Some Lines For parallel two-wire line: m 2D 120 2 D L ln ; C ; Zo ln 2D d d r ln D d d m = momr; = or; mo = 4x10-7 H/m; o = 8.854 pF/m D d For co-axial cable: m D 2 60 D L ln ; C ; Zo ln D 2 d r d ln d 170 Transmission-Line Wave Propagation Electromagnetic waves travel at < c in a transmission line because of the dielectric separating the conductors. The velocity of propagation is given by: v 1 1 c LC m r m/s Velocity factor, VF, is defined as: VF v 1 c r 171 Propagation Constant • Propagation constant, , determines the variation of V or I with distance along the line: V = Vse-x; I = Isex, where V , and I are the voltage and current at the S S source end, and x = distance from source. • = + j, where = attenuation coefficient (= 0 for lossless line), and = phase shift coefficient = 2/l (rad./m) 172 Incident & Reflected Waves • For an infinitely long line or a line terminated with a matched load, no incident power is reflected. The line is called a flat or nonresonant line. • For a finite line with no matching termination, part or all of the incident voltage and current will be reflected. 173 Reflection Coefficient The reflection coefficient is defined as: Er Ei or It can also be shown that: Ir Ii Z L Zo f Z L + Zo Note that when ZL = Zo, = 0; when ZL = 0, = -1; and when ZL = open circuit, = 1. 174 Voltage Standing Waves Vmax = Ei + Er l 2 Vmin = Ei - Er With a mismatched line, the incident and reflected waves set up an interference pattern on the line known as a standing wave. Vmax 1 + The standing wave ratio is : SWR V 1 min 175 Other Formulas When the load is purely resistive: (whichever gives an SWR > 1) Zo ZL SWR or Zo ZL Return Loss, RL = Fraction of power reflected = ||2, or -20 log || dB So, Pr = ||2Pi Mismatched Loss, ML = Fraction of power transmitted/absorbed = 1 - ||2 or -10 log(1-||2) dB So, Pt = Pi (1 - ||2) = Pi - Pr 176 Simple Antennas • An isotropic radiator would radiate all electrical power supplied to it equally in all directions. It is merely a theoretical concept but is useful as a reference for other antennas. • A more practical antenna is the half-wave dipole: l/2 Balanced Feedline Symbol 177 Half-Wave Dipole • Typically, the physical length of a half-wave dipole is 0.95 of l/2 in free space. • Since power fed to the antenna is radiated into space, there is an equivalent radiation resistance, Rr. For a real antenna, losses in the antenna can be represented by a loss resistance, Rd. Its efficiency is then: Pr Rr PT Rr + Rd 178 3-D Antenna Radiation Pattern 179 Gain and Directivity • Antennas are designed to focus their radiation into lobes or beams thus providing gain in selected directions at the expense of energy reductions in others. • The ideal l/2 dipole has a gain of 2.14 dBi (i.e. dB with respect to an isotropic radiator) • Directivity is the gain calculated assuming a lossless antenna 180 EIRP and Effective Area • When power, PT, is applied to an antenna with a gain GT (with respect to an isotropic radiator), then the antenna is said to have an effective isotropic radiated power, EIRP = PTGT. • The signal power delivered to a receiving antenna with a gain GR is PR = PDAeff where PD is the power density, and Aeff is the effective area. EIRP l2GR PD ; Aeff 2 4r 4 181 Impedance and Polarization • A half-wave dipole in free space and centre-fed has a radiation resistance of about 70 . • At resonance, the antenna’s impedance will be completely resistive and its efficiency maximum. If its length is < l/2, it becomes capacitive, and if > l/2, it is inductive. • The polarization of a half-wave dipole is the same as the axis of the conductor. 182 Ground Effects • Ground effects on antenna pattern and resistance are complex and significant for heights less than one wavelength. This is particularly true for antennas operating at HF range and below. • Generally, a horizontally polarized antenna is affected more by near ground reflections than a vertically polarized antenna. 183 Folded Dipole • Often used - alone or with other elements - for TV and FM broadcast receiving antennas because it has a wider bandwidth and four times the feedpoint resistance of a single dipole. 184 Monopole or Marconi Antenna Main characteristics: vertical and l/4 good ground plane is required omnidirectional in the horizontal plane 3 dBd power gain impedance: about 36 185 Loop Antennas Main characteristics: very small dimensions bidirectional greatest sensitivity in the plane of the loop very wide bandwidth efficient as RX antenna with single or multi-turn loop 186 Antenna Matching • Antennas should be matched to their feedline for maximum power transfer efficiency by using an LC matching network. • A simple but effective technique for matching a short vertical antenna to a feedline is to increase its electrical length by adding an inductance at its base. This inductance, called a loading coil, cancels the capacitive effect of the antenna. • Another method is to use capacitive loading. 187 Inductive and Capacitive Loading Inductive Loading Capacitive Loading 188 Collinear Array all elements lie along a straight line, fed in phase, and often mounted with main axis vertical result in narrow radiation beam omnidirectional in the horizontal plane 189 2-Way Mobile Communications • 1) Mobile radio, half-duplex, one-to-many, no dial tone: – e.g. CB, amateur (ham) radio, aeronautical, maritime, public safety, emergency, and industrial radios • 2) Mobile Telephone, Full-duplex, one-to-one: – Analogue cellular (AMPS) using FDMA or TDMA – Digital cellular (PCS) using TDMA, FDMA, and CDMA – Personal communications satellite service (PCSS) using both FDMA and TDMA 190 Mobile Telephone Systems • Mobile telephone began in the early 1980s first as the MTS (Mobile Telephone Service) at 40 MHz and later as the IMTS (Improved MTS) at 150 and 450 MHz. • Narrowband FM and relatively high transmit power were used. • Limited channels (total of only 33) and interference were problems. 191 Advanced Mobile Phone System • AMPS divide area into cells with low power transmitters in each cell. • Max. 4 W ERP for mobile radios; max. 600 mW for portable phones; to reduce interference min. power needed for communications is used at all times. • Base station: 869.040 – 893.970 MHz; mobile unit’s frequency is 45 MHz below. • Total of 790 duplex voice channels and 42 control channels available at 30 kHz each. • Channels are divided in 7- or 12-cell repeated pattern and frequencies are reused 192 Block Diagram Of Analogue Cell Phone Antenna Speaker RF amp Duplexer mixer Frequency synthesizer IF amp IF detector De-emphasis Audio amp Display Microprocessor Keypad Data RF power amp FM modulator Audio preamp & Pre-emphasis Mic 6 mW – 3W 193 7-Cell Pattern 6 5 4 1 3 3 7 2 5 6 1 4 • Each cell has a base station. • All cell sites in a region are tied to a mobile switching centre (MSC) or mobile telephone switching office (MTSO) which in turn is connected to other MSCs. In a real situation, the cells are more likely to be approximately circular, with some overlap. 194 Cellular Radio Network BSC: Base Station Controller MSC: Mobile Switching Centre BSC To other MSCs BSC MSC To other BSCs BSC BSC BSC Gateway MSC To Public Switched Telephone Network BSC BSC MSC BSC 195 Cell-Site Control • BSC assigns channels and power levels, transmitting signaling tones, etc. • MSC routes calls, authorizing calls, billing, initiating handoffs between cells, holds location and authentication registers, connects mobile units to the PSTN, etc. • Sometimes BSC and MSC are combined. • Cells can be subdivided into mini and micro cells to increase subscriber capacity in a region. 196 Digital Cellular Telephone • The United States Digital Cellular (USDC) system is backward compatible with the AMPS frequency allocation scheme but using digitized signals and PSK modulation. • It uses TDMA (Time-Division Multiple Access) to increase the number of subscribers threefold with the same 50-MHz frequency spectrum. • It provides higher security and better signal quality. • TDMA Service in the 1900 MHz band is also in use since there is no room in the 800 MHz band for expansion. 197 Code-Division Multiple-Access System • CDMA is a totally digital cellular telephone system. • It is more commonly found in the 1900 MHz PCS band with up to 11 CDMA RF channels. • Each CDMA RF channel has a bandwidth of 1.25 MHz, using a single carrier modulated by a 1.2288 Mb/s bitstream using QPSK. • Each RF channel can provide up to 64 traffic channels. • It uses a spread-spectrum technique so all frequencies can be used in all cells – soft handoff possible. • Each mobile is assigned a unique spreading sequence to reduce RF interference. 198 Global System For Mobile Communications • GSM uses frequency-division duplexing and a combination of TDMA and FDMA techniques. • Base station frequency: 935 MHz to 960 MHz; mobile frequency: 45 MHz below • 1800 MHz is allocated for PCS in Europe while North America utilizes the 1900 MHz band. • RF channel bandwidth is 200 kHz but each can hold 8 voice/data channels. 199 Personal Communications Satellite System • PCSS uses either low earth-orbit (LEO) or medium earth-orbit (MEO) satellites. • Advantages: can provide telephone services in remote and inaccessible areas quickly and economically. • Disadvantages: high risk due to high costs of designing, building and launching satellites; also high cost for terrestrial-based network and infrastructure. Mobile unit is more bulky and expensive than conventional cellular telephones. 200