2. 1 ANALOGUE TELECOMMUNICATIONS

3. 2 MAIN TOPICS (Part I) 1)Introduction to Communication Systems 2)Filter Circuits 3)Signal Generation 4)Amplitude Modulation 5)AM Receivers 6)AM Transmitters

4. 3 MAIN TOPICS (Part II) 7)Single-Sideband Communications Systems 8)Angle Modulation Transmission 9)Angle Modulated Receivers & Systems 10)Introduction To Transmission Lines & Antennas 11)Mobile Telecommunications

5. 4 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: SourceTransmitterReceiverRecipient

6. 5 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

7. 6 Modulation Modulation is the process of impressing information onto a high-frequency carrier for transmission. Reasons for modulation: –t–to prevent mutual interference between stations –t–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

8. 7 Frequency Bands BAND Hz  ELF30 - 300  AF300 - 3 k  VLF3 k - 30 k  LF30 k - 300 k  MF300 k - 3 M  HF3 M - 30 M BAND Hz  VHF30M-300M  UHF300M - 3 G  SHF3 G - 30 G  EHF30 G - 300G Wavelength, = c/f

9. 8 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 log 2 (1+ S/N) in bps where S/N = signal-to-noise power ratio

10. 9 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

11. 10 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

12. 11

13. 12 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)=C o +  A n cos  n  t  B n sin n  t (Fourier series)

14. 13 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 time- domain. HA square-wave has only odd harmonics

15. 14 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).

16. 15 Dual-Gate MOSFET Mixer Good dynamic range and fewer unwanted o/p frequencies.

17. 16 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: f1f1 f2f2 f 1 + f 2

18. 17 Equations for Balanced Mixer Let the inputs be v 1 = sin  1 t and v 2 = sin   t. A balanced mixer acts like a multiplier. Thus its output, v o = Av 1 v 2 = A sin  1 t sin  2 t. Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)] Therefore, v o = A/2[cos(  1 -  2 )t-cos(      t].  The last equation shows that the output of the balanced mixer consists of the sum and difference of the input frequencies.

19. 18 Balanced Ring Diode Mixer Balanced mixers are also called balanced modulators.

20. 19 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

21. 20 Internal Noise Thermal Noise is produced by the random motion of electrons in a conductor due to heat.Noise power, P N = kTB where T = absolute temperature in o K k = Boltzmann’s constant, 1.38x10 -23 J/ o K B = noise power bandwidth in Hz Noise voltage,

22. 21 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.

23. 22 Noise Spectrum of Electronic Devices Device Noise Shot and Thermal Noises Excess or Flicker Noise Transit-Time or High-Frequency Effect Noise 1 kHzf hc f

24. 23 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: In FM receivers, SINAD = (S+N+D)/(N+D) is usually used instead of SNR.

25. 24 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)

26. 25 Equivalent Noise Temperature and Cascaded Stages The equivalent noise temperature is very useful in microwave and satellite receivers. T eq = (F - 1)T o where T o is a ref. temperature (often 290 o K) When two or more stages are cascaded, the total noise factor is:

27. 26 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.

28. 27 Radio-Frequency Amplifiers

29. 28 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 (f o ), bandwidth (B), and quality factor (Q), of a parallel resonant circuit are:

30. 29 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. R C and C 2 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-to- base junction capacitance into a part of the output tuning capacitance.

31. 30 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.

32. 31 Stagger-Tuned IF Amplifiers

33. 32 Amplifier Classes An amplifier is classified as: Class A if it conducts current throughout the full input cycle (i.e. 360 o ). 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.

34. 33 Class B Push-Pull RF Amplifier

35. 34 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.

36. 35 Class C Amplifier (cont’d)

37. 36 Frequency Multipliers  One of the applications of class C amplifiers is in “frequency multiplication”. The basic block diagram of a frequency multiplier: High Distortion Device + Amplifier Tuning Filter Circuit Input fifi Output N x f i

38. 37 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 f i ; tuning to the 3rd harmonic triples f i ; etc.

39. 38 Waveforms for Frequency Multipliers

40. 39 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.

41. 40 Hazeltine Neutralization

42. 41 Review of Filter Types & Responses 4 major types of filters: low-pass, high-pass, band pass, and band- reject or band-stop 0 dB attenuation in the passband (usually) 3 dB attenuation at the critical or cutoff frequency, f c (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:

43. 42 Review of Filters (cont’d) Bandwidth of a filter: BW = f cu - f cl Phase shift: 45 o /pole at f c ; 90 o /pole at >> f c 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

44. 43 Low-Pass Filter Response VoVo fcfc f0 1 0.707 BW Gain (dB) 0 -20 -40 -60 fcfc 10f c 100f c 1000f c -20 dB/dec -40 dB/dec -60 dB/dec LPF with different roll-off ratesBasic LPF response f Ideal Passband BW = f c

45. 44 High-Pass Filter Response VoVo fcfc f 0 1 0.707 Gain (dB) 0 -20 -40 -60 0.01f c 0.1f c fcfc -20 dB/dec -40 dB/dec -60 dB/dec Passband Basic HPF responseHPF with different roll-off rates f

46. 45 Band-Pass Filter Response V out 1 0.707 f fofo f c1 f c2 BW BW = f c2 - f c1 Centre frequency: Quality factor: Q is an indication of the selectivity of a BPF. Narrow BPF: Q > 10. Wide-band BPF: Q < 10. Damping Factor:

47. 46 Gain (dB) 0 -3 f fofo f c1 f c2 BW Passband Pass band Band-Stop Filter Response Also known as band-reject, or notch filter. Frequencies within a certain BW are rejected. Useful for filtering interfering signals.

48. 47 Filter Response Characteristics AvAv f Chebyshev Butterworth Bessel

49. 48 Damping Factor Frequency selective RC circuit + _ V in V out R1R1 R2R2 General diagram of active filter The damping factor (DF) of an active filter sets the response characteristic of the filter. Its value depends on the order (# of poles) of the filter. (See Table on next slide for DF values.)

50. 49 Values For Butterworth Response Order 1 st Stage2 nd Stage Poles DF Poles DF 11 optional 221.414 32111 421.84820.765

51. 50 Active Filters Advantages over passive LC filters : – Op-amp provides gain – high Z in and low Z out 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.

52. 51 Single-pole Active LPF + _ V in V out R1R1 R2R2 R C Roll-off rate for a single-pole filter is -20 dB/decade. A cl is selectable since DF is optional for single-pole LPF

53. 52 Sallen-Key Low-Pass Filter + _ V in V out R1R1 R2R2 RARA RBRB CBCB CACA Sallen-Key or VCVS (voltage-controlled voltage-source) second- order low-pass filter Selecting R A = R B = R, and C A = C B = C : The roll-off rate for a two-pole filter is -40 dB/decade. For a Butterworth 2nd- order response, DF = 1.414; therefore, R 1 /R 2 = 0.586.

54. 53 Cascaded Low-Pass Filter + _ V in R1R1 R2R2 R A1 R B1 C B1 C A1 + _ R3R3 R4R4 R A2 C A2 Third-order (3-pole) configuration V out 2 poles1 pole Roll-off rate: -60 dB/dec

55. 54 Single-Pole High-Pass Filter + _ V in V out R1R1 R2R2 R C Roll-off rate, and formulas for f c, and A cl are similar to those for LPF. Ideally, a HPF passes all frequencies above f c. However, the op-amp has an upper-frequency limit.

56. 55 Sallen-Key High-Pass Filter + _ V in V out R1R1 R2R2 RARA RBRB CBCB CACA Again, formulas and roll-off rate are similar to those for 2nd-order LPF. To obtain higher roll- off rates, HPF filters can be cascaded. Basic Sallen-Key second-order HPF

57. 56 BPF Using HPF and LPF + _ V in R1R1 R2R2 R A1 C A1 + _ V out R3R3 R4R4 R A2 C A2 f A v (dB) 0 -3 fofo f c1 f c2 LP response HP response -20 dB/dec

58. 57 More On Bandpass Filter If BW and f o are given, then: A 2 nd order BPF obtained by combining a LPF and a HPF: BiFET op-amp has FETs at input stage and BJTs at output stage.

59. 58 Notes On Cascading HPF & LPF Cascading a HPF and a LPF to yield a band-pass filter can be done as long as f c1 and f c2 are sufficiently separated. Hence the resulting bandwidth is relatively wide. Note that f c1 is the critical frequency for the HPF and f c2 is for the LPF. Another BPF configuration is the multiple-feedback BPF which has a narrower bandwidth and needing fewer components

60. 59 Multiple-Feedback BPF + _ V in V out R3R3 R1R1 R2R2 C2C2 C1C1 Making C 1 = C 2 = C, Q = f o /BW A o < 2Q 2 Max. gain: R 1, C 1 - LP section R 2, C 2 - HP section

61. 60 Broadband Band-Reject Filter A LPF and a HPF can also be combined to give a broadband BRF: 2-pole band-reject filter

62. 61 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. R I = R F for unity gain and is often chosen to be >> R1.

63. 62 Multiple-Feedback Band-Stop Filter + _ V in V out R3R3 R1R1 R2R2 C2C2 C1C1 R4R4 The multiple-feedback BSF is very similar to its BP counterpart. For frequencies between f c1 and f c2 the op-amp will treat V in as a pair of common-mode signals thus rejecting them accordingly. When C 1 = C 2 =C

64. 63 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.

65. 64 Signal Generation - Oscillators Barkhausen criteria for sustained oscillations: ¬The closed-loop gain, |BA V | = 1. ­The loop phase shift = 0 o or some integer multiple of 360 o at the operating frequency. A V = open-loop gain B = feedback factor/fraction AVAV B Output

66. 65 Basic Wien-Bridge Oscillator _ + + _ C2C2 C1C1 R4R4 R3R3 R1R1 R2R2 Voltage Divider Lead-lag circuit V out R1R1 R4R4 R2R2 R3R3 C2C2 C1C1 Two forms of the same circuit

67. 66 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 R 3 =R 4 =X C1 =X C2. In order to oscillate, the non-inverting amplifier must have a closed- loop gain of 3, which can be achieved by making R 1 = 2R 2 When R 3 = R 4 = R, and C 1 = C 2 = C, the resonant frequency is:

68. 67 Phase-Shift Oscillator + _ RfRf C1C1 C2C2 C3C3 R1R1 R2R2 R3R3 V out Each RC section provides 60 o of phase shift. Total attenuation of the three-section RC feedback, B = 1/29. Choosing R 1 = R 2 = R 3 = R, C 1 = C 2 = C 3 = C, the resonant frequency is:

69. 68 Hartley Oscillators

70. 69 Colpitts Oscillator

71. 70 Clapp Oscillator The Clapp oscillator is a variation of the Colpitts circuit. C 4 is added in series with L in the tank circuit. C 2 and C 3 are chosen large enough to “swamp” out the transistor’s junction capacitances for greater stability. C 4 is often chosen to be << either C 2 or C 3, thus making C 4 the frequency determining element, since C T = C 4.

72. 71 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:

73. 72

74. 73 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.

75. 74 Packaging, symbol, and characteristic of crystals

76. 75 Crystal-Controlled Oscillators Pierce Colpitts

77. 76 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.

78. 77 Phase-Locked Loop The PLL is the basis of practically all modern frequency synthesizer design. The block diagram of a simple PLL: Phase Detector LPF Loop Amplifier VCO frfr fofo VpVp Examples of a PLL I.C.: XR215, LM565, and CD4046

79. 78 Operation of PLL ¬Initially, the PLL is unlocked, i.e.,the VCO is at the free-running frequency, f o. ­Since f o is probably not the same as the reference frequency, f r, the phase detector will generate an error/control voltage, V p. ®V p is filtered, amplified, and applied to the VCO to change its frequency so that f o = f r. The PLL will then remain in phase lock.

80. 79 PLL Frequency Specifications Free-Running Frequency Capture Range Lock Range fofo f LC f LL f HC f HL f 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.

81. 80 Basic PLL Frequency Synthesizer 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. Phase comparator LPFVCO NN frfr f out = Nf r f c = f out /N

82. 81 Frequency Synthesizer Using Prescaling Phase comparator LPFVCO Prescaler  P or  (P+1) NN MM frfr f out =(NP+M)f r 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.

83. 82 AM Waveform e c = E c sin  c t e m = E m sin  m t AM signal: e s = (E c + e m ) sin  c t

84. 83 Modulation Index The amount of amplitude modulation in a signal is given by its modulation index: When E m = E c, m =1 or 100% modulation. Over-modulation, i.e. E m >E c, should be avoided because it will create distortions and splatter. where, E max = E c + E m ; E min = E c - E m (all pk values)

85. 84 Effects of Modulation Index m = 1m > 1 In a practical AM system, it usually contains many frequency components. When this is the case,

86. 85 AM in Frequency Domain The expression for the AM signal: e s = (E c + e m ) sin  c t can be expanded to: e s = E c sin  c t + ½ mE c [cos (  c -  m )t-cos (  c +  m )t] The expanded expression shows that the AM signal consists of the original carrier, a lower side frequency, f lsf = f c - f m, and an upper side frequency, f usf = f c + f m.

87. 86 AM Spectrum f fcfc EcEc f usf mE c /2 f lsf fmfm fmfm f usf = f c + f m ; f lsf = f c - f m ; E sf = mE c /2 Bandwidth, B = 2f m

88. 87 AM Power Total average (i.e. rms) power of the AM signal is: P T = P c + 2P sf, where P c = carrier power; and P sf = side-frequency power If the signal is across a load resistor, R, then: P c = E c 2 /(2R); and P sf = m 2 P c /4. So,

89. 88 AM Current The modulation index for an AM station can be measured by using an RF ammeter and the following equation: where I is the current with modulation and I o is the current without modulation.

90. 89 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 m T. For example:

91. 90 Block Diagram of AM TX

92. 91 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.

93. 92 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.

94. 93 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.

95. 94 AM Modulator Circuits

96. 95 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

97. 96 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.

98. 97 Trapezoidal Pattern (cont’d) Improper phase -V p >+V p m<1m=1m>1

99. 98 AM Receivers Basic requirements for receivers: Êability to tune to a specific signal Ëamplify the signal that is picked up Ìextract the information by demodulation Íamplify the demodulated signal.Two important receiver specifications: sensitivity and selectivity

100. 99 Tuned-Radio-Frequency (TRF) Receiver The TRF receiver is the simplest receiver that meets all the basic requirements.

101. 100 Drawbacks of TRF Receivers ÀDifficulty in tuning all the stages to exactly the same frequency simultaneously. ÁVery high Q for the tuning coils are required for good selectivity  BW=f o /Q. ÂSelectivity is not constant for a wide range of frequencies due to skin effect which causes the BW to vary with  f o.

102. 101 Superheterodyne Receiver Block diagram of basic superhet receiver:

103. 102 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.

104. 103 Mixer and Local Oscillator The mixer and LO frequency convert the input frequency, f c, to a fixed f IF : High-side injection: f LO = f c + f IF

105. 104 Autodyne Converter Sometimes called a self-excited mixer, the autodyne converter combines the mixer and LO into a single circuit:

106. 105 IF Amplifier, Detector, & AGC

107. 106 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.

108. 107 Diode Detector Waveforms

109. 108 Diagonal Clipping Distortion Diagonal clipping distortion is more pronounced at high modulation index or high modulation frequency.

110. 109 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.

111. 110 Shape Factor

112. 111 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:f image = f sig + 2f IF wheref sig = desired signal. An image signal must be rejected by tuning circuits prior to mixing.

113. 112 Image-Frequency Rejection Ratio For a tuned circuit with a quality factor of Q, its image-frequency rejection ratio is: In dB, IFRR (dB) = 20 log IFRR

114. 113 IF Transformers The transformers used in the IF stages can be either single-tuned or double-tuned. Single-tuned Double-tuned

115. 114 Loose and Tight Couplings For single-tuned transformers, tighter coupling means more gain but broader bandwidth:

116. 115 Under, Over, & Critical Coupling Double-tuned transformers can be over, under, critically, or optimally coupled:

117. 116 Coupling Factors Critical coupling factor k c is given by: where Q p, Q s = prim. & sec. Q, respectively..IF transformers often use the optimum coupling factor, k opt = 1.5k c, to obtain a steep skirt and flat passband. The bandwidth for a double-tuned IF amplifier with k = k opt is given by B = kf o..Overcoupling means k>k c ; undercoupling, k< k c

118. 117 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.

119. 118 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 P sf = m 2 P c /4, and P t =P c (1+m 2 /2 ), then at m=1, P t = 6 P sf. SSB also has a bandwidth reduction of half, which in turn reduces noise by half.

120. 119 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. BPFor AF Input Balanced Modulator Carrier Oscillator DSB-SC USB LSB

121. 120 Waveforms for Balanced Modulator V 1, f c V 2, f m VoVo f f c +f m f c -f m

122. 121 Mathematical Analysis of Balanced Modulator V 1 = A 1 sin  c t; V 2 = A 2 sin  m t V o = V 1 V 2 = A 1 A 2 sin  c t sin  m t = ½A 1 A 2 {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 )

123. 122 LIC Balanced Modulator 1496

124. 123 Filter for SSB Filters with high Q are needed for suppressing the unwanted sideband. f a = f c - f 2 f b = f c - f 1 f d = f c + f 1 f e = f c + f 2 where X = attenuation of sideband, and  f = f d - f b

125. 124 Typical SSB TX using Filter Method

126. 125 SSB Waveform

127. 126 Generating SSB - Phasing Method This method is based on the fact that the lsf and the usf are given by the equations: cos {(  c -  m )t} = ½(cos  c t cos  m t + sin  c t sin  m t) cos {(  c +  m )t} = ½(cos  c t cos  m t - sin  c t sin  m t) 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 90 o. The 2nd equation is similar except for the (-) sign.

128. 127 Diagram for Phasing Method Balanced Modulator 1 Balanced Modulator 2 + 90 o phase shifter 90 o phase shifter Modulating signal E m cos  m t SSB output E c cos  c t Carrier oscillator

129. 128 Phasing vs Filtering Method Advantages of phasing method : ÀNo high Q filters are required. ÁTherefore, lower f m can be used. ÂSSB at any carrier frequency can be generated in a single step. Disadvantage: Difficult to achieve accurate 90 o phase shift across the whole audio range.

130. 129 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. where V p = peak signal voltage and R L = load resistance

131. 130 Non-coherent SSB BFO RX

132. 131 Coherent SSB BFO Receiver RF amplifier and preselector RF mixer IF amp. & bandpass filter IF mixer Carrier recovery and frequency synthesizer RF input signal Demod. info RF LO BFO RF SSBRCIF SSBRC

133. 132 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.

134. 133 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.

135. 134 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.

136. 135 Comparison of FM or PM with AM Advantages over AM: 1)better SNR, and more resistant to noise 2)efficient - class C amplifier can be used, and less power is required to angle modulate 3)capture effect reduces mutual interference Disadvantages: 1)much wider bandwidth is required 2)slightly more complex circuitry is needed

137. 136 Frequency Shift Keying (FSK) Carrier Modulating signal FSK signal

138. 137 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.

139. 138 Frequency Modulation Carrier Modulating Signal FM signal

140. 139 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.

141. 140 Frequency Deviation The amount by which the frequency of the FM signal varies with respect to its resting value (f c ) is known as frequency deviation:  f = k f e m, where k f is a system constant, and e m is the instantaneous value of the modulating signal amplitude. Thus the frequency of the FM signal is: f s (t) = f c +  f = f c + k f e m (t)

142. 141 Maximum or Peak Frequency Deviation If the modulating signal is a sine wave, i.e., e m (t) = E m sin  m t, then f s = f c + k f E m sin  m t. The peak or maximum frequency deviation:  = k f E m The modulation index of an FM signal is: m f =  / f m Note that m f can be greater than 1.

143. 142 Relationship between FM and PM For PM, phase deviation,  = k p e m, and the peak phase deviation,  max = m p = m f. Since frequency (in rad/s) is given by: the above equations suggest that FM can be obtained by first integrating the modulating signal, then applying it to a phase modulator.

144. 143 Equation for FM Signal If e c = E c sin  c t, and e m = E m sin  m t, then the equation for the FM signal is: e s = E c sin (  c t + m f sin  m t) This signal can be expressed as a series of sinusoids: e s = E c {J o (m f ) sin  c t - J 1 (m f )[sin (  c -  m )t - sin (  c +  m )t] + J 2 (m f )[sin (  c - 2  m )t + sin (  c + 2  m )t] - J 3 (m f )[sin (  c - 3  m )t + sin (  c + 3  m )t] + ….}

145. 144 Bessel Functions The J’s in the equation are known as Bessel functions of the first kind: m f J o J 1 J 2 J 3 J 4 J 5 J 6... 01 0.5.94.24.03 1.77.44.11.02 2.40.0.52.43.20.06.02 5.50.0-.34-.12.26.40.32.19...

146. 145 Notes on Bessel Functions Theoretically, there is an infinite number of side frequencies for any m f 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 m f = 2.4, 5.5, 8.65, etc. These points are useful for determining the deviation and the value of k f of an FM modulator system.

147. 146 Graph of Bessel Functions

148. 147 FM Side-Bands Each (J) value in the table gives rise to a pair of side- frequencies. The higher the value of m f, the more pairs of significant side- frequencies will be generated.

149. 148 Power and Bandwidth of FM Signal Regardless of m f, 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 f m,where n is the number of pairs of side-frequencies. If m f > 6, a good estimate of the BW is given by Carson’s rule:BW = 2(  + f m (max) )

150. 149 Narrowband & Wideband FM FM systems with a bandwidth < 15 kHz, are considered to be NBFM. A more restricted definition is that their m f < 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 m f are called WBFM.

151. 150 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, e m is pre-emphasized before frequency modulating e c.

152. 151 Pre-emphasis circuit In FM broadcasting, the high frequency components are boosted by passing the modulating signal through a HPF with a 75  s time constant before modulation.   = R 1 C = 75  s.

153. 152 De-emphasis Circuit At the FM receiver, the signal after demodulation must be de-emphasized by a filter with similar characteristics as the pre- emphasis filter to restore the relative amplitudes of the modulating signal.

154. 153 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: L+R (mono) kHz L-R.0515233853607467 19 kHz Pilot Carrier SCA (optional)

155. 154 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.

156. 155 Block Diagram of FM Transmitter FM Modulator Buffer Pre-emphasis Audio Frequency Multiplier(s) Driver Power Amp Antenna

157. 156 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.

158. 157 Crosby AFC System An LC oscillator operated as a VCO with automatic frequency control is known as the Crosby system.

159. 158 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.

160. 159 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.

161. 160 Example of Indirect FM Generator Armstrong Modulator

162. 161 Block Diagram of FM Receiver

163. 162 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.

164. 163 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.

165. 164 S-curve Characteristics of FM Detectors f IF   fifi vovo EmEm

166. 165 PLL FM Detector PLL and quadrature detectors are commonly found in modern FM receivers.  Phase Detector LPF Demodulated output VCO FM IF Signal

167. 166 Quadrature Detector Both the quadrature and the PLL detector are conveniently found as IC packages.

168. 167 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.

169. 168 Transmission Line Equivalent Circuit R L R L C G C G L L C C “Lossy” Line Lossless Line ZoZo ZoZo

170. 169 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, Z o, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, Z L = Z o.

171. 170 Formulas for Some Lines D d D d For parallel two-wire line: For co-axial cable:  =  o  r ;  =  o  r ;  o = 4  x10 -7 H/m;  o = 8.854 pF/m

172. 171 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: m/s Velocity factor, VF, is defined as:

173. 172 Propagation Constant Propagation constant, , determines the variation of V or I with distance along the line: V = V s e -x  ; I = I s e - x , where V S, and I S are the voltage and current at the source end, and x = distance from source.  =  + j , where  = attenuation coefficient (= 0 for lossless line), and  = phase shift coefficient = 2  / (rad./m)

174. 173 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.

175. 174 Reflection Coefficient The reflection coefficient is defined as : It can also be shown that: Note that when Z L = Z o,  = 0; when Z L = 0,  = -1; and when Z L = open circuit,  = 1.

176. 175 Standing Waves V min = E i - E r With a mismatched line, the incident and reflected waves set up an interference pattern on the line known as a standing wave. The standing wave ratio is : V max = E i + E r  Voltage

177. 176 Other Formulas When the load is purely resistive: (whichever gives an SWR > 1) Return Loss, RL = Fraction of power reflected = |  | 2, or -20 log |  | dB So, P r = |  | 2 P i Mismatched Loss, ML = Fraction of power transmitted/absorbed = 1 - |  | 2 or -10 log(1-|  | 2 ) dB So, P t = P i (1 - |  | 2 ) = P i - P r

178. 177 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: Balanced Feedline Symbol /2

179. 178 Half-Wave Dipole Typically, the physical length of a half-wave dipole is 0.95 of /2 in free space. Since power fed to the antenna is radiated into space, there is an equivalent radiation resistance, R r. For a real antenna, losses in the antenna can be represented by a loss resistance, R d. Its efficiency is then:

180. 179 3-D Antenna Radiation Pattern

181. 180 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 /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

182. 181 EIRP and Effective Area When power, P T, is applied to an antenna with a gain G T (with respect to an isotropic radiator), then the antenna is said to have an effective isotropic radiated power, EIRP = P T G T. The signal power delivered to a receiving antenna with a gain G R is P R = P D A eff where P D is the power density, and A eff is the effective area.

183. 182 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 /2, it is inductive. The polarization of a half-wave dipole is the same as the axis of the conductor.

184. 183 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.

185. 184 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.

186. 185 Monopole or Marconi Antenna Main characteristics:  vertical and /4  good ground plane is required  omnidirectional in the horizontal plane  3 dBd power gain  impedance: about 36 

187. 186 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

188. 187 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.

189. 188 Inductive and Capacitive Loading Inductive LoadingCapacitive Loading

190. 189 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

191. 190 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

192. 191 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.

193. 192 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

194. 193 Block Diagram Of Analogue Cell Phone Duplexer RF power amp RF amp mixer FM modulator Frequency synthesizer Microprocessor IF amp IF detector De-emphasis Audio amp Audio preamp & Pre-emphasis Keypad Display Data Antenna Speaker 6 mW – 3W Mic

195. 194 7-Cell Pattern 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. 1 2 3 4 5 5 6 7 4 3 6 1 In a real situation, the cells are more likely to be approximately circular, with some overlap.

196. 195 Cellular Radio Network BSC MSC Gateway MSC To Public Switched Telephone Network To other MSCs BSC: Base Station Controller MSC: Mobile Switching Centre To other BSCs BSC

197. 196 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.

198. 197 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.

199. 198 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.

200. 199 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.

201. 200 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.