ANALOG COMMUNICATIONS

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					ANALOGUE TELECOMMUNICATIONS




                              1
                MAIN TOPICS (Part I)

1)   Introduction to Communication Systems
2)   Filter Circuits
3)   Signal Generation
4)   Amplitude Modulation
5)   AM Receivers
6)   AM Transmitters




                                             2
                 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




                                                      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                    BAND    Hz
 ELF 30 - 300                 VHF 30M-300M
 AF           300 - 3 k       UHF 300M - 3 G
 VLF 3 k - 30 k               SHF 3 G - 30 G
 LF           30 k - 300 k    EHF 30 G - 300G
 MF           300 k - 3 M
 HF           3 M - 30 M




                              •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 time-
  domain.
 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
    Excess or                                     Effect Noise
    Flicker Noise




                 Shot and Thermal Noises


                                                            f
         1 kHz                              fhc
                                                                    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:


          S               PS          VS
            (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     A1 A 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:




                  1  fo    RL
      fo        ; B ; Q
           2 LC     Q     XL
                                                                    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-to-
  base 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:




               High
Input                                  Tuning       Output
             Distortion
                                        Filter
  fi         Device +                               N x fi
                                       Circuit
             Amplifier


                                                                   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 band-
    reject 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
                                f 
                                 c

                                                                            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
                                  0
    Vo         Ideal
                                 -20
   1
                                 -40
0.707
         Passband                -60
            BW
   0                fc       f              fc    10fc 100fc 1000fc        f

    Basic LPF response                 LPF with different roll-off rates


                                                                               43
                         High-Pass Filter Response

                           Gain (dB)
                                    0
    Vo
                                   -20
   1
                                   -40
0.707
                    Passband       -60

   0           fc              f                   0.01fc 0.1fc   fc         f
         Basic HPF response              HPF with different roll-off rates


                                                                                 44
                   Band-Pass Filter Response


    Vout                    Centre frequency:   fo     f c1 f c 2
   1
                              Quality factor: Q  f o
0.707                                            BW
                                     Q is an indication of the
                  BW                 selectivity of a BPF.
                                     Narrow BPF: Q > 10.
                                f    Wide-band BPF: Q < 10.
            fc1   fo fc2
           BW = fc2 - fc1            Damping Factor: DF  1 Q


                                                                     45
                 Band-Stop Filter Response

                                • Also known as band-reject,
Gain (dB)                         or notch filter.
                                • Frequencies within a certain
   0                              BW are rejected.
  -3                            • Useful for filtering interfering
                                  signals.
       Pass
                     Passband
       band

                            f
            fc1 fo fc2

              BW

                                                                46
     Filter Response Characteristics

Av
                   Chebyshev



                        Bessel

                           Butterworth


                                   f


                                         47
                              Damping Factor


                                                The damping factor (DF)
         Frequency                              of an active filter sets
Vin       selective                      Vout   the response characteristic
                          +
         RC circuit       _                     of the filter.
                                                                R1
                                    R1                 DF  2 
                                                                R2

                                    R2          Its value depends on the
                                                order (# of poles) of the
                                                filter. (See Table on next
      General diagram of active filter          slide for DF values.)

                                                                             48
Values For Butterworth Response

Order     1st Stage          2nd Stage

        Poles     DF       Poles    DF

 1       1      optional


 2       2      1.414

 3       2         1        1        1

 4       2      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                                    1
Vin                Vout           fc 
          +                            2 RC
      C   _
                                             R1
                  R1              Acl  1 +
                                             R2
                  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           Selecting RA = RB = R,
                                 and CA = CB = C :
      RA   RB
                                                 1
Vin                   +      Vout         fc 
           CB         _                        2 RC

                            R1     The roll-off rate for a
                                   two-pole filter is
  Sallen-Key or VCVS        R2     -40 dB/decade.
  (voltage-controlled             For a Butterworth 2nd-
  voltage-source) second-         order response, DF = 1.414;
  order low-pass filter           therefore, R1/R2 = 0.586.

                                                            52
                  Cascaded Low-Pass Filter

                       CA1
                                     Roll-off rate: -60 dB/dec
      RA1   RB1
                                  RA2
Vin                     +
            CB1         _                     +              Vout
                                 CA2          _
                                R1
                                                        R3
                                R2
             2 poles                     1 pole         R4

              Third-order (3-pole) configuration

                                                                    53
              Single-Pole High-Pass Filter

                             • Roll-off rate, and formulas
      C                        for fc , and Acl are similar to
                               those for LPF.
Vin           +       Vout   • Ideally, a HPF passes all
              _                frequencies above fc.
          R
                               However, the op-amp has an
                     R1        upper-frequency limit.

                     R2




                                                             54
                 Sallen-Key High-Pass Filter

                 RA
                                    Again, formulas and
      CA    CB                      roll-off rate are similar
Vin                   +      Vout   to those for 2nd-order
                      _             LPF.
            RB
                            R1      To obtain higher roll-
                                    off rates, HPF filters
                            R2      can be cascaded.
      Basic Sallen-Key
      second-order HPF



                                                                55
                        BPF Using HPF and LPF

                   CA1
      Vin                                 RA2
                             +
                             _                        +        Vout
                  RA1
                                          CA2         _
                                     R1
Av (dB)                                                       R3
                                     R2
                                                              R4
 0
-3
                                                HP response
                                                LP response
                                 f
            fc1     fo fc2
                                                                      56
                     More On Bandpass Filter

If BW and fo are given, then:
             BW 2        BW         BW 2        BW
      f c1       + fo 
                      2
                            ; fc2       + fo +
                                             2

              4           2          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                Making C1 = C2 = C,
                        R2
                                                   1     R1 + R3
      R1    C2                               fo 
                        _                         2 C   R1 R2 R3
Vin                                  Vout
           R3           +                        Q = fo/BW

                                     Q               Q    Max. gain:
                             R1            ; R2 
                                  2 f oCAo         f oC       R2
  R1, C1 - LP section                      Q              Ao 
                             R3                               2R1
  R2, C2 - HP section             2 f oC (2Q  Ao )
                                               2
                                                                  2
                                                         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
                                    The multiple-feedback
                   R2               BSF is very similar to
      R1   C2                       its BP counterpart. For
Vin                _         Vout   frequencies between fc1
                                    and fc2 the op-amp will
                   +                treat Vin as a pair of
      R3
                R4 When             common-mode signals
                                    thus rejecting them
                    C1 = C2 =C      accordingly.
                           1
                  fo 
                       2 C R1R2

                                                          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                           Spectrum
        Generator          Filter        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:
                                                   Output
 The closed-loop gain, |BAV|       AV
  = 1.
 The loop phase shift = 0o or
  some integer multiple of
  360o at the operating
  frequency.
                                 AV = open-loop gain
                                        B
                                 B = feedback factor/fraction




                                                                64
                   Basic Wien-Bridge Oscillator



          R1                                        R4
Voltage                                  R1
               _                                    C1
Divider                                                   _
          R2   +              Vout
                    C1 R4                R2               +   Vout

          R3         Lead-lag            R3              C2
                   C2 circuit


                    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 closed-
    loop 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
                                                     Rf
      _        C1        C2    C3            Acl          29
                                    Vout             R3
      +                                    Choosing
                    R1        R2    R3     R1 = R2 = R3 = R,
                                           C1 = C2 = C3 = C,
                                           the resonant
                                           frequency is:
Each RC section provides 60o of
phase shift. Total attenuation of                  1
                                           fr 
the three-section RC feedback,                  2 6 RC
B = 1/29.
                                                                 67
                 Hartley Oscillators




   L1 + L2           1                         L2
B           fo           ; LT  L1 + L2   B
      L1          2 LT C1                     L1
                                                    68
          Colpitts Oscillator




   C1           1           C1C2
B    ; fo         ; CT 
   C2        2 LCT        C1 + C2
                                     69
                           Clapp Oscillator

                                                C2              1
                                          B          ; fo 
                                             C 2 + C3        2 LCT
                                                    1
                                          CT 
                                               1    1   1
                                                  +   +
                                               C 2 C3 C 4

   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              Vp                                  fo
      Phase                  Loop
                     LPF                    VCO
     Detector               Amplifier


•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         LPF         VCO        fout = Nfr


                             N
       fc = fout/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                                        fout
          comparator           LPF          VCO
                                                         =(NP+M)fr

                                  Prescaler
              N
                                 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




                              AM signal:
 ec = Ec sin wct         es = (Ec + em) sin wct
em = Em sin wmt
                                                  82
                    Modulation Index

 • The amount of amplitude modulation in a signal is
   given by its modulation index:


                 Em    Em ax  Em in
              m    or
                 Ec    Em ax + Em in
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 + m2 + ...+ mn
                        2         2

                                                     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              fm
                                           f
     flsf            fc            fusf

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:



                              m2
                  I  Io   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                    2
                    mT                mT
       PT  PC (1 +    ); I  I o 1 +
                     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 high-
  level 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     m>1
                            Improper
   Vm ax  Vm in                     -Vp>+Vp
m                            phase
   Vm ax + Vm in
                                               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




     B60 dB
SF 
     B6 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     2
                                           where,
                f image        f sig
           x             
                 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 pQs
   where Qp, Qs = prim. & sec. Q, respectively.
IF transformers often use the optimum coupling
  factor, kopt = 1.5kc , to obtain a steep skirt and
flat passband. The bandwidth for a double-tuned
  IF amplifier with k = kopt is given by B = kfo.
Overcoupling means k>kc; undercoupling, k< kc
                                                       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           or
 AF
Input            Carrier                           LSB
                Oscillator                                 119
                  Waveforms for Balanced Modulator


V2, fm            Vo




         V1, fc




                       f
    fc-fm fc+fm
                                                     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 side-
  frequency (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
              4f            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                        Balanced Modulator 1
  signal
                      Carrier
Em cos wmt
                     oscillator
                                          Ec cos wct

                                                       +    SSB
             90o phase               90o phase             output
              shifter                 shifter



                              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.


                              2
                         Vp
                 PEP 
                        2 RL
        where Vp = peak signal voltage
          and RL = load resistance
                                                     129
Non-coherent SSB BFO RX




                          130
                   Coherent SSB BFO Receiver


           RF SSBRC                IF SSBRC
         RF amplifier             IF amp. &     IF     Demod.
             and        RF mixer   bandpass
RF                                             mixer   info
          preselector                filter
input
signal                        RF LO

                          Carrier recovery   BFO
                           and frequency
                            synthesizer

                                                           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)   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




                                                           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 )            or  (t )   w (t )dt
                  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 Jo      J1    J2    J3    J4  J5  J6 . . .
0     1
0.5   .94   .24    .03
1     .77   .44    .11    .02
2.4   0.0   .52    .43    .20   .06   .02
5.5   0.0   -.34   -.12   .26   .40   .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 side-
  frequencies.
• 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 pre-
  emphasis 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                       SCA
     L+R                               (optional)
    (mono)          L-R        L-R
                                                    kHz
  .05      15 23          38         53 60 67 74
                                                      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                Frequency
Modulator           Multiplier(s)                    Antenna


           Buffer                   Driver   Power
                                              Amp
          Pre-emphasis


  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.


            Phase
           Detector
FM IF                                  Demodulated
Signal        f               LPF        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      L        R   L           L        L

Zo                             Zo
     C       G   C       G            C        C


          “Lossy” Line                Lossless Line

              R + jwL                      L
         Zo                          Zo 
              G + jwC                      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
    D      L  ln      ; C         ; Zo      ln
                   d        ln
                                2D          r    d
d
                                    d
        m = momr;  = or; mo = 4x10-7 H/m; o = 8.854 pF/m
            For co-axial cable:
    D
                 m D             2        60    D
            L     ln ; C           ; Zo     ln
                2 d            ln
                                   D         r d
d
                                     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:
                1    1    c
             v                      m/s
                LC   m   r

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 = Ise-
  x, 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        Ir
                       or
                    Ei        Ii

 It can also be shown that:       Z L  Zo
                                           f
                                  Z L + Zo

Note that when ZL = Zo,  = 0; when ZL = 0,  = -1;
and when ZL = open circuit,  = 1.

                                                    174
                 Standing Waves
 Voltage



                      Vmax = Ei + Er
                                       Vmin = Ei - Er
             l
             2
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:         ZL    Zo
                                     SWR     or
(whichever gives an SWR > 1)               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

                                            Symbol
    Balanced Feedline
                                                                   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 
                  4r 2
                                  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
                   mixer          IF        IF                       Audio
          RF amp                                     De-emphasis
                                 amp     detector                     amp

                                                           Display
                   Frequency
      Duplexer                         Microprocessor
                   synthesizer
                                                           Keypad
                                                    Data

    RF power         FM                   Audio preamp
                                                                        Mic
      amp          modulator              & Pre-emphasis
    6 mW – 3W

                                                                                 193
                              7-Cell Pattern

                                     • Each cell has a base station.
                                     • All cell sites in a region are
                                       tied to a mobile switching
           6       3                   centre (MSC) or mobile
       5       7       6               telephone switching office
           1       5                   (MTSO) which in turn is
       4       2       1
           3       4                   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      BSC    BSC      BSC   BSC     BSC    BSC


To other                         Gateway           MSC
                 MSC
 MSCs                             MSC

           To other BSCs                                BSC
                             To Public Switched
                             Telephone Network
                                                              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

				
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