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1
ANALOGUE TELECOMMUNICATIONS
2
MAIN TOPICS (Part I)
1) Introduction to Communication Systems2) Filter Circuits3) Signal Generation4) Amplitude Modulation5) AM Receivers6) AM Transmitters
3
MAIN TOPICS (Part II)
7) Single-Sideband Communications Systems8) Angle Modulation Transmission9) Angle Modulated Receivers & Systems10) Introduction To Transmission Lines & Antennas11) Mobile Telecommunications
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:
Source Transmitter Receiver Recipient
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
6
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
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Frequency Bands
BAND Hz ELF 30 - 300 AF 300 - 3 k VLF 3 k - 30 k LF 30 k - 300 k MF 300 k - 3 M HF 3 M - 30 M
BAND Hz VHF 30M-300M UHF 300M - 3 G SHF 3 G - 30 G EHF 30 G - 300G
•Wavelength, = c/f
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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 Bwhere 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
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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
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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
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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)=Co+An cosnt Bn sin n t (Fourier series)
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.
A square-wave has only odd harmonics
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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).
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Dual-Gate MOSFET Mixer
Good dynamic range and fewer unwanted o/p frequencies.
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:
f1
f2
f1+ f2
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Equations for Balanced Mixer
Let the inputs be v1 = sin 1t and v2 = sin t.A balanced mixer acts like a multiplier. Thusits output, vo = Av1v2 = A sin 1t sin 2t.
Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)]Therefore, vo = 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.
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Balanced Ring Diode Mixer
Balanced mixers are also called balanced modulators.
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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
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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 oKk = Boltzmann’s constant, 1.38x10-23 J/oKB = noise power bandwidth in Hz
Noise voltage,
kTBR4VN
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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.
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Noise Spectrum of Electronic Devices
DeviceNoise
Shot and Thermal Noises
Excess orFlicker Noise
Transit-Time orHigh-FrequencyEffect Noise
1 kHz fhc
f
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:
N
S
N
S
V
Vlog20
P
P log 10 dB)(
N
S
In FM receivers, SINAD = (S+N+D)/(N+D)is usually used instead of SNR.
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)
25
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:
...AA
1F
A
1FFF
21
3
1
21T
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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.
27
Radio-Frequency Amplifiers
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 (fo) , bandwidth (B), and quality factor (Q), of a parallel resonant circuit are:
L
Loo X
RQ
Q
fB
LCf ;;
2
1
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. 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.
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.
31
Stagger-Tuned IF Amplifiers
32
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.
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Class B Push-Pull RF Amplifier
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.
35
Class C Amplifier (cont’d)
36
Frequency Multipliers
One of the applications of class C amplifiers is in “frequency multiplication”. The basic block diagram of a frequency multiplier:
High DistortionDevice +Amplifier
TuningFilter
Circuit
Inputfi
Output
N x fi
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 fi ; tuning to the 3rd harmonic triples fi ; etc.
38
Waveforms for Frequency Multipliers
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.
40
Hazeltine Neutralization
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, 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:
decc
fatdBattenxf
ffatdBatten )(.log)(.
42
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
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Low-Pass Filter Response
Vo
fcf0
1
0.707
BW
Gain (dB)
0
-20
-40
-60
fc 10fc 100fc 1000fc
-20 dB/dec
-40 dB/dec
-60 dB/dec
LPF with different roll-off ratesBasic LPF response
f
Ideal
Passband
BW = fc
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High-Pass Filter Response
Vo
fc f0
1
0.707
Gain (dB)
0
-20
-40
-60
0.01fc 0.1fc fc
-20 dB/dec
-40
dB/d
ec-6
0 dB
/dec
Passband
Basic HPF response HPF with different roll-off rates
f
45
Band-Pass Filter Response
Vout
1
0.707
ffofc1 fc2
BW
BW = fc2 - fc1
21 cco fff Centre frequency:
Quality factor:BW
fQ o
Q is an indication of theselectivity of a BPF.Narrow BPF: Q > 10.Wide-band BPF: Q < 10.
Damping Factor: QDF 1
46
Gain (dB)
0-3
ffofc1 fc2
BW
PassbandPassband
Band-Stop Filter Response
• Also known as band-reject, or notch filter.
• Frequencies within a certain BW are rejected.
• Useful for filtering interfering signals.
47
Filter Response Characteristics
Av
f
Chebyshev
Butterworth
Bessel
48
Damping Factor
Frequencyselective
RC circuit+_
Vin Vout
R1
R2
General diagram of active filter
The damping factor (DF)of an active filter setsthe response characteristicof the filter.
2
12R
RDF
Its value depends on theorder (# of poles) of thefilter. (See Table on nextslide for DF values.)
49
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
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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.
51
Single-pole Active LPF
+_
Vin Vout
R1
R2
R
C
2
11
2
1
R
RA
RCf
cl
c
Roll-off rate for a single-polefilter is -20 dB/decade.
Acl is selectable since DF isoptional for single-pole LPF
52
Sallen-Key Low-Pass Filter
+_
Vin Vout
R1
R2
RA RB
CB
CA
Sallen-Key or VCVS(voltage-controlledvoltage-source) second-order low-pass filter
RCfc 2
1
Selecting RA = RB = R,and CA = CB = C :
The roll-off rate for atwo-pole filter is-40 dB/decade.For a Butterworth 2nd-order response, DF = 1.414;therefore, R1/R2 = 0.586.
53
Cascaded Low-Pass Filter
+_
Vin
R1
R2
RA1 RB1
CB1
CA1
+_
R3
R4
RA2
CA2
Third-order (3-pole) configuration
Vout
2 poles 1 pole
Roll-off rate: -60 dB/dec
54
Single-Pole High-Pass Filter
+_
Vin Vout
R1
R2
R
C
• Roll-off rate, and formulas for fc , and Acl are similar to those for LPF.
• Ideally, a HPF passes all frequencies above fc. However, the op-amp has an upper-frequency limit.
55
Sallen-Key High-Pass Filter
+_
Vin Vout
R1
R2
RA
RB
CBCA
Again, formulas androll-off rate are similarto those for 2nd-orderLPF.
To obtain higher roll-off rates, HPF filterscan be cascaded.
Basic Sallen-Keysecond-order HPF
56
BPF Using HPF and LPF
+_
Vin
R1
R2
RA1
CA1
+_
Vout
R3
R4
RA2
CA2
f
Av (dB)
0-3
fofc1 fc2
LP responseHP response
-20 dB/dec
-20
dB/d
ec
57
More On Bandpass Filter
If BW and fo are given, then:
24;
242
2
22
2
1
BWf
BWf
BWf
BWf ococ
A 2nd order BPF obtained by combining a LPF and a HPF:
BiFET op-amphas FETs atinput stage andBJTs at outputstage.
58
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
59
Multiple-Feedback BPF
+
_Vin Vout
R3
R1
R2C2
C1 Making C1 = C2 = C,
321
31
2
1
RRR
RR
Cfo
)2(2
;2
23
21
oo
ooo
AQCf
QR
Cf
QR
CAf
QR
1
2
2R
RAo
Q = fo/BW
Ao < 2Q2
Max. gain:
R1, C1 - LP sectionR2, C2 - HP section
60
Broadband Band-Reject Filter
A LPF and a HPF can also be combined to give a broadbandBRF:
2-pole band-reject filter
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.RI = RF for unity gain and is often chosen to be >> R1.
62
Multiple-Feedback Band-Stop Filter
+
_Vin Vout
R3
R1
R2C2
C1
R4
The multiple-feedbackBSF is very similar toits BP counterpart. Forfrequencies between fc1
and fc2 the op-amp willtreat Vin as a pair ofcommon-mode signalsthus rejecting themaccordingly.
212
1
RRCfo
When C1 = C2 =C
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:
SweepGenerator Filter
Spectrumanalyzer
The sweep generator outputs a sine wave whose frequency increases linearly between two preset limits.
64
Signal Generation - Oscillators
• Barkhausen criteria for sustained oscillations:
The closed-loop gain, |BAV| = 1.
The loop phase shift = 0o or some integer multiple of 360o at the operating frequency.
AV = open-loop gain
B = feedback factor/fraction
AV
B
Output
65
Basic Wien-Bridge Oscillator
_
++
_
C2
C1 R4
R3
R1
R2
VoltageDivider
Lead-lagcircuit
Vout
R1
R4
R2
R3 C2
C1
Vout
Two forms of the same circuit
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 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:
RCfr 2
1
67
Phase-Shift Oscillator
+
_
Rf
C1 C2 C3
R1 R2 R3
Vout
Each RC section provides 60o ofphase shift. Total attenuation ofthe three-section RC feedback, B = 1/29.
Choosing R1 = R2 = R3 = R,C1 = C2 = C3 = C,the resonantfrequency is:
RCfr
62
1
293
R
RA fcl
68
Hartley Oscillators
21
1
;2
1LLL
CLf T
T
o 1
21
L
LLB
1
2
L
LB
69
Colpitts Oscillator
21
21
2
1
2
1
CC
CCC;
LCf;
C
CB T
T
o
70
Clapp Oscillator
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.
432
32
2
1111
2
1;
CCC
C
LCf
CC
CB
T
T
o
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:
b
oV
V
CC
21
72
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.
74
Packaging, symbol, and characteristic of crystals
75
Crystal-Controlled Oscillators
Pierce Colpitts
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.
77
Phase-Locked Loop
• The PLL is the basis of practically all modern frequency synthesizer design.
• The block diagram of a simple PLL:
PhaseDetector
LPFLoop
AmplifierVCO
fr foVp
•Examples of a PLL I.C.: XR215, LM565, and CD4046
78
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.
79
PLL Frequency Specifications
Free-RunningFrequency
Capture Range
Lock Range
fofLCfLL fHC fHLf
There is a limit on how far apart the free-runningVCO frequency and the reference frequency can be
for lock to be acquired or maintained.
80
Basic PLL Frequency Synthesizer
For output frequencies in the VHF range and higher,a prescaler is required. The prescaler is a fixed dividerplaced ahead of the programmable divide by N counter.
Phasecomparator LPF VCO
N
frfout = Nfr
fc = fout/N
81
Frequency Synthesizer Using Prescaling
Phasecomparator LPF VCO
PrescalerP or (P+1)
N
M
fr fout
=(NP+M)fr
2-modulus prescaler divides by P+1 when M counter is non zero;it divides by P when M counter reaches zero. N counter countsdown (N-M) times. E.g. of I.C. prescaler: LMX5080 for UHFoperation.
82
AM Waveform
ec = Ec sin ctem = Em sin mt
AM signal:es = (Ec + em) sin ct
83
Modulation Index
• The amount of amplitude modulation in a signal is given by its modulation index:
minmax
minmax
EE
EEor
E
Em
c
m
When Em = Ec , m =1 or 100% modulation.
Over-modulation, i.e. Em>Ec , should be avoidedbecause it will create distortions and splatter.
where, Emax = Ec + Em; Emin = Ec - Em (all pk values)
84
Effects of Modulation Index
m = 1 m > 1
In a practical AM system, it usually contains manyfrequency components. When this is the case,
222
21 ... nT mmmm
85
AM in Frequency Domain
• The expression for the AM signal: es = (Ec + em) sin ct
can be expanded to:es = Ec sin ct + ½ mEc[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, flsf = fc - fm, and an upper side frequency, fusf = fc + fm.
86
AM Spectrum
ffc
Ec
fusf
mEc/2mEc/2
flsf
fmfm
fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2
Bandwidth, B = 2fm
87
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 = Ec
2/(2R); and Psf = m2Pc/4. So,
)2
1(2m
PP cT
88
AM Current
• The modulation index for an AM station can be measured by using an RF ammeter and the following equation:
21
2mII o
where I is the current with modulation and Io is the current without modulation.
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 mT. For example:
21);
21(
22T
oT
CT
mII
mPP
90
Block Diagram of AM TX
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.
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.
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.
94
AM Modulator Circuits
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
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.
97
Trapezoidal Pattern (cont’d)
Improperphase
-Vp>+Vp
minmax
minmax
VV
VVm
m<1 m=1 m>1
98
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
99
Tuned-Radio-Frequency (TRF) Receiver
• The TRF receiver is the simplest receiver that meets all the basic requirements.
100
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.
101
Superheterodyne Receiver
Block diagram of basic superhet receiver:
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.
103
Mixer and Local Oscillator
• The mixer and LO frequency convert the input frequency, fc, to a fixed fIF:
High-side injection: fLO = fc + fIF
104
Autodyne Converter
• Sometimes called a self-excited mixer, the autodyne converter combines the mixer and LO into a single circuit:
105
IF Amplifier, Detector, & AGC
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.
107
Diode Detector Waveforms
108
Diagonal Clipping Distortion
Diagonal clipping distortion is more pronounced athigh modulation index or high modulation frequency.
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.
110
Shape Factor
dB
dB
B
BSF
6
60
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: fimage = fsig + 2fIF
wherefsig = desired signal.
• An image signal must be rejected by tuning circuits prior to mixing.
112
Image-Frequency Rejection Ratio
• For a tuned circuit with a quality factor of Q, its image-frequency rejection ratio is:
image
sig
sig
image
f
f
f
fx
wherexQIFRR
,1 22
In dB, IFRR(dB) = 20 log IFRR
113
IF Transformers
• The transformers used in the IF stages can be either single-tuned or double-tuned.
Single-tuned Double-tuned
114
Loose and Tight Couplings
• For single-tuned transformers, tighter coupling means more gain but broader bandwidth:
115
Under, Over, & Critical Coupling
• Double-tuned transformers can be over, under, critically, or optimally coupled:
116
Coupling Factors
• Critical coupling factor kc is given by:
sp
cQQ
k1
where Qp, Qs = prim. & sec. Q, respectively.IF transformers often use the optimum coupling
factor, kopt = 1.5kc , to obtain a steep skirt andflat passband. The bandwidth for a double-tuned
IF amplifier with k = kopt is given by B = kfo.Overcoupling means k>kc; undercoupling, k< kc
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.
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 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.
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.
BPF orAF
Input
BalancedModulator
CarrierOscillator
DSB-SCUSB
LSB
120
Waveforms for Balanced Modulator
V1, fc
V2, fm Vo
ffc+fm
fc-fm
121
Mathematical Analysis of Balanced Modulator
• V1 = A1sin ct; V2 = A2sin mt
• Vo = V1V2 = A1A2sin ct sin mt
= ½A1A2{cos(c- m)t – cos(c+ m)t}
• The equation above shows that the output of the balanced modulator consists of a lower side-frequency (c - m) and an upper side-frequency (c+ m)
122
LIC Balanced Modulator 1496
123
Filter for SSB
• Filters with high Q are needed for suppressing the unwanted sideband.
fa = fc - f2
fb = fc - f1
fd = fc + f1
fe = fc + f2
f
dBXantifQ c
4
)20/log( where X = attenuation ofsideband, and f = fd - fb
124
Typical SSB TX using Filter Method
125
SSB Waveform
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 ct cos mt + sin ct sin mt)
cos {(c + m)t} = ½(cos ct cos mt - sin ct sin mt)
• 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.
127
Diagram for Phasing Method
Balanced Modulator 1
Balanced Modulator 2
+90o phaseshifter
90o phaseshifter
Modulatingsignal
Em cos mt
SSBoutput
Ec cos ct
Carrieroscillator
128
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.
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.
L
p
R
VPEP
2
2
where Vp = peak signal voltageand RL = load resistance
130
Non-coherent SSB BFO RX
131
Coherent SSB BFO Receiver
RF amplifierand
preselectorRF mixer
IF amp. &bandpass
filter
IFmixer
Carrier recoveryand frequency
synthesizer
RFinputsignal
Demod.info
RF LO
BFO
RF SSBRC IF SSBRC
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.
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.
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.
135
Comparison of FM or PM with AM
Advantages over AM:
1) better SNR, and more resistant to noise2) efficient - class C amplifier can be used, and less
power is required to angle modulate3) capture effect reduces mutual interferenceDisadvantages:
1) much wider bandwidth is required2) slightly more complex circuitry is needed
136
Frequency Shift Keying (FSK)
Carrier
Modulatingsignal
FSKsignal
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.
138
Frequency Modulation
Carrier
ModulatingSignal
FMsignal
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.
140
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)
141
Maximum or Peak Frequency Deviation
• If the modulating signal is a sine wave, i.e., em(t) = Emsin mt, then fs = fc + kfEmsin mt.
• The peak or maximum frequency deviation: = kf Em
• The modulation index of an FM signal is:mf = / fm
• Note that mf can be greater than 1.
142
Relationship between FM and PM
• For PM, phase deviation, = kpem, and the peak phase deviation, max = mp = mf.
• Since frequency (in rad/s) is given by:
dtttordt
tdt )()(
)()(
the above equations suggest that FM can be
obtained by first integrating the modulating
signal, then applying it to a phase modulator.
143
Equation for FM Signal
• If ec = Ec sin ct, and em = Em sin mt, then the equation for the FM signal is:
es = Ec sin (ct + mf sin mt)
• This signal can be expressed as a series of sinusoids: es = Ec{Jo(mf) sin ct
- J1(mf)[sin (c - m)t - sin (c + m)t]
+ J2(mf)[sin (c - 2m)t + sin (c + 2m)t]
- J3(mf)[sin (c - 3m)t + sin (c + 3m)t]
+ … .}
144
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 10.5 .94 .24 .031 .77 .44 .11 .022.4 0.0 .52 .43 .20 .06 .025.5 0.0 -.34 -.12 .26 .40 .32 .19 . . .
145
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.
146
Graph of Bessel Functions
147
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.
148
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( + fm (max) )
149
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.
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, em is pre-emphasized before frequency modulating ec.
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.
= R1C = 75 s.
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.
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)
kHzL-R L-R
.05 15 23 38 53 6074
67
19 kHz PilotCarrier SCA
(optional)
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.
155
Block Diagram of FM Transmitter
FMModulator
Buffer
Pre-emphasis
Audio
FrequencyMultiplier(s)
Driver PowerAmp
Antenna
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.
157
Crosby AFC System
• An LC oscillator operated as a VCO with automatic frequency control is known as the Crosby system.
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.
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.
160
Example of Indirect FM Generator
ArmstrongModulator
161
Block Diagram of FM Receiver
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.
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.
164
S-curve Characteristics of FM Detectors
fIF
fi
vo
Em
165
PLL FM Detector
• PLL and quadrature detectors are commonly found in modern FM receivers.
PhaseDetector
LPFDemodulated
output
VCO
FM IFSignal
166
Quadrature Detector
• Both the quadrature and the PLL detector are conveniently found as IC packages.
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.
168
Transmission Line Equivalent Circuit
R L R L
C G C G
L L
C C
“Lossy” Line Lossless Line
CjG
LjRZo
C
LZo
ZoZo
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, Zo, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, ZL = Zo.
170
Formulas for Some Lines
D
d
D
d
d
DZ
dD
Cd
DL
r
o
2ln
120;
2ln
;2
ln
For parallel two-wire line:
For co-axial cable:
d
DZ
dD
Cd
DL
r
o ln60
;ln
2;ln
2
= or; = or; o = 4x10-7 H/m; o = 8.854 pF/m
171
Transmission-Line Wave Propagation
Electromagnetic waves travel at < c in a transmissionline because of the dielectric separating the conductors.The velocity of propagation is given by:
r
c
LCv
11m/s
Velocity factor, VF, is defined as:
rc
vVF
1
172
Propagation Constant
• Propagation constant, , determines the variation of V or I with distance along the line: V = Vse-x; I = Ise-x, where VS, and IS 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)
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.
174
Reflection Coefficient
The reflection coefficient is defined as:
i
r
i
r
I
Ior
E
E
It can also be shown that:
oL
oL
ZZ
ZZ
Note that when ZL = Zo, = 0; when ZL = 0, = -1;and when ZL = open circuit, = 1.
175
Standing Waves
Vmin = Ei - Er
With a mismatched line, the incident and reflectedwaves set up an interference pattern on the line known as a standing wave.The standing wave ratio is :
1
1
min
max
V
VSWR
Vmax = Ei + Er
Vol
tage
176
Other Formulas
When the load is purely resistive:(whichever gives an SWR > 1) L
o
o
L
Z
Zor
Z
ZSWR
Return Loss, RL = Fraction of power reflected= ||2, or -20 log || dBSo, Pr = ||2Pi
Mismatched Loss, ML = Fraction of powertransmitted/absorbed = 1 - ||2 or -10 log(1-||2) dBSo, Pt = Pi (1 - ||2) = Pi - Pr
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
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, Rr. For a real antenna, losses in the antenna can be represented by a loss resistance, Rd. Its efficiency is then:
dr
r
T
r
RR
R
P
P
179
3-D Antenna Radiation Pattern
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
181
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.
4;
4
2
2R
effD
GA
r
EIRPP
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 becomes capacitive, and if > /2, it is inductive.
• The polarization of a half-wave dipole is the same as the axis of the conductor.
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.
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.
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
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
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.
188
Inductive and Capacitive Loading
Inductive LoadingCapacitive
Loading
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
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
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.
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
193
Block Diagram Of Analogue Cell Phone
Duplexer
RF poweramp
RF amp mixer
FMmodulator
Frequencysynthesizer
Microprocessor
IFamp
IFdetector
De-emphasisAudioamp
Audio preamp& Pre-emphasis
Keypad
Display
Data
AntennaSpeaker
6 mW – 3W
Mic
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.
12
34
55
67
4
36
1
In a real situation, the cells aremore likely to be approximatelycircular, with some overlap.
195
Cellular Radio Network
BSC BSC BSCBSC
MSCMSC GatewayMSC
To Public SwitchedTelephone Network
To otherMSCs
BSC: Base Station ControllerMSC: Mobile Switching Centre
To other BSCs
BSC BSC BSC
BSC
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.
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.
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.
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.
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.
