2. MAIN TOPICS
Introduction to Communication Systems
Amplitude Modulation
AM Receivers
AM Transmitters
Suppressed-Carrier AM Systems
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 modulation: AM, FM, and PM
6. 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 = a constant of the system
t = time available
B = channel bandwidth
7. 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, λ = c/f
8. Types of Signal Distortion
Types of distortion in communications:
harmonic distortion
intermodulation distortion
nonlinear frequency response
nonlinear phase response
noise
interference
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. Jean Baptiste Joseph Fourier (1768-1830)
Had crazy idea (1807):
Any periodic function can
be rewritten as a weighted sum
of Sines and Cosines of
different frequencies.
Don’t believe it?
– Neither did Lagrange,
Laplace, Poisson and other
big wigs
– Not translated into English
until 1878!
But it’s true!
– called Fourier Series
– Possibly the greatest tool
used in Engineering
11. A Sum of Sinusoids
Our building block:
Add enough of them to get
any signal f(x) you want!
How many degrees of
freedom?
What does each control?
Which one encodes the
coarse vs. fine structure of
the signal?
)+φωxAsin(
22. angular frequency ( )iux
e−
Note that these are derived using
Fourier Transform Pairs (I)
23.
24. 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+ΣAn cos nω t + ΣBnsin nω t (Fourier series)
25. 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
26. External Noise
Equipment / Man-made Noise is generated
by any equipment that operates with
electricity
Atmospheric Noise is often caused by
lightning
Space Noise is strongest from the sun and,
at a much lesser degree, from other stars
27. Noise Spectrum of Electronic Devices
Device
Noise
Shot and Thermal Noises
Excess or
Flicker Noise
Transit-Time
Noise
1 kHz fhc
f
28. Frequency Multipliers
One of the applications of class C
amplifiers is in “frequency multiplication”.
The basic block diagram of a frequency
multiplier:
High
Distortion
Device +
Amplifier
Tuning
Filter
Circuit
Input
fi
Output
N x fi
29. 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.
33. 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
111
1
2
1
;
CCC
C
LC
f
CC
C
B
T
T
o
++
=
=
+
=
π
34. 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
o
V
V
C
C
21+
=
35.
36. 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).
37. 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
38. Equations for Balanced Mixer
Let the inputs be v1= sin ω1t and v2= sin ω2t.
A balanced mixer acts like a multiplier. Thus
its output, vo = Av1v2 = A sin ω1t sin ω2t.
Since sin X sin Y = 1/2[cos(X-Y) -
cos(X+Y)]
Therefore, vo = A/2[cos(ω1-ω2)t-cos(ω1+ω2)t].
The last equation shows that the output of
the balanced mixer consists of the sum and
39. Balanced Ring Diode Mixer
Balanced mixers are also called balanced modulators.
40. AM Waveform
ec = Ec sin ωct
em = Em sin ωmt
AM signal:
es = (Ec + em) sin ωct
41. Modulation Index
The amount of amplitude modulation in a
signal is given by its modulation index:
minmax
minmax
EE
EE
or
E
E
m
c
m
+
−
=
When Em = Ec , m =1 or 100% modulation.
Over-modulation, i.e. Em>Ec , should be avoided
because it will create distortions and splatter.
where, Emax = Ec + Em; Emin = Ec - Em (all pk values)
42. Effects of Modulation Index
m = 1 m > 1
In a practical AM system, it usually contains many
frequency components. When this is the case,
22
2
2
1 ... nT mmmm +++=
43. AM in Frequency Domain
The expression for the AM signal:
es = (Ec + em) sin ωct
can be expanded to:
es = Ec sin ωct + ½ mEc[cos (ωc-ωm)t-cos (ωc+ωm)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.
45. 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 = m2
Pc/4. So,
)
2
1(
2
m
PP cT +=
46. AM Current
The modulation index for an AM station
can be measured by using an RF ammeter
and the following equation:
2
1
2
m
II o +=
where I is the current with modulation and
Io is the current without modulation.
47. 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
1);
2
1(
22
T
o
T
CT
m
II
m
PP +=+=
48. 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
49. by H Chan, Mohawk College
Tuned-Radio-Frequency (TRF) Receiver
The TRF receiver is the simplest receiver
that meets all the basic requirements.
51. 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.
52. Mixer and Local Oscillator
The mixer and LO frequency convert the
input frequency, fc, to a fixed fIF:
High-side injection: fLO = fc + fIF
53. 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.
56. 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.
57. 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.
58. Image Frequency Rejection
For a tuned circuit with a quality factor of
Q, then the image frequency rejection is:
image
sig
sig
image
f
f
f
f
x
wherexQIR
−=
+= ,1 22
In dB, IR (dB) = 20 log IR
61. 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.
62. 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.
63. 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.
65. 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
66. 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.
68. 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 = m2
Pc/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.
69. 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 or
AF
Input
Balanced
Modulator
Carrier
Oscillator
DSB-SC
USB
LSB
71. 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
dBXantif
Q c
∆
=
4
)20/log( where X = attenuation of
sideband, and f = fd - fb
