4.5 - SSB Receivers
The major
advantage of
SSB for voice
transmission is
the elimination
of the carrier.
However, the
carrier is needed
at the receiver in
order to get the
original
intelligence!
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Mixer SSB Demodulator
Recall that a mixer will output sum and difference of the
two input frequencies.
When the SSB input and a local oscillator fC are applied
to the mixer, the output after the low-pass will the the
intelligence signal.
COM455 1001 Sideband Systems
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1
Restoring Carrier at SSB
Receiver
One of the methods to have the carrier restored at the
receiver for demodulation is to use residual pilot
carrier transmitted with the SSB signal to retain
correct carrier fC and phase info.
Ex. ACSSB Tx signal
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SSB Receiver Block Diagram
Similar to AM superheterodyne receiver, a SSB receiver
employs a mixer to down convert RF to IF.
To demodulate the SSB signal, a SSB receiver uses
another mixer to “detect” the intelligence signal.
COM455 1001 Sideband Systems
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2
4.5 - SSB Receivers
The major
advantage of
SSB for voice
transmission is
the elimination
of the carrier.
However, the
carrier is needed
at the receiver in
order to get the
original
intelligence!
COM455 1001 Sideband Systems
1
Mixer SSB Demodulator
Recall that a mixer will output sum and difference of the
two input frequencies.
When the SSB input and a local oscillator fC are applied
to the mixer, the output after the low-pass will the the
intelligence signal.
COM455 1001 Sideband Systems
COM455 1001 Communications Fundamentals
2
1
Restoring Carrier at SSB
Receiver
One of the methods to have the carrier restored at the
receiver for demodulation is to use residual pilot
carrier transmitted with the SSB signal to retain
correct carrier fC and phase info.
Ex. ACSSB Tx signal
COM455 1001 Sideband Systems
3
SSB Receiver Block Diagram
Similar to AM superheterodyne receiver, a SSB receiver
employs a mixer to down convert RF to IF.
To demodulate the SSB signal, a SSB receiver uses
another mixer to “detect” the intelligence signal.
COM455 1001 Sideband Systems
COM455 1001 Communications Fundamentals
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2
Topic 5 Principles of Frequency
Modulation
You will learn...
Introduction to Frequency Modulation
FM Analysis
FM Noise Suppression
FM Generation
FM Slope Detector
PLL and Applications
FM Superheterodyne Receiver
Stereo FM
COM455 1001 Principles of FM
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5.1 - Introduction to FM
There are three parameters of
a carrier that may carry
information:
– Amplitude
– Frequency
– Phase
AM
FM
PM
FM and PM are very closely
related, and are termed
Angle Modulation (as both
refer to changes in phase
angle of the carrier)
COM455 1001 Principles of FM
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1
Modulating Signal & FM Signal
Intelligence (Modulating)
Signal (f i)
Maximum Intelligence
Amplitude
Minimum Intelligence
Amplitude
Unmodulated Carrier
Signal at the Centre
Frequency (fC)
Maximum Carrier
Frequency ( fC + δ )
Frequency
Modulated (FM)
Signal
Minimum Carrier
Frequency ( fC - δ )
Constant Carrier
Amplitude
COM455 1001 Principles of FM
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Frequency Modulation
Power in an FM signal does not vary with
modulation.
FM signals do not have an envelope that
reproduces the modulation.
Ideally, the deviation (the amount of change) of
the centre frequency should be directly
proportional to the intelligence amplitude.
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2
Frequency Deviation δ
The maximum change in carrier fC due to
modulation is called the frequency deviation, fd
or δ.
The intelligence amplitude determines the
amount of carrier frequency deviation.
(up to +/- δ)
The intelligence frequency fi determines the rate
of carrier frequency deviation.
(how fast the changing)
COM455 1001 Principles of FM
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FM Modulation Index
Modulation index measures the extent to which a
carrier is varied by the intelligence.
v(t ) = EC sin(ωC t + m f sin ωi t )
Modulation index mf is inversely proportional to
the modulation/intelligence frequency.
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3
5.2 - FM Analysis
FM and PM signals have similar equations regarding
composition.
v(t ) = EC sin(ωC t + m f sin ωi t )
v(t ) = EC sin(ωC t + m p sin ωi t )
COM455 1001 Principles of FM
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FM Frequency Spectrum
For FM, the bandwidth and sideband frequencies
with significant amplitude depend on both deviation
and modulating frequency.
COM455 1001 Principles of FM
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4
Bessel Functions
The FM frequency components of an FM (PM)
signal can be expressed in Bessel functions.
v (t ) = EC sin(ωC t + m f sin ωi t )
Bessel functions represent normalized (EC = 1)
voltages for the various components as
J1(mf) - Amplitude for fC+/-fi
f C (t ) = J 0 ( m f ) cos ωC t
− J 1 (m f )[cos(ωC − ωi )t − cos(ωC + ωi )t ]
+ J 2 ( m f )[cos(ωC − 2ωi )t + cos(ωC + 2ωi )t ]
− J 3 ( m f )[cos(ωC − 3ωi )t − cos(ωC + 3ωi )t ]
+ ...
COM455 1001 Principles of FM
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The BW of FM
Therefore, the bandwidth of an FM signal cab be
determined by the number of significant sidebands
with significant amplitude.
Example 5-3: The BW of an FM with fi = 10kHz
and max deviation δ = 20 kHz is found as follows:
COM455 1001 Principles of FM
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5
Carson’s Rule to Predict BW
Calculating the bandwidth of an FM signal is simple,
but tedious using Bessel functions.
Carson’s Rule provides an adequate approximation
for determining FM signal bandwidth:
BW ≈ 2(δ MAX + f i MAX )
In FM, since the δMAX is unchanged, increasing
modulating frequency reduces modulation index so
it reduces the number of sidebands with significant
amplitude. On the other hand, increasing modulating
frequency increases the frequency separation
between sidebands.
COM455 1001 Principles of FM
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Broadcast FM
In North America, standard broadcast FM allows a
maximum intelligence frequency (fiMAX) of 15kHz
and has a maximum deviation (δ
δMAX) of 75kHz.
This leads to an allocated BW of 200kHz, which
corresponds to 2 δMAX plus two 25kHz guard bands.
Carson’s Rule: BW ≈ 2(δ
δMAX+ fiMAX) = 180 kHz.
When a station broadcasts with δMAX , it’s referred as
100 % modulation in FM. (% modulation=δ
δ/δ
δMAX)
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6
Deviation Ratio (DR)
Another way to describe the modulation index is by
deviation ratio (DR).
DR =
f dev(max)
max freq dev
=
max input freq
f i (max)
For standard broadcast FM, the deviation ratio (DR)
allowed is 75kHz/15kHz = 5. For broadcast
television (NTSC) audio signal, the allowed DR is
25kHz/15kHz = 1.67
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Narrowband and Wideband FM
There are no theoretical limits to the modulation
index or the frequency deviation of an FM signal.
The limits are a practical compromise between
signal-to-noise ratio and bandwidth.
Government regulations limit the bandwidth of FM
transmissions in terms of δMAX and fiMAX
FM systems with DR ≥ 1 are considered to be
wideband FM systems, while systems with DR < 1
are considered to be narrowband FM systems. A
strict definition of the term narrowband FM refers to
a signal with mf of less than 0.5
Narrowband FM (NBFM) is used for voice
transmissions. Wideband FM (WBFM) is used for
most other transmissions.
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7
5.3 - FM Noise Suppression
One of the original reasons for developing FM was to
give improved performance in the presence of noise,
which is still one of the major advantages over AM.
COM455 1001 Principles of FM
1
AM Noise Susceptibility
Noise Spikes
on AM Signal
Noise Spikes
on Detected Signal
D
COM455 1001 Principles of FM
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C
R
2
1
Limiting Action of FM Receivers
FM receivers reduce further the effects of noise spikes
by limiting the signal amplitude of the received signal.
Limiter
BPF
COM455 1001 Principles of FM
When an FM signal is clipped
in the receiver, the detected
signal is said to be quieting.
3
FM Noise Analysis
Noise must introduce phase or freq variation to cause
problems with FM signal.
One way to approach the problem of FM and noise is
think of noise as a phasor of random amplitude and
phase angle.
COM455 1001 Principles of FM
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2
Preemphasis/Deemphasis
Pre-emphasis is can be
used before modulation
to reduce the effect of
noise on high frequency
components in the
modulating signal. At
the receiver, deemphasis must be
added.
COM455 1001 Principles of FM
5
5.4 - FM Generation
FM Transmitters typically use the following
components and configurations to generate FM
signal:
– Direct-FM Modulators
– Frequency Multipliers
– Phase-Locked Loop FM Generators
– Indirect-FM Modulators
– Digital FM Modulators
COM455 1001 Principles of FM
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3
Varactor Diode Modulator
Capacitance of diode varies with reverse bias voltage
When Ei applied, the change in itsamplitude it causes
the resonant frequency of the tank to vary since D1 is
in parallel
with C1
- producing
FM
COM455 1001 Principles of FM
7
5.5 - FM Slope Detector
The easiest FM detector (discriminator) to
understand is the slope detector. It uses a LC tank
that is detuned from
the carrier fC (or fIF)
so that the output
amplitude will increase
or decrease as the
frequency of the
received FM signal
varies.
Output is then
applied to…
COM455 1001 Principles of FM
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4
5.6 - PLL and Applications
One of the most important devices used in communication
systems today is the Phase Locked Loops (PLL).
PLL is a close-loop system that use negative feedback to
maintain constant output frequency. More specifically, it
locks on the phase and/or frequency of an input signal.
Some examples of PLL applications are:
–
–
–
–
–
–
–
FM modulation and demodulation
Frequency synthesis and multiplication
Data synchronization and conditioning
Voltage-to-Frequency conversion
Tone decoding
Frequency Shift Keying (FSK)
Motor Speed Control
You can find PLLs used in CD/DVD players, modems, GPS
devices, PCs, cell phones, radio, etc.
COM455 1001 Principles of FM
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Three Possible States of PLL
The PLL has three possible states of operation
–
–
–
Free-running
Capture
Locked (or tracking)
COM455 1001 Principles of FM
COM455 1001 Communications Fundamentals
(when fVCO is far away from fin)
(when fVCO is close to fin)
( fVCO = fin)
10
5
PLL Example
Example 6-2: A PLL is set up by the external components
so that its VCO free-runs at 10MHz. The VCO does not
change until the input is within 50kHz of 10MHz. After
reaching tracking/locked condition, the VCO follows the
input to 200kHz of 10MHz before the VCO starts to freerun again. Determine the lock & capture ranges of the PLL.
Solution:
The capture occured at 50kHz from the free - run frequency,
when symmetric, the capture range is 50kHz x 2 = 100kHz.
COM455 1001 Principles of FM
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Various PLL Block Diagrams
A PLL can be represented in different diagrams.
ve
fref
Phase
detector
Loop
Filter
Amp
fref
vco
fo
fo
φ
COM455 1001 Principles of FM
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A
12
6
PLL FM Demodulator
When an input FM signal within the capture range of
the PLL, the filtered error voltage output is the
demodulated output.
Demodulated
signal
fIN
FM
input
Phase
detector
Loop
Filter
Ø
LPF
Amp
Amp
vco
fVCO
Audio
output
vco
COM455 1001 Principles of FM
13
Example of FM Receiver Using PLL
The VCO is acting as the FM signal source as the
input of the FM receiver (the PLL).
Generating FM Signal
COM455 1001 Principles of FM
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Demodulating FM Signal
14
7
Frequency Synthesizer
The frequency synthesizer provides stable, accurate
and tunable oscillators that use only a single crystal.
fREF
fOUT
φ
XTAL
÷N
The frequency synthesizer is
comprised of a PLL which
has a frequency divider
placed in its feedback loop.
COM455 1001 Principles of FM
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PLL FM Transmitter
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8