Amplitude Modulation
Basic AM
• Modulation
– Amplitude
– Spectrum
• Carrier
• Sideband
v(t )  (1  m  cosmt )  EC cosCt
C
C  m
– Modulation index
• 0<m<1
• How to measure?
– Modulated carrier vs baseband signal
• What happens when m>1?
Waveforms
• Base Band and Modulation
– m = 100%
– m = 120%
• Math
• Real
Spectrum
• Double Side Band Full Carrier
– Spectrum
• LSB, USB, Carrier
Efficiency
• Efficiency in Using Bandwidth?
– Two side bands
• Efficiency in Using Power?
– Carrier
V 2 Ec2  (sin ct )2  Ec2
Pc 


R
R
2R
– Side band
m 2 Pc
Plsb  Pusb 
4
– Information in side band
– Carrier for synchronization
Variations of AM
• Bandwidth Efficiency
– Single Side Band
– Vestigial Side Band
• Power Efficiency
– Reduced Carrier
– Suppressed Carrier
• Complication
– Circuits: generation, tuning, detection
– Filter, tone variation, carrier recovery
AM Radio
• Source
– Audio, 20-20 kHz
• Carrier Frequency
– 525 – 1705 kHz
• Performance
– 5 kHz bandwidth
– 250-500 km range
– 50 µV sensitivity
AM Radio
Commercial AM Radio
• Circuit Diagram
Commercial Radio
• Circuit Board
Block Diagram
Crucial Components
• RF Amplifier
– High dynamic range
• Autodyne Oscillator and Mixer
– Tuning
• Loop Antenna
– LC resonator
• Intermediate Frequency Amplifiers
– 455 kHz
– Tank circuit (double tuner) with flat center band
• Diode RF Rectifier
– Envelope detection
• Automatic Gain Control
– Strong vs weak stations
• Audio Amplifier
• Speaker
Heterodyne
• Incoming RF
vRF (t )  (1  m  cosmt )  EC cosCt
– RF amplifier
– Bandpass filter
• Local Oscillator
• Mixer
vLO (t )  ELO cos(C  IF )t
– Nonlinear device or balanced mixer
vIF (t )  (1  m sin mt ) EC ELO cos(IF )t
– Bandpass filter
– Signal proportional to local oscillator strength
• Intermediate Frequency (AM at 455 kHz)
– No further tuning
• EMI from Local Oscillator
Detection
• Baseband Signal
• Coherent Detection
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–
–
–
–
–
vRF (t )  m  A(t )
vRF (t )  (1  m  A(t ))  EC cosCt
AM modulation
vLO (t )  ELO cosCt
Local oscillator
vM (t )  (1  m  A(t ))  EC ELO cosCt  cosCt
Mixer
vLP (t )  (1  m  A(t ))  EC ELO / 2
LP filter
vO (t )  m  A(t )  EC ELO / 2
AC coupling
High sensitivity for low
S/N
Envelope Detection
• Circuit
– RF rectifier
– Filter
– RC time constant
– Simple for high S/N
• Waveform
Detection
• Square Law Detector
–
–
–
–
Modulation
Nonlinearity
RF filter
Low pass and AC
coupling
vRF (t )  (1  m  A(t ))  EC cosCt
vsq (t )  (1  m  A(t ))2  EC2 cos2 Ct
vRF LF (t )  (1  m  A(t ))2  EC2 / 2
vLF (t )  m  A(t )  EC2
Performance
•
•
•
•
Sensitivity
Fidelity or Bandwidth
Dynamic Range
Selectivity
– Adjacent channel
• Image Rejection
– IF
• Spurious Response
– Harmonics
• Squelch
• Noises
– Static noise from transmission media
– Motor boating noise from drift in frequency
Lab and Projects
• Virtual Laboratory
– http://www.mathworks.com/matlabcentral/fileexchange/load
Category.do
(Search: FM AM)
– Matlab project
• http://www.cse.ucsc.edu/classes/ee151/Winter01/
– Software Radio (http://www.wpi.edu/Pubs/E-project/Available/Eproject-042805-113037/unrestricted/DASR_MQP_REPORT.pdf)
• Simple AM Projects
• Experiments
– Agilent 33120 AM modulation
– MC1496
http://www.onsemi.com/pub/Collateral/AN531-D.PDF
AM Transmitter
• History
• Block Diagram
• Automatic Level Control
– m<1
AM Modulator
• Balanced Mixer
• Other Circuit
– Base band signal modulates oscillator current
Power Amplifier
Impedance Matching
• Maximize Power to the Load
• Impedance
– Z=V/I
• Change V – I Relation
– Transformer
• High current, low voltage
• Low current, high voltage
– RLC Networks
• Frequency dependent impedance
Impedance Matching
• MIT Open Courseware
– http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-andComputer-Science/6-776Spring-2005/025E9FB7-46C3-471898EA-CBD23EEC9D98/0/lec4.pdf
• Berkeley RLC Pi and T LC Network
Calculator
– http://bwrc.eecs.berkeley.edu/Research/RF/projects/60GHz/matc
hing/ImpMatch.html
• Role of Antenna
– Impedance transformation between circuit and
free space
Short Wave Radio
• Terrestrial Coverage, HAM
– http://n5xu.ece.utexas.edu/hamradio/
• Regenerative Receiver Project
– http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm
HAM
• http://www.arrl.org/hamradio.html
• http://www.qsl.net/dl4yhf/
– Spectral analysis of audio signals
• http://www.ac6v.com/
– Antenna projects
• http://www.radio.org/linux/
– Linux HAM
AM Stereo
• Two Channels
– L+R
– L-R
• Quadrature Amplitude Modulation
– Sin, Cos: orthogonal
– Increase Information Capacity Without
Increasing Bandwidth
• Motorola CQUAM System
– Details
– Chip
Modern Application of AM
• Advanced Television Systems
Committee (ATSC)
• 8-VSB Digital Signaling
AM Noise Analysis
• DSBFC Signal and Noise
e(t )  (1  m  A(t ))  EC cosCt nC cos(Ct ) nS sin(Ct )
–
–
–
–
–
Baseband signal power: SM  m2 A2 EC2 / 2
Carrier power: SC  EC2 / 2
Noise spectral density: N0 / 2
Double sideband bandwidth: 2  2W
2
2
Total noise power: N  2W  N0  nc / 2  ns / 2
• Coherent Detection
eD (t )  (1  m  A(t ))  EC ELO / 2 nC ELO / 2
– In phase noise only
S / N  m2  A2  EC2 / 2WN 0
– S/N
AM Noise Analysis
• Envelope Detection
– Signal Dominant Regime
e(t )  [(1  m  A(t )) EC )  nC ]2  ns2  (1  m  A(t )) EC  nC
• Similar to coherent detection
– Noise Dominant Regime
• Noise large
• Signal small with random phase
e(t )  nC2  ns2  (1  m  A(t )) EC cos 
• Numerical Analysis
AM Review
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•
•
•
AM Waveform
Modulation Index Limit
Double Sideband, Full Carrier
Detection Circuits
– Coherent detection
– Envelope detection
• Variations
– SSB, RC or SC
– Vestigial sideband
• AM Transmitter
– Impedance matching
– Efficiency