Basic AM
Modulation
Amplitude
Spectrum
Carrier
Sideband
v (t ) (1 m cos mt ) EC cos C t
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 c t )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 V 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 C t
RF amplifier
Bandpass filter
Local Oscillator
Mixer
vLO (t ) E LO cos(C IF )t
Nonlinear device or balanced mixer
v (t ) (1 m sin t ) E E cos( )t
Bandpass filter
Signal proportional to local oscillator strength
IF
LO
IF
Intermediate Frequency (AM at 455 kHz)
No further tuning
EMI from Local Oscillator
Detection
Baseband Signal
Coherent Detection
vRF (t ) m A(t )
vRF (t ) (1 m A(t )) EC cos C t
AM modulation
vLO (t ) ELO cos C t
Local oscillator
vM (t ) (1 m A(t )) EC ELO cos C t cos C t
Mixer
vLP (t ) (1 m A(t )) EC E LO / 2
LP filter
vO (t ) m A(t ) EC E LO / 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 C t
vsq (t ) (1 m A(t )) 2 EC2 cos2 C t
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-2005/025E9FB7-46C3-4718-98EA-C
BD23EEC9D98/0/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 C t n C cos(C t ) n S sin(C t )
Baseband signal power: SM m 2 A2 EC2 / 2
Carrier power: SC EC2 / 2
Noise spectral density: N 0 / 2
Double sideband bandwidth: 2 2W
2
2
N
2
W
n
/
2
n
/2
0
c
s
Total noise power:
Coherent Detection
eD (t ) (1 m A(t )) EC ELO / 2 n C E LO / 2
In phase noise only
2
2
2
S
/
N
E
/ 2WN 0
C
S/N
AM Noise Analysis
Envelope Detection
Signal Dominant Regime
e(t ) [(1 m A(t )) EC ) n C ]2 ns2 (1 m A(t )) EC n C
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
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
