Amplitude Modulation

Basic AM

• Modulation– Amplitude– Spectrum

• Carrier• Sideband

– Modulation index • 0 < m < 1• How to measure?

– Modulated carrier vs baseband signal

• What happens when m>1?

tEtmtv CCm cos)cos1()(

mC

C

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

– Side band

– Information in side band– Carrier for synchronization

R

E

R

tE

R

VP cccc 2

)(sin 2222

4

2c

usblsb

PmPP

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– RF amplifier– Bandpass filter

• Local Oscillator• Mixer

– Nonlinear device or balanced mixer– Bandpass filter– Signal proportional to local oscillator strength

• Intermediate Frequency (AM at 455 kHz)– No further tuning

• EMI from Local Oscillator

tEtmtv CCmRF cos)cos1()(

tEtv IFCLOLO )cos()(

tEEtmtv IFLOCmIF )cos()sin1()(

Detection• Baseband Signal• Coherent Detection

– AM modulation– Local oscillator– Mixer– LP filter– AC coupling– High sensitivity for low

S/N

tEtAmtv CCRF cos))(1()(

tEtv CLOLO cos)(

ttEEtAmtv CCLOCM coscos))(1()(

2/))(1()( LOCLP EEtAmtv

2/)()( LOCO EEtAmtv

)()( tAmtvRF

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

tEtAmtv CCRF cos))(1()( tEtAmtv CCsq 222 cos))(1()(

2/))(1()( 22CLFRF EtAmtv

2)()( CLF EtAmtv

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/loadCategory.do

(Search: FM AM)

– Matlab project• http://www.cse.ucsc.edu/classes/ee151/Winter01/

– Software Radio (http://www.wpi.edu/Pubs/E-project/Available/E-project-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 AmplifierImpedance 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-and-Computer-Science/6-776Spring-2005/025E9FB7-46C3-4718-98EA-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

– Baseband signal power:– Carrier power: – Noise spectral density: – Double sideband bandwidth: – Total noise power:

• Coherent Detection– In phase noise only– S/N

)sin()cos(cos))(1()( tntntEtAmte CSCCCC

2/2CC ES

2/0N

2/2/2 220 sc nnNWN

W22

0222 2// WNEAmNS C

2/222CM EAmS

2/2/))(1()( LOCLOCD EnEEtAmte

AM Noise Analysis

• Envelope Detection

– Signal Dominant Regime

• Similar to coherent detection

– Noise Dominant Regime• Noise large• Signal small with random phase

• Numerical Analysis

CCsCC nEtAmnnEtAmte ))(1(])))(1[()( 22

cos))(1()( 22CsC EtAmnnte

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