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Transcript of Ref am
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