Amplitude Modulation

1
Amplitude Modulation
Bandpass Signals
Assume that m(t) is a normalized signal with
bandwidth of W
Average power of x(t) is defined as
2
Amplitude Modulation

• Double-Sideband Transmitted Carrier (DSBTC)

• Double-Sideband Suppressed Carrier (DSBSC)

• Single-Sideband (SSB)

• Vestigial Sideband (VSB)

3
Amplitude Modulation
Double-Sideband Transmitted Carrier (DSBTC)
the index of modulation
Three cases
desired condition ( normal modulation)
100-percent modulation
over modulation
4
Amplitude Modulation
Double-Sideband Transmitted Carrier (DSBTC)
m(t)
Amax Ac(1a)
Amin Ac(1-a)
alt1
agt1
5
Amplitude Modulation
Double-Sideband Transmitted Carrier (DSBTC)
Thus
6
Amplitude Modulation
Double-Sideband Transmitted Carrier (DSBTC)
M(f)
2W
Carrier
Xc(f)
Lower sideband
Upper sideband
fc-W
fcW
fc
-fc-W
-fcW
-fc
2W
7
Amplitude Modulation
Double-Sideband Suppressed Carrier (DSBSC)
m(t)
8
Amplitude Modulation
Double-Sideband Suppressed Carrier (DSBSC)
M(f)
2W
Xc(f)
fc-W
fcW
fc
-fc-W
-fcW
-fc
2W
9
Amplitude Modulation
Modulators

• Product modulator
• Square-law modulator
• Balanced modulator
• Ring modulator
• Switching modulator

10
Amplitude Modulation
Product Modulators
DSB-SC
m(t)
DSB-TC
11
Amplitude Modulation
Product Modulators
m(t)
12
Amplitude Modulation
Product Modulators
13
Amplitude Modulation
Square-law Modulator
Nonlinear element
m(t)
Filter
Nonlinear element approximation
Square-law modulator
14
Amplitude Modulation
Square-law Modulator
Square-law modulator
15
Amplitude Modulation
Balanced Modulator
Balanced modulator
16
Amplitude Modulation
Ring Modulator
Ring modulator
17
Amplitude Modulation
Switching Modulator
18
Amplitude Modulation
AM Transmitter
19
Amplitude Modulation
Single-Sideband (SSB)
20
Amplitude Modulation
Double-Sideband Suppressed Carrier (DSBSC)
X(f)
2W
Xc(f)
fc-W
fcW
fc
-fc-W
-fcW
-fc
2W
21
Amplitude Modulation
Single-Sideband (SSB)
X(f)
2W
Xc(f)
Upper sideband
fcW
fc
-fc-W
-fc
W
22
Amplitude Modulation
Single-Sideband (SSB)
X(f)
2W
Xc(f)
Lower sideband
fc-W
fc
-fcW
-fc
W
23
Amplitude Modulation
Single-Sideband (SSB)
24
Amplitude Modulation
Vestigial Sideband (VSB)
25
Amplitude Modulation
Vestigial Sideband (VSB)
26
Amplitude Modulation
Demodulators
1) Coherent demodulation

• Gated demodulator
• Square-law demodulator
• Quadrature receiver

2) Incoherent demodulator

• Square-law detector
• Rectifier detector
• Envelope Detector

27
Amplitude Modulation
Demodulators
1) Coherent demodulation
y(t)
yD(t)
xc(t)
BPF
sync
28
Amplitude Modulation
Broadcast AM and Superheterodyne Receiver
Frequency Bands
VLF 3-30 kHz LF 30-300 kHz MF 300-3,000 kHz (AM
540-1,600 kHz) HF 3-30 MHz VHF 30-300 MHz (FM
88.1-107.9 MHz VHF-TV54-216 MHz)
UHF 300-3,000 MHz (UHF-TV470-890
MHz) SHF 3-30 GHz EHF 30-300 GHz
AM fm 5kHz, carrier spacing 10kHz, IF
455kHz,107 stations FM fm 15kHz, carrier
spacing 200kHz, IF 10.7MHz
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Amplitude Modulation
Broadcast AM and Superheterodyne Receiver
30
Amplitude Modulation
Television
Picture AM Sound FM
31
Amplitude Modulation
Television