tvs, by ya bao 1 amplitude modulation 1.defining am a carrier frequency whose amplitude is varied in...
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TVS, By Ya Bao 1
AMPLITUDE MODULATION1. DEFINING AM
• A carrier frequency whose amplitude is varied in proportion to the instantaneous amplitude of a modulating voltage
2. GENERATING THE AM
nonlinear device: diode or transistor biased in its nonlinear region
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3. ANALYSIS OF THE AM WAVE
tfVv ccc 2sin
tffVm
tffVm
tfVv sccscccc )(2cos2
)(2cos2
2sin
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4. Different Carriers and AM
Carriers are spaced at 20 kHz, beginning at 100kHz. Each carrier is modulated by a signal with 5kHz bandwidth. Is there interference from sideband overlap?
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5. MODULATION INDEX AND SIGNAL POWER
minmax
minmax
VV
VV
V
Vm
c
m
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Moduiation Index and Power
R
V
R
V
R
VP cccarrc 2
)2/( 222
42
2m
R
VPP cUSBLSB
)2
1()2
1(2
222 mP
m
R
VP c
CT
21
P
P 2
c
T m )1(2
C
T
P
Pm
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Current Calculations
21
2m
I
I
c
T
ExampleA carrier of 1000 W is modulated with a resulting modulation index of 0. 8. What is the total power?
What is the carrier power if the total power is 1000 W and the modulation index is 0.95?
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6. SUPPRESSED CARRIERS AND SINGLE SIDEBAND (SSB)
6.1 Signal Magnitudes and Ranges
decibel (dB)
O
I
P
PdB log10
O
I2O
2I
V
Vlog20
V
V10logfor volts)( dB
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The dB gain is
Ratio Power Voltage
(dB) (dB)
1 0 0
10 10 20
100 20 40
1,000 30 60
10,000 40 80
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dB REFERENCE VALUES
• dBm (reference is 1 mW),
• dBV (reference is 1 V),
• dBW (reference is 1 W).
• Example:
A signal of 200 mW could be expressed in dBm as
An audio amplifier might have an output of 7 dBW. This corresponds to
A signal of 8.2 V corresponds to
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6.2 Double Sideband Suppressed Carrier (DSBSC)
When the carrier is reduced, this is called double-sideband suppressed-carrier AM, or DSB-SC. If the carrier could somehow be removed or reduced, the transmitted signal would consist of two information-bearing sidebands, and the total transmitted power would be information
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• For a 100-W total available power, compare the power in the sidebands when modulation is standard AM with a modulation index of 100%, versus a suppressed carrier design where 90% of the carrier power is suppressed. How many times greater is the sideband power in the suppressed carrier case?
• A 500-W DSB-SC system with 100% modulation suppresses 50% of the carrier, and the suppressed carrier power goes to the sidebands. How much power is in the sidebands, and how much is in the carrier? By how many dB has the sideband power increased?
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6.3 Single-Sideband (SSB)
• suppressing the carrier and one of the sidebands
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6.4 Filtering the SSB LSB or USB • Dual Conversion: up-converting the mod
ulating frequency twice and selecting the upper or lower sideband for transmission.
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7. Filtering the SSB LSB or USB
Their quality or "Q" factor: A filter is said to have a high Q if it selects or rejects a narrow range of frequencies compared with its centre frequency. Q is defined as centre frequency fo/3dB bandwidth.
Type of the filters
LC filters ceramic mechanical crystal
Order of Qs
200 2000 10,000 50,000
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Dual Conversion
up-converting the mod ulating frequency twice and selecting the upper or lower sideband for transmission.
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8. VESTIGIAL SIDEBAND
• In VSB, a portion (or vestige) of the lower sideband is transmitted along with the full carrier power and upper-sideband content.
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AM: Features and Drawbacks:
the AM signal is greatly affected by noise
impossible to determine absolutely the original signal level
conventional AM is not efficient in the use of transmitter power
AM is useful where a simple, low-cost receiver and detector is desired
The noise is either not a problem or can be tolerated in the ap plication.