AMPLITUDE MODULATION

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AMPLITUDE MODULATION

• DEFINING AM
• A carrier frequency whose amplitude is varied in
  proportion to the instantaneous amplitude of a
  modulating voltage
• GENERATING THE AM
• nonlinear device diode or transistor biased in
  its nonlinear region

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ANALYSIS OF THE AM WAVE
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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
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Moduiation Index and Power
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Current Calculations
Example A 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)

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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 modulating
  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 modulating 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 application.