COMMUNICATION SYSTEM EECB353 Chapter 2 Part II AMPLITUDE MODULATION

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COMMUNICATION SYSTEM EECB353Chapter 2 Part
IIAMPLITUDE MODULATION

• Anas Bin Muhamad Bostamam
• Dept of Electrical Engineering
• Universiti Tenaga Nasional
• http//metalab.uniten.edu.my/shafinaz

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AM Voltage Distribution

• Mathematically an unmodulated carrier can be
  described as
• where vc(t) time-varying voltage for the
  carrier
• Ec peak carrier amplitude (volts)
• fc carrier frequency (hertz)
• But Vmax Ec Em , then the instantaneous
  modulated amplitude
• where amplitude of modulated wave
• Em peak change in the amplitude of the
  envelope (volts)
• fm frequency of the modulating signal
  (hertz)

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AM Voltage Distribution

• Em mEc
• constant modulating signal
• unmodulated carrier

Carrier Signal
Upper side freq signal
Lower side freq signal
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AM Voltage Distribution
Vam

• Notes from DSBFC eqn, Vam
• Amplitude of the carrier Ec is unaffected by the
  modulation process
• Amplitude of the upper (Eusf) and lower side
  frequencies (Elsf )depend on both carrier
  amplitude, Ec and coefficient of modulation,m.

Voltage Spectrum for AM DSBFC wave
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Example 3

• One input to a conventional AM modulator is a
  500kHz carrier with an amplitude of 20Vp. The
  second input is a 10kHz modulating signal that is
  of sufficient amplitude to cause a change in the
  output wave of ?7.5Vp. Determine
• Upper and lower side frequencies.
• Modulation coefficient and percent modulation
• Peak amplitude of the modulated carrier and the
  upper and lower side frequency voltages.
• Maximum and minimum amplitudes of the envelope.
• Expression for the modulated wave.
• Draw the output spectrum.
• Sketch the output envelope.

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Example 4

• If the modulated wave has the equation,
• find
• (a) the carrier freq
• (b) the usf and lsf
• (c) the modulating signal freq
• (d) the peak amplitude of the carrier signal
• (e) the upper and lower side signal peak
  amplitude
• (f) the change In peak amplitude of the modulated
  wave
• (g) the coefficient of modulation.

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AM Power Distribution

• The average power dissipated in a load by
  unmodulated carrier is equal to the rms carrier
  voltage, Ec squared divided by the load
  resistance, R.
• Mathematically, power in unmodulated carrier, Pc
  is

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AM Power Distribution

• The upper and lower sideband powers
• where mEc/2 is the peak voltage of usf and lsf.
• Then,
• Total transmitted power in DSBFC AM envelope

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AM Power Distribution
Power Spectrum for AM DSBFC wave

• Note
• Carrier power in the modulated signal is the same
  in the unmodulated signal i.e carrier power is
  unaffected by the modulation process.
• The total power in an AM envelope increase with
  modulation (i.e as m ?, Pt ?).
• Major disadvantage of AM DSBFC is most of the
  power is wasted in the carrier. (It does not
  contain info, info is contained in the
  sidebands).

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Efficiency of AM

• Efficiency, E is defined as the percentage of
  total power that conveys information i.e it is
  the percentage of total transmitted power that is
  in the sidebands.

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Example

• Draw the power spectrum for a given expression
  and determine the modulation index and
  efficiency. (Assume R1?)

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Example

• Determine the maximum sideband power if the
  carrier output is 1 kW and calculate the total
  maximum transmitted power.

Since ESF mEc/2, It is obvious that the max SB
power occurs when m 1 or 100, and also when m
1, each side freq is ½ the carrier
amplitude. Since power is proportional to the
square of voltage, each SB has ¼ of the carrier
power i.e ¼ x 1kW, or 250W. Therefore, total SB
power is 250W x 2 500W. And the total
transmitted power is 1kW 500W 1.5kW
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Importance of High-percentage Modulation
Table Effective transmission at 50 versus 100
modulation

• Notes
• Even though the total transmitted power has only
  fallen from 1.5kW to 1.125kW, the effective
  transmission has only ¼ the strength at 50
  modulation as compared to 100.
• Because of these considerations, most AM
  transmitter attempts to maintain between 90 and
  95 percent modulation as a compromise between
  efficiency and the chance of drifting into
  overmodulation.

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Example

• A DSBFC AM Signal with a 1kW carrier was
  modulated to a depth of 60. How much power is
  contained in the upper sideband?
• Given then,
• but, total power

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Exercise
The total power of an AM transmitter is measured
to be 850W. What is the total output sideband
power if it has a percent modulation of 100?
Calculate the efficiency.
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Example 5

• For an AM DSBFC wave with a peak unmodulated
  carrier voltage Vc 10Vp, a load resistance RL
  10?, and a modulation coefficient, m 1,
  determine
• Powers of the carrier and the upper and lower
  sidebands.
• Total sideband power.
• Total power of the modulated wave.
• Draw the power spectrum.
• Repeat steps (a) to (d) for a modulation index, m
  0.5.

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Example 6

• A 1.5MHz carrier signal is modulated with 3.4kHz
• modulating signal. The modulated carrier voltage
  is
• 28Vmax and 14Vmin across a 100O resistive load
• impedance. Determine
• a) Peak amplitude of the unmodulated carrier.
• b) Coefficient of modulation
• c) Carrier power.
• d) Sideband power
• e) Total power
• f) Upper and lower sideband frequencies.
• g) Bandwidth.