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Power of a PM Signal


Power of a PM Signal

Power of a PM (Phase Modulated) Signal

1. PM Signal Form

A phase modulated signal is written as:

s(t) = Ac cos(ωc t + kp m(t))

Where:

  • Ac = carrier amplitude
  • kp = phase sensitivity
  • m(t) = message signal

2. Total Power of PM Signal

PPM = Ac2 / (2R)

This is the same as carrier power.

3. Key Concept

  • Amplitude remains constant
  • Only phase changes
  • Power depends on amplitude, so it remains constant

4. Power Distribution

  • Total power is constant
  • Distributed among:
    • Carrier
    • Infinite sidebands (theoretical)
  • As modulation increases:
    • Carrier power decreases
    • Sideband power increases
    • Total power remains unchanged

5. Key Observations

  • Power is independent of modulation index
  • Same behavior as FM
  • More efficient than AM

Summary

  • PM Power Formula: P = Ac2 / (2R)
  • Power remains constant
  • Only spectral distribution changes

Comparison

Modulation Power Behavior
AM Varies with modulation index
FM Constant
PM Constant

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