*Question G8A05:
What type of modulation varies the instantaneous power level of the RF signal?*  

*Correct Answer is  D: Amplitude modulation*  

### AI Response: (may contain errors)  

In the context of radio frequency (RF) signals, modulation is the process of varying a property of the carrier wave to transmit information. Let's examine the choices:

1. **Amplitude Modulation (AM)**:  
   - **Correct Answer**: Amplitude modulation varies the instantaneous power level of the RF signal. In AM, the amplitude (or strength) of the carrier wave is varied in proportion to the information signal being sent. This means that the power level of the RF signal changes as the amplitude changes.
   - **Formula**: The modulated signal can be represented as:  
     $$s(t) = [A_c + m(t)] \cdot \cos(2\pi f_c t)$$  
     where:
     - $A_c$ = amplitude of the carrier wave
     - $m(t)$ = message signal (information)
     - $f_c$ = carrier frequency

2. **Phase Modulation (PM)**:  
   - **Incorrect**: Phase modulation varies the phase of the carrier wave, not the amplitude. The power level remains constant, but the phase of the wave changes according to the information signal.

3. **Frequency Modulation (FM)**:  
   - **Incorrect**: Frequency modulation varies the frequency of the carrier wave. Like phase modulation, the power level remains constant, and only the frequency changes with the information signal.

4. **Power Modulation**:  
   - **Incorrect**: This is not a standard type of modulation used in RF communications. The term is misleading as it suggests varying power directly, but in practice, modulation types like AM, FM, and PM are used to encode information.

In summary, amplitude modulation is the only type of modulation among the choices that directly varies the instantaneous power level of the RF signal by changing its amplitude.