*Question G7C09:
What is the phase difference between the I and Q RF signals that software-defined radio (SDR) equipment uses for modulation and demodulation?*  

*Correct Answer is  B: 90 degrees*  

### AI Response: (may contain errors)  

In software-defined radio (SDR) systems, the I (In-phase) and Q (Quadrature) signals are used for modulation and demodulation. These signals are essential for representing complex signals in a way that can be easily processed by digital systems.

1. **Correct Answer: 90 degrees**  
   - The I and Q signals are separated by a phase difference of 90 degrees. This means that the Q signal is a quarter of a cycle (or 90 degrees) out of phase with the I signal. This orthogonal relationship allows for the representation of both amplitude and phase information of a signal, which is crucial for complex modulation schemes like QAM (Quadrature Amplitude Modulation).

2. **Why the other choices are incorrect:**
   - **Zero degrees (Choice A):** If the phase difference were zero degrees, the I and Q signals would be identical, and you would not be able to separate the two components needed for complex signal representation.
   - **180 degrees (Choice C):** A 180-degree phase difference would mean the signals are in opposite phases, which does not provide the orthogonal relationship needed for I/Q modulation.
   - **45 degrees (Choice D):** A 45-degree phase difference would not provide the necessary orthogonality. The signals would not be fully independent, which is required for accurate signal representation and processing.

3. **Useful Concept:**
   - The orthogonality of I and Q signals (90-degree phase difference) allows for the separation of the real and imaginary components of a signal, which is fundamental in digital signal processing for SDRs. This is why the 90-degree phase difference is critical for the proper functioning of SDR systems.