*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 phase difference between the I (In-phase) and Q (Quadrature) RF signals is crucial for modulation and demodulation. 

### Correct Answer: 90 degrees

- **Explanation**: 
  - The I and Q signals are used to represent a single RF signal in a way that allows for efficient modulation schemes, such as Quadrature Amplitude Modulation (QAM).
  - The I signal is the real part, while the Q signal is the imaginary part of the complex representation of the signal. 
  - A phase difference of 90 degrees (or $\frac{\pi}{2}$ radians) ensures that the I and Q signals are orthogonal, meaning they do not interfere with each other. This orthogonality allows for the simultaneous transmission of two signals without distortion.

### Why the Other Choices are Incorrect:

- **A: Zero**  
  - If the phase difference were zero, the I and Q signals would be identical, leading to no ability to distinguish between them. This would not allow for effective modulation.

- **C: 180 degrees**  
  - A 180-degree phase difference would mean that the I and Q signals are completely out of phase, effectively canceling each other out. This would not work for modulation as it would not allow for the representation of different states.

- **D: 45 degrees**  
  - A 45-degree phase difference does not provide the necessary orthogonality between the I and Q signals. This would lead to interference and a loss of the ability to accurately demodulate the signal.

### Summary:
- The I and Q signals must be 90 degrees apart to maintain orthogonality, allowing for effective modulation and demodulation in SDR systems. This is a fundamental principle in digital communications.