*Question G7C04:
How is a product detector used?*  

*Correct Answer is  D: Used in a single sideband receiver to extract the modulated signal*  

### AI Response: (may contain errors)  

A product detector is a crucial component in single sideband (SSB) receivers, and here's why the correct answer is D: "Used in a single sideband receiver to extract the modulated signal."

### Explanation of the Correct Answer:
- **Product Detector in SSB Receivers**: 
  - A product detector works by mixing the incoming SSB signal with a locally generated carrier signal. This process demodulates the SSB signal, allowing the original audio signal to be recovered.
  - The mixing process produces two new frequencies: the sum and the difference of the original frequencies. The difference frequency corresponds to the original audio signal, which is what we want to extract.

### Why the Other Choices are Incorrect:
- **Choice A: Used in test gear to detect spurious mixing products**  
  - While product detectors can be used in test equipment, their primary function is not to detect spurious products but to demodulate signals. This choice does not accurately describe the main application of a product detector.

- **Choice B: Used in a transmitter to perform frequency multiplication**  
  - Frequency multiplication is typically achieved using non-linear devices like frequency multipliers, not product detectors. Product detectors are not used in transmitters for this purpose.

- **Choice C: Used in an FM receiver to filter out unwanted sidebands**  
  - FM receivers use different techniques, such as phase-locked loops (PLLs) or discriminators, to demodulate FM signals. Product detectors are not the standard method for filtering sidebands in FM.

### Summary:
- The product detector is essential for demodulating SSB signals in receivers, making choice D the correct answer. 
- Understanding the specific applications of different types of detectors will help you in the exam and practical radio operations.