*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 type of demodulator used in radio receivers, particularly for single sideband (SSB) and CW (continuous wave) signals. Here's why the correct answer is D:

1. **Correct Answer Explanation (D):**
   - In a single sideband (SSB) receiver, the product detector is used to extract the modulated signal. It mixes the incoming SSB signal with a locally generated carrier signal to produce an audio frequency output. This process effectively demodulates the SSB signal, allowing the original audio information to be recovered.

2. **Why Other Choices Are Incorrect:**
   - **Choice A:** A product detector is not used in test gear to detect spurious mixing products. Test equipment typically uses spectrum analyzers or other specialized circuits to identify spurious signals.
   - **Choice B:** A product detector is not used in a transmitter for frequency multiplication. Frequency multiplication is typically achieved using nonlinear devices like frequency multipliers, not product detectors.
   - **Choice C:** A product detector is not used in an FM receiver to filter out unwanted sidebands. FM receivers use different demodulation techniques, such as frequency discriminators or phase-locked loops, to recover the modulated signal.

3. **Useful Concepts:**
   - The product detector works by mixing the incoming RF signal with a local oscillator signal. The mixing process results in sum and difference frequencies, and the desired audio signal is extracted from the difference frequency.
   - This is particularly useful for SSB signals, where the carrier is suppressed, and the product detector helps in recreating the carrier to demodulate the signal effectively.

Understanding the role of a product detector in SSB receivers is crucial for recognizing its function in demodulating signals without a carrier.