*Question E4C07:
What does the MDS of a receiver represent?*  

*Correct Answer is  B: The minimum discernible signal*  

### AI Response: (may contain errors)  

The correct answer to the question about what the MDS of a receiver represents is:

**B: The minimum discernible signal.**

### Explanation of the Correct Answer:
- **Minimum Discernible Signal (MDS)** refers to the weakest signal level that a receiver can detect and still distinguish it from the noise floor. It is a critical parameter in receiver performance, as it determines the sensitivity of the receiver.
- MDS is typically expressed in dBm (decibels relative to 1 milliwatt) and is influenced by factors such as the receiver's noise figure and bandwidth.

### Why the Other Choices are Incorrect:
- **A: The meter display sensitivity**
  - This term does not specifically relate to the performance of a receiver in terms of signal detection. It generally refers to how sensitive a meter is to changes in input, not the ability to discern weak signals.
  
- **C: The modulation distortion specification**
  - This choice refers to the distortion that can occur during the modulation process, affecting the quality of the transmitted signal. It does not pertain to the receiver's ability to detect weak signals.
  
- **D: The maximum detectable spectrum**
  - This term suggests a limit on the range of frequencies a receiver can detect, which is not the same as MDS. MDS focuses on the minimum signal level rather than the frequency range.

### Useful Formulas or Rules of Thumb:
- A common rule of thumb is that the MDS can be estimated using the formula:

$$
MDS = -174 \, \text{dBm} + 10 \log_{10}(B) + NF
$$

where:
- $B$ = bandwidth in Hz
- $NF$ = noise figure of the receiver in dB

This formula helps in understanding how bandwidth and noise figure affect the minimum discernible signal of a receiver.