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

Choices:  
A: The meter display sensitivity  
B: The minimum discernible signal  
C: The modulation distortion specification  
D: The maximum detectable spectrum    

Correct Answer is  B: The minimum discernible signal  

Correct answer: B — The minimum discernible signal

1) Why B is correct
- MDS is the weakest input signal level that a receiver can reliably distinguish from its own noise, usually defined for a specific bandwidth and a small output SNR (often about 3 dB).
- It effectively represents the receiver’s sensitivity limit or noise floor in practical terms.

2) Why the others are incorrect
- A: Meter display sensitivity — Not a standard receiver spec; MDS is unrelated to a front-panel meter.
- C: Modulation distortion specification — Distortion specs are IMD, THD, SINAD, etc., not MDS.
- D: Maximum detectable spectrum — Not a recognized term; MDS is about minimum signal level, not spectrum span.

3) Useful formulas and rules of thumb
- Thermal noise power:
  $$N = kTB$$
  where $k$ is Boltzmann’s constant, $T$ is temperature in kelvins, and $B$ is bandwidth in Hz.

- In dBm:
  $$N_{dBm} \approx -174\ \text{dBm/Hz} + 10\log_{10}(B) + NF$$
  where $B$ is in Hz and $NF$ is the receiver noise figure in dB.

- MDS approximation (for a chosen output SNR criterion):
  $$MDS \approx N_{dBm} + SNR_{min}$$
  Often $SNR_{min} \approx 3\ \text{dB}$ for CW in narrow bandwidths.

- Bandwidth rule of thumb: Halving $B$ lowers noise by 3 dB, improving MDS by 3 dB (more negative is better).
- Example (500 Hz BW, NF = 6 dB):
  $$N \approx -174 + 10\log_{10}(500) + 6 \approx -174 + 27 + 6 = -141\ \text{dBm}$$
  $$MDS \approx -141 + 3 \approx -138\ \text{dBm}$$
  Typical high-performance HF receivers show MDS around −135 to −140 dBm in 500 Hz.