Question E4D14:
What power level does a receiver minimum discernible signal of -100 dBm represent?  

Choices:  
A: 100 microwatts  
B: 0.1 microwatt  
C: 0.001 microwatts  
D: 0.1 picowatts    

Correct Answer is  D: 0.1 picowatts  

To convert dBm to power, use:
$$P_{\text{mW}} = 10^{\frac{\text{dBm}}{10}}$$
$$P_{\text{W}} = 10^{\frac{\text{dBm}-30}{10}}$$

Applying to −100 dBm:
- In milliwatts: $$P_{\text{mW}} = 10^{\frac{-100}{10}} = 10^{-10}\ \text{mW}$$
- In watts: $$P_{\text{W}} = 10^{-13}\ \text{W}$$
- Convert to picowatts: $$10^{-13}\ \text{W} = 0.1\ \text{pW}$$

Therefore, the correct answer is D: 0.1 picowatts.

Why the others are incorrect:
- A: 100 microwatts  
  - $$100\ \mu\text{W} = 0.1\ \text{mW} \Rightarrow \text{dBm} = 10\log_{10}(0.1) = -10\ \text{dBm}$$
- B: 0.1 microwatt  
  - $$0.1\ \mu\text{W} = 10^{-7}\ \text{W} = 10^{-4}\ \text{mW} \Rightarrow \text{dBm} = 10\log_{10}(10^{-4}) = -40\ \text{dBm}$$
- C: 0.001 microwatts  
  - $$0.001\ \mu\text{W} = 10^{-9}\ \text{W} = 10^{-6}\ \text{mW} \Rightarrow \text{dBm} = 10\log_{10}(10^{-6}) = -60\ \text{dBm}$$

Useful rules of thumb:
- 0 dBm = 1 mW.
- Every −10 dB is a factor of 10 less power.
- Benchmarks:  
  - −30 dBm ≈ 1 µW  
  - −60 dBm ≈ 1 nW  
  - −90 dBm ≈ 1 pW  
  - −100 dBm ≈ 0.1 pW