Question E4D01:
What is meant by the blocking dynamic range of a receiver?  

Choices:  
A: The difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression  
B: The minimum difference in dB between the levels of two FM signals that will cause one signal to block the other  
C: The difference in dB between the noise floor and the third-order intercept point  
D: The minimum difference in dB between two signals which produce third-order intermodulation products greater than the noise floor    

Correct Answer is  A: The difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression  

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Blocking dynamic range (BDR) describes how well a receiver handles a very strong off-frequency signal without losing sensitivity to a weak desired signal.

Why the correct answer is correct (A):
- BDR is defined as the difference (in dB) between the receiver’s noise floor and the level of an off-channel signal that causes 1 dB of gain compression (i.e., “blocking” the receiver).
- In other words:
  $$\text{BDR} = P_{\text{block}} - N_{\text{floor}}$$
  where:
  - $P_{\text{block}}$ = input level of a strong, frequency-offset signal that causes 1 dB gain compression
  - $N_{\text{floor}}$ = receiver noise floor (MDS)

Why the other choices are incorrect:
- B: Describes FM capture effect (co-channel FM signals), not receiver blocking or gain compression.
- C: Refers to third-order intercept (IP3) and noise floor; that’s tied to intermodulation dynamic range, not blocking dynamic range.
- D: Describes the threshold where third-order intermodulation products exceed the noise floor; again, that’s intermodulation dynamic range, not blocking.

Useful formulas and rules of thumb:
- Noise floor (per dBm):
  $$N_{\text{floor}} \approx -174 + 10\log_{10}(B) + NF \quad \text{(dBm)}$$
  where:
  - $B$ = receiver bandwidth in Hz
  - $NF$ = noise figure in dB
- Large BDR is better; typical high-performance HF receivers have BDRs > 120 dB (measured at a specified frequency offset such as 2 kHz or 20 kHz).
- $P_{\text{block}}$ is related to the receiver’s input 1 dB compression point but is measured with a strong off-frequency signal at a defined offset.

This clarifies that BDR is about strong-signal blocking via gain compression, not intermodulation products or FM capture.