Question E4D06:
What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?  

Choices:  
A: Reciprocal mixing  
B: Quieting  
C: Desensitization  
D: Cross modulation interference    

Correct Answer is  C: Desensitization  

Correct answer: C — Desensitization

1) Why it’s correct
- Desensitization (“desense”) is the loss of receiver sensitivity when a strong off-frequency or nearby signal drives the RF front end or AGC, causing gain compression or raising the effective noise floor. The result is that a weak on-frequency signal that was previously copyable is no longer heard.

Useful sensitivity relation:
- Minimum Discernible Signal (MDS) without interference:
  $$\text{MDS} \approx -174\,\text{dBm/Hz} + 10\log_{10}(B) + NF$$
- With desense, the effective noise floor is raised:
  $$\text{MDS}' = \text{MDS} + \Delta$$
  where $\Delta$ is the desense due to the strong nearby signal.

2) Why the other choices are incorrect
- A: Reciprocal mixing
  - This is degradation from LO phase noise: a strong adjacent signal mixes with the LO’s phase noise sidebands, adding noise into the passband. It reduces SNR but is specifically tied to LO phase-noise mechanisms, not general front-end overload/AGC “desense.”
- B: Quieting
  - An FM effect where increasing signal strength reduces noise at the detector output. It’s an improvement in SNR with stronger signals, not a loss of sensitivity.
- D: Cross modulation interference
  - Nonlinear transfer of the modulation from a strong AM signal onto the desired signal. That’s distortion/modulation transfer, not a broad reduction in sensitivity.

3) Useful rules of thumb
- Blocking Dynamic Range (BDR) quantifies desense susceptibility:
  $$\text{BDR} = P_{\text{block}} - \text{MDS}$$
  where $P_{\text{block}}$ is the interfering signal level (at a specified frequency offset) that causes 1 dB gain reduction.
- Mitigation: use preselectors/roofing filters, enable ATT, reduce preamp gain, increase physical/antenna separation, and choose receivers with high IP3, high BDR, and low LO phase noise.