Question E4D07:
Which of the following reduces the likelihood of receiver desensitization?  

Choices:  
A: Insert attenuation before the first RF stage  
B: Raise the receiver’s IF frequency  
C: Increase the receiver’s front-end gain  
D: Switch from fast AGC to slow AGC    

Correct Answer is  A: Insert attenuation before the first RF stage  

To reduce receiver desensitization, you want to prevent strong signals from overloading the first RF stages (RF amp/mixer). The correct choice is A.

Why A is correct (Insert attenuation before the first RF stage):
- A small RF attenuator at the input lowers all incoming signals, reducing the chance of front-end overload and intermodulation. This directly combats desense.
- For third-order intermodulation, an input attenuator of $A$ dB reduces the IMD products relative to the desired signal by about $2A$ dB.  
  - Fundamental drops by $A$ dB (linear), IMD3 drops by $3A$ dB (cubic), so IMD-to-signal improves by $2A$ dB.
- Practical tip: Engaging the receiver’s RF attenuator (e.g., 6–10 dB) often restores linear operation in strong-signal environments.

Why the others are incorrect:
- B: Raise the receiver’s IF frequency  
  - Changing IF may affect image rejection, but desense is caused by RF front-end overload. IF choice doesn’t prevent the first RF stage/mixer from being driven too hard.
- C: Increase the receiver’s front-end gain  
  - More gain makes overload and AGC action happen sooner, worsening desense.
- D: Switch from fast AGC to slow AGC  
  - AGC timing affects how quickly gain changes, not the intrinsic overload limit. Slow AGC can even prolong reduced gain after a strong signal, effectively worsening the perceived desense.

Useful formulas and rules of thumb:
- Third-order IMD improvement with input attenuation:  
  $$\Delta(IMD_{\text{rel}}) \approx 2A \text{ dB}$$
  where $A$ is the input attenuation in dB.
- Desense mechanisms to watch:  
  - 1 dB compression (P1dB) exceeded → gain compresses.  
  - Third-order intermod (IP3) → spurious responses rise near desired signal.
- Rule of thumb: If strong nearby signals are present, try an RF attenuator, a band-pass preselector, or reduce preamp gain to keep the first RF stage linear.