Question E4D09:
What is the purpose of the preselector in a communications receiver?  

Choices:  
A: To store frequencies that are often used  
B: To provide broadband attenuation before the first RF stage to prevent intermodulation  
C: To increase the rejection of signals outside the band being received  
D: To allow selection of the optimum RF amplifier device    

Correct Answer is  C: To increase the rejection of signals outside the band being received  

Correct answer: C — To increase the rejection of signals outside the band being received

Why C is correct:
- A preselector is a tunable band-pass (or tracking) filter placed at the receiver input. Its job is to pass the desired band while rejecting out-of-band signals, including strong nearby transmitters and image frequencies. This improves strong-signal handling and reduces desense and intermod in the mixer/LNA.

Why the others are incorrect:
- A: Storing often-used frequencies is a memory/VFO function, not a filtering (preselector) function.
- B: “Broadband attenuation” is just an RF attenuator (pad). It reduces all signals equally, including the desired one. A preselector is selective, not broadband; it attenuates mainly out-of-band signals.
- D: Choosing the RF amplifier device is a design choice; a preselector doesn’t select components—it filters signals.

Useful formulas and rules of thumb:
- Band-pass selectivity (quality factor):
  $$Q = \frac{f_0}{\text{BW}_{3\,\text{dB}}}$$
  Higher $Q$ → narrower bandwidth → better out-of-band rejection.

- Superhet image frequency (high-side or low-side injection):
  $$f_{\text{image}} = f_{\text{RF}} \pm 2 f_{\text{IF}}$$
  The preselector helps suppress the image response at $f_{\text{image}}$.

- Intermodulation reduction:
  - If out-of-band interferers are reduced by $\Delta$ dB before the mixer, third-order IMD products drop by about $3\Delta$ dB.

- Noise figure impact:
  - A passive preselector ahead of the first gain stage adds roughly its insertion loss to the receiver noise figure (rule of thumb: every 1 dB of loss ≈ +1 dB NF). Hence, balance selectivity versus loss.