Question E4C02:
Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals?  

Choices:  
A: A front-end filter or preselector  
B: A narrow IF filter  
C: A notch filter  
D: A properly adjusted product detector    

Correct Answer is  A: A front-end filter or preselector  

Correct answer: A — A front-end filter or preselector

1) Why A is correct
- A front-end filter/preselector is an RF band-pass filter placed before the first active device (RF amp/mixer). It greatly attenuates strong out‑of‑band signals so they don’t drive the front end into overload, desense, or intermodulation.
- Key idea: stop unwanted energy before it hits nonlinear stages. If an interfering signal is outside the passband, the preselector provides attenuation $A(f)$ so the interfering power at the device is reduced by:
  $$P_\text{after} = P_\text{before} \times 10^{-A(f)/10}$$
- Bandwidth of a tuned preselector is roughly:
  $$BW \approx \frac{f_0}{Q}$$
  Higher $Q$ → narrower passband → better rejection of strong out-of-band signals.

2) Why the others are incorrect
- B: A narrow IF filter
  - It sits after the first mixer. Strong out‑of‑band signals can already cause front‑end overload or intermod products before reaching the IF, so an IF filter won’t prevent those problems.
- C: A notch filter
  - A notch filter rejects a very narrow, specific frequency (often in-band). It doesn’t provide broad out-of-band rejection needed to keep the front end linear in the presence of strong off‑channel signals.
- D: A properly adjusted product detector
  - The product detector demodulates SSB/CW at audio/IF. Its adjustment doesn’t affect front‑end linearity or out-of-band attenuation.

3) Useful rules of thumb
- Best cure for strong out‑of‑band interference: filter it before the first active stage.
- Preselector effectiveness improves blocking dynamic range and reduces intermodulation by keeping undesired RF out of nonlinear devices.
- Approximate preselector attenuation benefit: every 10 dB of extra out‑of‑band rejection reduces interfering voltage by about a factor of $\sqrt{10} \approx 3.16$ and power by a factor of 10.