Question E4C09:
Which of the following choices is a good reason for selecting a high IF for a superheterodyne HF or VHF communications receiver?  

Choices:  
A: Fewer components in the receiver  
B: Reduced drift  
C: Easier for front-end circuitry to eliminate image responses  
D: Improved receiver noise figure    

Correct Answer is  C: Easier for front-end circuitry to eliminate image responses  

Correct answer: C — Easier for front-end circuitry to eliminate image responses

1) Why C is correct
- In a superheterodyne, the IF is set by mixing the RF with the LO:  
  $$f_{IF} = |f_{LO} - f_{RF}|$$
- An image signal at frequency $f_{IMG}$ will also mix to the same IF. The image lies:
  $$f_{IMG} = f_{RF} \pm 2f_{IF}$$
- The separation between the desired signal and its image is:
  $$\Delta f_{image} = |f_{IMG} - f_{RF}| = 2f_{IF}$$
- A higher IF makes $2f_{IF}$ larger, pushing the image farther away. That makes it much easier for the RF front-end tuned circuits (preselector) to attenuate the image.

Quick example:
- If $f_{RF} = 14\,\text{MHz}$ and $f_{IF} = 455\,\text{kHz}$, then $\Delta f_{image} = 910\,\text{kHz}$ (only about 6.5% of 14 MHz — hard to reject with simple RF tuning).
- If $f_{IF} = 9\,\text{MHz}$, then $\Delta f_{image} = 18\,\text{MHz}$ — far easier to reject.

2) Why the other choices are incorrect
- A: Fewer components in the receiver  
  Not inherently true. High IF can still require robust IF filtering and stable LO generation. Many designs use more components (e.g., dual conversion: high first IF for image rejection, low second IF for selectivity).
- B: Reduced drift  
  Drift is primarily tied to LO stability, not the IF value. Higher-frequency oscillators can drift more, so a higher IF does not guarantee reduced drift.
- D: Improved receiver noise figure  
  Noise figure is dominated by the RF front end and mixer. The choice of IF does not by itself improve NF; in some cases, higher IF can even increase mixer conversion loss.

3) Useful formulas and rules of thumb
- IF definition:  
  $$f_{IF} = |f_{LO} - f_{RF}|$$
- Image frequencies:  
  $$f_{IMG} = f_{RF} \pm 2f_{IF}$$
- Image separation:  
  $$\Delta f_{image} = 2f_{IF}$$
- Rule of thumb: Use a high first IF to push images far away (easy RF image rejection), then a low second IF for steep, narrow selectivity.