Question E4C14:
What is the purpose of the receiver IF Shift control?  

Choices:  
A: To permit listening on a different frequency from the transmitting frequency  
B: To change frequency rapidly  
C: To reduce interference from stations transmitting on adjacent frequencies  
D: To tune in stations slightly off frequency without changing the transmit frequency    

Correct Answer is  C: To reduce interference from stations transmitting on adjacent frequencies  

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IF Shift (often called Passband Tuning) slides the IF filter’s passband relative to the signal without changing your receive or transmit frequency. This lets you place an interfering signal on the steep skirt of the filter while keeping the desired signal inside the passband.

Why C is correct (reduce adjacent-channel interference):
- By shifting the passband, you can attenuate a nearby interfering station that’s just outside your channel while preserving the wanted signal. This is exactly what IF Shift is for.

Why the other choices are incorrect:
- A: “Listen on a different frequency from the transmit frequency” describes split operation or RIT/XIT (Receiver/Transmitter Incremental Tuning), not IF Shift. IF Shift doesn’t change your RX or TX frequency.
- B: “Change frequency rapidly” is done with the VFO or memories. IF Shift only moves the filter passband, not the tuned frequency.
- D: “Tune in stations slightly off frequency without changing the transmit frequency” is RIT/Clarifier. IF Shift can improve copy, but it does not actually retune the receiver to the station; it changes which parts of the spectrum your IF filter passes.

Useful relationships and rules of thumb:
- Let the SSB filter bandwidth be $B$ (e.g., $B \approx 2.4\,\text{kHz}$). If an interferer is offset by $f_i$ Hz, choose an IF shift $\Delta f$ so the desired signal remains inside the passband while the interferer is pushed toward or beyond the filter edge:
  - Keep desired: $|0 - \Delta f| \le B/2$
  - Push QRM out: $|f_i - \Delta f| > B/2$
- Typical bandwidths: SSB $B \approx 2.1\text{–}2.8\,\text{kHz}$; CW $B \approx 250\text{–}500\,\text{Hz}$.
- Rule of thumb:
  - Use IF Shift/PBT to reduce adjacent-channel QRM.
  - Use RIT/Clarifier to tune a slightly off-frequency station without moving your TX frequency.
  - Use split for intentionally different TX/RX frequencies.

Correct answer: C
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