Broadcast superheterodyne receiver image frequency: A broadcast superheterodyne receiver without an RF amplifier has a loaded Q of the antenna coupling circuit equal to 100, with an intermediate frequency (IF) of 455 kHz. If the tuned signal is at 25 MHz, what will be the corresponding image frequency?

Introduction / Context:
At higher frequencies, image rejection becomes difficult without an RF amplifier stage. This question demonstrates how to calculate the image frequency for a tuned broadcast receiver.

Given Data / Assumptions:

• Tuned signal frequency f_s = 25 MHz.
• Intermediate frequency IF = 455 kHz.
• No RF amplifier stage is present, so image response is significant.

Concept / Approach:

For a superheterodyne receiver: Local oscillator frequency f_lo = f_s + IF. Image frequency f_i = f_lo + IF = f_s + 2IF.

Step-by-Step Solution:

Verification / Alternative check:

Alternative: f_i = f_s + 2IF = 25.000 + 0.910 = 25.910 MHz. Matches.

Why Other Options Are Wrong:

• 24.09 MHz and 24.54 MHz: represent lower side frequencies, not correct images.
• 25.45 MHz: this is the LO frequency, not the image.
• 26.50 MHz: unrelated.

Common Pitfalls:

• Forgetting that the image is separated by 2*IF, not IF.
• Confusing LO frequency with image frequency.

Final Answer:

25.91 MHz.