In a superheterodyne receiver with an intermediate frequency (IF) of 450 kHz, if the receiver is tuned to a signal frequency of 1200 kHz, what is the corresponding image frequency?

Difficulty: Medium

Correct Answer: 2100 kHz

Explanation:


Introduction / Context:
The concept of image frequency is central to understanding the operation of superheterodyne receivers. The image frequency is an undesired signal that can also be converted to the same intermediate frequency (IF), potentially causing interference if not rejected by the RF front-end.


Given Data / Assumptions:

  • Signal frequency fs = 1200 kHz.
  • Intermediate frequency fi = 450 kHz.
  • Local oscillator frequency chosen as flo = fs + fi = 1650 kHz (high-side injection).


Concept / Approach:
The image frequency fsi is defined as fsi = flo + fi or fsi = flo − fi depending on injection side. For high-side injection (LO above signal), fsi = flo + fi = 1650 + 450 = 2100 kHz.


Step-by-Step Solution:

flo = fs + fi = 1200 + 450 = 1650 kHz.fsi = flo + fi = 1650 + 450 = 2100 kHz.Therefore, image frequency = 2100 kHz.


Verification / Alternative check:

Alternative formula: fsi = fs + 2fi = 1200 + 900 = 2100 kHz (same result).


Why Other Options Are Wrong:

750 and 990 kHz correspond to differences, not images.1650 kHz is the LO frequency, not the image.1350 kHz is arbitrary, not related to IF image equation.


Common Pitfalls:

Confusing LO frequency with image frequency.Using wrong formula: fsi = fs − 2fi (applies only for low-side injection).


Final Answer:

2100 kHz

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion