Question

The occurrence of double spotting indicates that _________.

• A. the IF is too high
• B. the selectivity is poor
• C. image rejection capability of the receiver is inadequate
• D. the local oscillator frequency is less than that of the incoming signal

Hint:

Double spotting happens when the receiver can't block image frequencies properly—enhance image rejection to fix this.

Answer 1

The Correct Option is C

Double spotting is a phenomenon observed in superheterodyne receivers where the same station is heard at two different points on the tuning dial. This typically happens when the receiver is not adequately rejecting the image frequency.
What is Image Frequency?
When a signal is received at frequency \(f_s\), and the local oscillator is tuned to \(f_{LO}\), the intermediate frequency \(f_{IF}\) is given by: \[ f_{IF} = |f_{LO} - f_s| \] However, any other signal at a frequency: \[ f_{image} = f_{LO} + f_{IF} \] can also produce the same intermediate frequency when mixed in the receiver. If the receiver lacks sufficient filtering, this image signal can also be processed, leading to the same output—this causes double spotting.
Why does it happen?
Inadequate image frequency rejection causes the receiver to respond to both the actual signal and its image. This leads to the same station being heard at two nearby frequencies—known as "double spotting".
Incorrect Options Explained:
Option (1): High IF may help with image rejection, not cause double spotting
Option (2): Poor selectivity causes adjacent channel interference, not double spotting.
Option (4): LO being lower does not directly cause double spotting.
Final Answer: (3) image rejection capability of the receiver is inadequate