Subject: Principles of Communication Engineering

Difficulty : Medium

Marks : 05

Double spotting

• Double spotting is a condition where the same desired signal is detected at two nearby points on the receiver tuning dial.
• One point is the desired point while the other is called the spurious or image point.
• It can be used to determine the IF of an unknown receiver.
• Poor front-end selectivity and inadequate image frequency rejection leads to double spotting.
• Double spotting is undesirable since the strong signal might mask and overpower the weak signal at the spurious point in the frequency spectrum.
• Double spotting can be counter acted by improving the selectivity of RF amplifier and increasing the value of IF.

• Consider an incoming strong signal of 1000 kHz and local oscillator tuned at 1455 kHz. Thus a signal of 455 kHz is produced at the output of the mixer which is the IF frequency.

  Now consider the same signal but with 545kHz tuned local oscillator. Again we get 455 kHz signal at the output.

  Therefore the same 1000 kHz signal will appear at 1455 kHz as well as 545 kHz on the receiver dial and the image will not get rejected. This is known as Double spotting phenomenon.

• It is also known as Adjacent channel selectivity.