lord loh. said:
I would like to know what exactly are image signals (in the context of superhetrodyne receivers) and what is their origin?
My textbook says that it is to be eliminated in the RF amplifier because if it reaches the mixer, it can never be eliminated.
The same textbook says that, the tuned stages of the IF amplifier eliminates the image signals as the lower local oscillator makes the image frequency to deviate further more from the desired IF.
These somewhat contradictory explanations have left me confused...Please help...
I would appreciate any explanations, etext, weblinks or anything that helps me clerify this idea.
Image frequency is very easy to understand.
A double-balanced mixer gives two outputs, the sum and the difference, so for a specific local oscillator frequency the input to the mixer can be either higher than the local oscillator, or lower than it, by an amount equal to the IF frequency.
For an example:
The local oscillator is running at 100MHz, and the IF is 10.7MHz. This gives reception of two frequencies 100+10.7=110.7MHz (sum) and 100-10.7=89.3MHz (difference). In this case only 89.3MHz is in the FM broadcast band, so the front end tuning should be tuned to 89.3MHz as well - attenuating the unwanted 110.7MHz signal (if there was one).
You will notice that the wanted signal and image frequency are 21.4MHz apart, this is double the IF frequency - for the image rejection to be effective, the two frequencies need to be as far apart as possible, which is why 10.7MHz is used for VHF, and 455KHz for much lower frequencies.
Very high end receivers often use much higher IF's, often higher than the tuning range of the receiver! - in this way the image is shifted too far away to give any problems, and a simple low-pass filter is all that's required on the input.
And YES, you MUST get rid of the image before the mixer, it's the front end tuning which does this!. The IF frequency simply sets where the image is, at twice the IF frequency away from the desired frequency.