• In the broadcast AM receivers the total oscillator frequency is higher than the incoming signal frequency by intermediate frequency i.e.

  $f_0=f_s+I.F.$ …………………….. (1)

  OR

  I.F. =$( f_0-f_s)$ ------------- (2)

• If two frequency components $f_1$ and $f_2$ are mixed in a mixer then mixer output consists of following components. If ($f_1$> $f_2$)

• Output of mixer: $f_1 , f_2 (f_1- f_2) and ( f_1+ f_2)$…………...(3)

• Out of which the difference component i.e. $( f_1- f_2)$ is selected using a tuned circuit after the mixer.

• Now assume that the local oscillator frequency is set to “$f_0$ " and an unwanted signal at frequency $f_{si}$ =$(f_0+IF)$ manages to reach at the input of the mixer along with the desired signal frequency $f_s$ . Then the mixer output consists of the four frequency components corresponding to $f_{si}$ as follows:

  $f_{si},(f_0+IF),(2f_0+IF)$and IF……………………………… (4)

• Where the last component at IF is the difference between $f_{si}$ and $f_0$. This component will also be amplified by the IF amplifier along with the desired signal at frequency $f_s$.

• This will create interference because both the stations corresponding to carrier frequencies $f_s$ and $f_{si}$ will be tuned at the same position.

• This unwanted signal at frequency $f_{si}$ is known as “image frequency” and it is said to be the “image” of the signal frequency $f_s$. The relation between $f_s$ and $f_{si}$ is:

  Image frequency: $f_{si} =f_s+2IF$

Remedy:

• The image frequency must be rejected by the receiver. The image rejection depends on the front end selectivity of the receiver i.e. the selectivity of the RF circuit.

• The image rejection must be achieved before the IF stage because once it reaches the IF stage it cannot be removed.

• The use of RF amplifier thus improves the image frequency rejection.