Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

Image frequency

• In a standard broadcast receiver (and , in fact , in the vast majority of all receivers made ) the local oscillator frequency is made higher than the incoming signal frequency for reasons that will become apparent
• It is made equal at all times to the signal frequency plus the intermediate frequency.
• Thus $f_0=f_s+f_i $ OR $f_0=f_s−f_i $ no matter what the signal frequency may be When $f_0$ and $f_s$ are mixed , the difference frequency , which is one of the by-products , is equal to $f_i$ .
• As such, it is the only one passed and amplified by the IF stage.
• If a frequency $f_si$ manages to reach the mixer, such that $f_si =f_o+f_i$ , that is , $f_si=f_s+2f_i$, then this frequency will also produce $f_i$ when mixed with $f_0$.
• Unfortunately, this spurious intermediate-frequency signal will also be amplified by the IF stage and will therefore provide interference.
• This has the effect of two stations being received simultaneously and is naturally undesirable.
• The term fsi is called image frequency and is defined as the signal frequency plus twice the intermediate frequency .Reiterating , we have $f_si=f_s+2f_i$
• The rejection of an image frequency by a single –tuned circuit , i.e., the ratio of the gain at the signal frequency to the gain at the image frequency , is given by

$α =√1+Q2ρ2$ where $ρ=f_si/f_s−f_s/f_si$ Q=loaded Qo ftuned circuit

• If the receiver has an RF stage, then there are two tuned circuits , both tuned to $f_s$ .The rejection of each will be calculated by the same formula , and the total rejection will be product of the two.
• Whatever applies to gain calculations applies also to those involving rejection.
• Image –rejection depends on the front-end selectivity of the receiver and must be achieved before the IF stage.
• Once the spurious frequency enters the first IF amplifier, it becomes impossible to remove it from the wanted signal.
• It can be seen that if $f_si/f_s$ is large , as it is in the AM broadcast band , the use of an RF stage is not essential for good image-frequency rejection , but it does become necessary above about 3MHz.

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.