When The Image Matters

Sometimes you find out what you thought to knew about a subject is just utterly wrong. When it comes to image frequency for a long time I have believed that the Image_F = Desired_F + or – 2 * LO_F. Which is essentially true, the image can be plus or minus but I was ignoring one critical piece of information and that is the position of the Local Oscillator in respect to the desired frequency.

So there is only 1 image frequency, not 2, if the LO is below the Desired frequency then the image frequency is Image_F = Desired_F – 2 * LO_F and if the LO is above the Desired frequency then Image_F = Desired_F + 2 * LO_F. Which now means that using a 455Khz IF for AM Broadcast Receiver actually makes sense if using a high side LO to do the mixing in a superhet design.

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