As I understand it, the last mixer stage in a superheterodyne transmitter outputs the desired signal, an image of the signal, and the LO frequency. But a non-tunable RF filter is usually responsable the 2 undesired outputs.

The final RF filter would have to reject the LO signal for any given center frequency that the transmitter can be tuned to.

Does this mean that, for example, a tunable transmitter with an IF of 455 kHz would be limited to a range of about 455 kHz without tuning/switching the final RF filter?

I'm thinking that, at the lowest tunable frequency, LO is at the center frequency + the IF, or 455 kHz above the center frequency. The RF filter would have to reject this, however, that means it would also reject signals with a center frequency if it were tuned 455 kHz higher.

This seems like a pretty big limitation, given the popularity of 455 kHz as an IF, but is that all correct?

  • 3
    $\begingroup$ Hi, please read through this comprehensive article on Wikipedia first. It completely covers the question of band limitations, high side and low side mixing, etc, even explaining why 455 kHz is used for MW receivers and 10.7 MHz IF for FM. Your question starts from some incorrect assumptions, maybe the 455 Hz vs kHz. $\endgroup$
    – tomnexus
    Jan 19 at 14:19
  • 1
    $\begingroup$ Thanks for the correction, I had meant kHz instead of Hz. Also @tomnexus thanks for pointing me to the Wikipedia article, it did answer my questions about the RF filter. $\endgroup$ Jan 19 at 21:06


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