Which two performance aspects are typically considered when selecting an intermediate frequency?

• A. Image rejection and selectivity
• B. Cross-modulation distortion and interference
• C. Noise figure and distortion
• D. Stability and image response

Answer: (A)

In a superheterodyne receiver, choosing the intermediate frequency is about how the receiver handles two key performance aspects: image rejection and selectivity. The image frequency is another RF signal that, when mixed with the local oscillator, can fall into the same intermediate frequency and masquerade as the desired signal. If the image isn’t sufficiently attenuated before the mixing stage, it can interfere with reception. A larger intermediate frequency generally makes this image easier to reject with the RF front end, because the image is farther away in frequency and easier to suppress with preselector filters.

Selectivity is about how sharply the receiver can distinguish the desired signal from nearby signals, and this is set by the filters in the intermediate frequency stage. The bandwidth and shape of the IF filter determine how well adjacent channels are suppressed. The chosen intermediate frequency influences how feasible it is to achieve the required filter performance with available technology, so selectivity is tightly tied to that choice.

Other factors like noise figure, distortion, cross-modulation, or stability are important to receiver performance in other contexts, but they aren’t the two primary considerations when selecting the intermediate frequency.