Receiver behavior at higher RF frequencies As the tuned RF frequency increases in a superheterodyne receiver, which of the following effects generally becomes worse and needs more careful design?

Introduction / Context:
At higher RF frequencies, maintaining front-end performance becomes challenging due to component Q limits, stray capacitances, and fixed IF separation. Image rejection, in particular, depends on how well the RF tuned circuits attenuate signals spaced 2IF away from the desired frequency.

Given Data / Assumptions:

• Fixed IF (e.g., 455 kHz in AM or 10.7 MHz in FM receivers).
• Single or dual RF tuned circuits ahead of the mixer.
• Component Q decreases relatively as frequency increases and parasitics rise.

Concept / Approach:

Image frequency sits at a separation of 2IF from the desired RF. As RF increases, the fractional separation (2IF / fS) shrinks, making it harder for finite-Q RF circuits to reject the image because their skirts are not infinitely steep. Therefore, image rejection typically degrades unless more selective preselection or higher IF (or multiple conversions) is used.

Step-by-Step Solution:

Verification / Alternative check:

Shortwave and VHF receivers use higher IFs or dual conversion to improve image rejection at high frequencies, confirming the trend.

Why Other Options Are Wrong:

(a) tracking typically becomes harder, not easier; (b) tracking can degrade, but image rejection deterioration is the core limitation; (c) IF selectivity does not inherently worsen with RF frequency since IF is fixed; (e) audio fidelity is not the primary issue tied to RF tuning frequency.

Common Pitfalls:

Confusing adjacent-channel selectivity (IF issue) with image rejection (RF issue); assuming raising LO alone fixes the image without improving RF filtering.

Final Answer:

Image frequency rejection will be poor