In a superheterodyne receiver, which stage typically provides better selectivity: the RF stage or the IF stage?

Question

Solve this multiple-choice question and choose the correct option.

Accepted Answer

Introduction / Context:Selectivity refers to the ability to separate the desired signal from adjacent signals. In superheterodyne receivers, the IF stage is designed with fixed frequency filters that achieve higher Q-factors than tunable RF circuits, giving better selectivity. Given Data / Assumptions: Receiver tuned to RF frequency f_s. Intermediate frequency f_if fixed (e.g., 455 kHz). IF stage uses sharp bandpass filters. Concept / Approach:The RF stage is tunable and must cover a wide frequency range, so its Q is limited. The IF stage operates at a fixed frequency, allowing high-Q resonators and better adjacent channel rejection. Hence, IF stage selectivity surpasses RF stage selectivity. Step-by-Step Solution: RF stage selectivity depends on variable tuned circuits; bandwidth broadens at higher frequencies.IF stage bandwidth can be narrow and optimized (e.g., ±6 kHz for AM, ±100 kHz for FM).Thus, IF stage provides sharper filtering than RF stage. Verification / Alternative check: Receiver specifications list IF bandwidth as the main selectivity parameter, not RF bandwidth. Why Other Options Are Wrong: RF stage has poorer selectivity due to tuning range limitations.Same selectivity is impossible in practice. Common Pitfalls: Confusing sensitivity and selectivity or assuming both stages contribute equally—IF is dominant. Final Answer: The IF stage has better selectivity than RF stage

In a superheterodyne receiver, which stage typically provides better selectivity: the RF stage or the IF stage?

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