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

Difficulty: Easy

Correct Answer: The IF stage has better selectivity than RF stage

Explanation:


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
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