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In a radio receiver, how is the local oscillator frequency generally set in relation to the incoming radio frequency signal?

Difficulty: Easy

Correct Answer: Higher than the incoming frequency

Explanation:


Introduction / Context:
The choice of local oscillator frequency relative to the incoming RF signal is fundamental to superheterodyne receiver design. Most commercial receivers use high-side injection, where the LO is set higher than the incoming signal frequency.


Given Data / Assumptions:

  • Receiver tuned to signal frequency f_s.
  • Intermediate frequency f_if is fixed (e.g., 455 kHz).
  • Local oscillator frequency f_lo = f_s + f_if (high-side injection).


Concept / Approach:
By setting f_lo higher than f_s, the IF remains constant for all received channels. Although low-side injection (f_lo = f_s − f_if) is possible, it is less common because it complicates image frequency rejection.


Step-by-Step Solution:

Suppose f_s = 1000 kHz, f_if = 455 kHz.For high-side injection: f_lo = 1000 + 455 = 1455 kHz.Mixer output = |1455 − 1000| = 455 kHz (desired IF).


Verification / Alternative check:

Commercial AM/FM receiver schematics confirm LO > signal frequency in most cases.


Why Other Options Are Wrong:

Lower frequency LO is less common and used only in special designs.Equal frequency LO produces zero IF, which is impractical.'Any of the above' is too broad; correct answer is high-side for most practical cases.Random variation is impossible for proper operation.


Common Pitfalls:

Students often assume LO can be freely chosen, but in practice, design constraints favor LO > RF.


Final Answer:

Higher than the incoming frequency
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