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:
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
Discussion & Comments