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In superheterodyne receivers, what is the primary function of the local oscillator?

Difficulty: Easy

Correct Answer: Shift the frequency of the received modulated carrier to the IF band

Explanation:


Introduction / Context:
The local oscillator is a critical component of the superheterodyne receiver, which is the most widely used receiver architecture in modern communication systems. Its primary purpose is to enable frequency translation so that the desired incoming RF signal can be processed more efficiently.


Given Data / Assumptions:

  • Receiver architecture: superheterodyne.
  • Incoming RF carrier frequency f_s.
  • Local oscillator generates frequency f_lo.
  • Intermediate frequency (IF) chosen as standard value (e.g., 455 kHz).


Concept / Approach:
The mixer multiplies the incoming RF signal with the local oscillator signal. This produces sum and difference frequencies. The difference frequency is selected as the IF, which remains constant regardless of the tuned station. This makes subsequent amplification and filtering easier and more selective.


Step-by-Step Solution:

Incoming frequency f_s = RF.Local oscillator frequency f_lo = f_s ± f_if.Mixer output = f_lo + f_s and f_lo − f_s.Filter selects IF = |f_lo − f_s|.


Verification / Alternative check:

For AM broadcast: f_s = 1000 kHz, IF = 455 kHz, f_lo = 1455 kHz. The IF is fixed at 455 kHz regardless of tuned frequency.


Why Other Options Are Wrong:

Detection of modulating signal occurs later in detector stage, not oscillator.Amplification is done by RF/IF amplifiers, not oscillator.Noise filtering is performed by tuned circuits and IF filters, not oscillator.Synchronization is relevant in communication but not the role of LO here.


Common Pitfalls:

Confusing local oscillator with detector or RF amplifier stages.


Final Answer:

Shift the frequency of the received modulated carrier to the IF band
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