What is the purpose of a local oscillator (LO) in a superheterodyne receiver?

• A. The LO mixes with the RF signal to produce an intermediate frequency (IF) that is easier to filter and amplify with fixed characteristics.
• B. The LO boosts the RF signal before mixing.
• C. The LO filters the RF signal to the desired channel.
• D. The LO converts IF back to audio directly.

Answer: (A)

In a superheterodyne receiver the local oscillator provides a frequency that, when mixed with the received RF signal, creates a fixed intermediate frequency. The mixer outputs both sum and difference frequencies, and the difference that matches the IF is selected for further processing. Having a fixed IF lets the subsequent IF filter and amplifier stages be designed with fixed characteristics, giving reliable, sharp selectivity across the whole band. The LO’s job isn’t to boost the RF signal, nor to filter the RF signal, and it doesn’t convert IF directly to audio; those tasks are handled by other stages after the mixing and demodulation.