In a superheterodyne receiver, what is the role of the local oscillator?

• A. To generate a reference bypass filter
• B. To provide a frequency that mixes with the incoming RF signal to produce an intermediate frequency
• C. To demodulate the signal directly at baseband
• D. To amplify the RF signal before mixing

Answer: (B)

The local oscillator in a superheterodyne receiver provides a frequency that mixes with the received RF signal to produce a fixed intermediate frequency. Through the mixer, the RF signal and the LO generate sum and difference frequencies, and the difference (the intermediate frequency) is chosen to be a constant, easy-to-filter and amplify stage. By keeping the IF constant, the receiver can use high-quality, selective filters and amplifiers at a single frequency while the LO is tuned to translate different RF channels into that same IF. The LO isn’t used to demodulate or directly amplify the signal; its job is to enable the frequency conversion that makes the rest of the receiver more selective and stable.