What is reciprocal mixing?

Reciprocal mixing occurs in a receiver when phase noise from its own local oscillator (LO) mixes with a strong, off-frequency signal. This unwanted mixing process creates noise that falls right into the receiver's intermediate frequency (IF) passband. Normally, a mixer combines a pure LO signal with the desired RF signal to create the IF. However, no oscillator is perfectly pure; they all generate some phase noise, which spreads out from the center frequency. If a strong adjacent signal mixes with this phase noise, it effectively raises the noise floor of the receiver and can completely mask a weak desired signal. Other options describe different phenomena. For example, two out-of-band signals generating an in-band spurious signal is known as intermodulation, not reciprocal mixing. Therefore, reciprocal mixing strictly relates to the receiver's LO phase noise.