LO

A local oscillator is a signal generator built into a radio receiver or transceiver that produces a stable, controllable frequency. Its output mixes with an incoming radio signal to shift that signal down to a lower intermediate frequency (IF), where it can be filtered, amplified, and demodulated. Without an LO, a receiver cannot separate a desired signal from the clutter of other frequencies in the air.

The LO frequency must be offset from the desired signal's frequency by exactly the IF bandwidth. If you want to receive a 2.4 GHz signal and your IF is 400 MHz, your LO will run at either 2.0 GHz or 2.8 GHz, depending on whether you use low-side or high-side injection. The choice affects image rejection and spurious response performance. The LO's frequency stability directly limits receiver tuning accuracy and frequency resolution.

Construction and performance

LOs are typically crystal oscillators, voltage-controlled oscillators (VCOs), or phase-locked loops (PLLs) tuned by a frequency synthesizer. Crystal oscillators offer excellent stability but fixed frequency. VCOs are tunable but drift with temperature and supply voltage; a PLL wraps a VCO in feedback to lock it to a stable reference. Phase noise is the critical performance metric: low phase noise keeps the LO's energy concentrated at its nominal frequency, minimizing noise floor degradation in the receiver. A noisy LO degrades sensitivity and dynamic range.

In superheterodyne receivers, the mixer receives both the LO signal and the incoming RF. Their nonlinear interaction generates sum and difference frequencies; the receiver selects the desired IF component and rejects the rest. The LO's power level matters: too weak and conversion loss increases; too strong and the mixer becomes nonlinear in undesired ways, generating unwanted harmonics and intermodulation products.

Microwave and RF receivers often use multiple LOs in cascade. A first LO might downconvert from 10 GHz to 1 GHz; a second LO then converts from 1 GHz to 100 MHz, where final demodulation occurs. This approach trades complexity for flexibility and image rejection. Modern software-defined radios shift the LO directly to baseband or near-baseband, capturing the signal in its entirety for digital signal processing.