9.2.3 LO

The LO provides the reference frequency for frequency conversion. When tuning across a band, the LO frequency must always remain offset from the desired RF by exactly the IF. Frequency stability of the LO determines tuning accuracy and receiver drift. In an AM broadcast receiver, coarse tuning by ear suffices, but SSB or ISB communications receivers require LO stability better than ±20 Hz.

Early receivers used crystal oscillators for fixed frequencies or switched-crystal banks for multiple channels. Modern tunable receivers employ frequency synthesizers—most commonly phase-locked loops (PLLs) or direct digital synthesizers (DDS)—to generate precise, agile LO signals with sub-hertz resolution.

Ganging and tracking. To simplify tuning, the RF and LO stages are mechanically or electronically ganged, so a single control simultaneously adjusts both resonant circuits, maintaining the IF offset. Tracking accuracy ensures that the frequency difference between the two circuits remains constant across the band.
Desirable LO characteristics. An ideal LO should:
Maintain high frequency stability and spectral purity.
Provide a mixing signal stronger than the RF input (typically 10–20 dB higher) to drive the nonlinear mixer efficiently.
Deliver constant amplitude across its tuning range.
Exhibit minimal phase noise and spurious outputs.
Prevent radiation of the LO signal through the antenna.

In high-performance receivers, LO phase noise is a dominant factor in reciprocal mixing, where oscillator noise sidebands degrade weak-signal reception near strong interferers. The LO provides one of the two input signals required by the mixer, and its frequency accuracy, phase noise, and stability directly influence conversion accuracy, adjacent-channel performance, and reciprocal mixing behavior.