An oscillator generates a periodic signal without an external input. In RF systems, oscillators provide the local oscillator (LO) signal for frequency conversion and the carrier signal for transmitters. Phase noise (short-term frequency instability) is the key quality metric.

Oscillation Conditions

The Barkhausen criterion: a feedback amplifier oscillates when loop gain \(|A\beta| = 1\) and loop phase shift is 0° (or 360°). Common topologies: Colpitts (capacitive voltage divider), Hartley (inductive divider), Clapp, Pierce (crystal oscillators).

Phase Noise

Phase noise \(\mathcal{L}(f_m)\) (dBc/Hz) describes the spectral purity of an oscillator — the noise power in a 1 Hz bandwidth at offset \(f_m\) from the carrier, relative to the carrier power. The Leeson model:

Phase noise decreases with increasing loaded Q (\(Q_L\)) and oscillator power (\(P_s\)).

Voltage-Controlled Oscillator (VCO)

A VCO is an oscillator whose frequency is tuned by a control voltage, via a varactor diode (voltage-variable capacitor). Tuning sensitivity \(K_v\) (MHz/V) characterises the VCO. Higher \(K_v\) increases frequency range but also increases phase noise sensitivity to supply noise.

Phase-Locked Loop (PLL)

A PLL phase-locks a VCO to a stable reference oscillator (usually a crystal), allowing the VCO's tunable frequency range with the reference's spectral purity. Components: phase-frequency detector (PFD), charge pump, loop filter, VCO, frequency divider (÷N). Within the loop bandwidth, VCO phase noise is suppressed to near the reference noise floor.