VCO
A VCO produces a periodic output (sine, square, or triangle) whose frequency is a monotonic function of an analogue tuning voltage V_TUNE. Tuning sensitivity K_VCO (in MHz/V) and tuning range are key specs. Examples: Mini-Circuits ROS-2700 1.6–2.7 GHz, Analog Devices HMC830 25 MHz–3 GHz integrated, Crystek CVCO55BE 5–6 GHz integrated module.
A VCO uses a varactor diode (a reverse-biased diode whose junction capacitance varies with voltage) inside an LC resonator or a current-starved ring oscillator. In LC VCOs, varactor C tunes resonance frequency f = 1 / (2π√(LC)), giving a smooth tuning curve and low phase noise but limited range (~30 % BW). Ring oscillators built from current-starved CMOS inverters tune across a decade by varying inverter delay, at the cost of higher phase noise. Inside a PLL, a feedback loop holds the VCO at exactly N · f_REF.
In plain terms
A guitar string tuned by twisting a peg: the harder you twist (the higher the voltage), the higher the note (frequency); the VCO is the peg, and a PLL is the tuner that turns it for you.
Why designers use it
- Generate the tunable carrier inside cellular, Wi-Fi, GPS, and Bluetooth radios.
- Synthesise wide-range LO sweeps inside spectrum analysers and signal generators.
- Recover the embedded clock from high-speed serial data lanes (PCIe, USB, 100GbE).
- Provide a swept tone for FMCW radar transmit chirps in automotive sensors.
Best for
- PLL feedback
- Spectrum analysers
- FMCW radar
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- When ultra-low close-in phase noise is critical and tuning range is small — a fixed-frequency crystal oscillator outperforms a VCO by 20+ dB.
- Open-loop without PLL stabilisation — VCO drift over temperature and supply voltage is enormous; it must be locked.
Common mistakes
- Layout that couples switching-regulator noise into V_TUNE — the VCO turns the noise into FM sidebands (ripple at the regulator frequency around the carrier).
- Operating the VCO past its rated tuning range — at the edge K_VCO drops sharply, the loop bandwidth collapses, and the synth fails to lock.
Where you will find it
- An iPhone modem's main RF synthesiser uses an integrated LC VCO covering 4.5–6 GHz with two divide-by-2 stages to produce any cellular band: the VCO inside the AD9361-class transceiver is locked by a fractional-N PLL to a 19.2 MHz reference, hopping bands in tens of microseconds when handing off between LTE and 5G NR.
- A Tesla Model 3's FMCW automotive radar uses a 77 GHz VCO swept over 2 GHz in 100 µs to produce the chirp: the chirp linearity (set by the VCO + closed-loop linearisation) directly determines the radar's range resolution, and the radar measures forward-vehicle distance to within 10 cm even at 150 km/h.
- A Keysight E4438C signal generator's IF VCO covers 250–500 MHz inside a YIG-stabilised loop: the wide tuning is needed to synthesise modulated test signals across cellular and ISM bands, and the YIG-tracked loop scrubs VCO close-in phase noise to under −110 dBc/Hz at 10 kHz offset.
A short history
A voltage-controlled oscillator is an electronic oscillator whose oscillation frequency is controlled by a voltage input, which can be used for frequency or phase modulation. Harmonic VCOs typically use a resonator, such as an LC tank, with an amplifier, while relaxation oscillators can generate a sawtooth or triangular waveform instead. A VCO is also an integral part of a phase-locked loop.