A voltage-controlled oscillator generates an RF signal whose frequency is set by a DC tuning voltage. The relationship between voltage and frequency is characterized by Kvco (MHz/V). VCOs are the core of PLL synthesizers and direct-tuned transmitters.

Active Components

Voltage-Controlled Oscillator

Q: What VCO topologies exist?

LC tank (varactor-tuned): best phase noise, narrower tuning. Ring oscillator: wide tuning, easy CMOS integration, higher phase noise. YIG-tuned: ultra-wide bandwidth (octaves), excellent phase noise, but large and slow-tuning. DRO for fixed-frequency low phase noise.

Q: What is VCO phase noise?

Phase noise measures spectral purity in dBc/Hz at a specified offset from the carrier. Typical values: -90 to -120 dBc/Hz at 100 kHz offset for LC VCOs. Phase noise determines receiver sensitivity and transmitter spectral purity.

A voltage-controlled oscillator (VCO) generates an RF signal whose frequency varies with an applied DC tuning voltage. The tuning sensitivity Kvco (MHz/V) defines the voltage-to-frequency relationship. VCOs are the core element of PLL frequency synthesizers and direct-tuned transmitters. Key specs: phase noise, tuning range, output power, pushing, and pulling.

Category: Active Components

Related to: PLL, Phase Noise, Varactor, Kvco

Understanding Voltage-Controlled Oscillators

The VCO is one of the most critical active components in any RF system. It generates the local oscillator (LO) signal for frequency conversion, serves as the core of frequency synthesizers, and provides the carrier for direct-modulation transmitters.

Key Specifications

Tuning range: The frequency span achievable over the tuning voltage range. Octave-band (2:1) is common for wideband VCOs
Kvco: Tuning sensitivity in MHz/V. Lower Kvco reduces noise sensitivity but limits tuning range
Phase noise: Spectral purity in dBc/Hz. Typical: -90 to -120 dBc/Hz at 100 kHz offset
Output power: Typically 0 to +10 dBm for discrete VCOs
Pushing: Frequency change per volt of supply variation (MHz/V)
Pulling: Frequency change under load mismatch (MHz for 12 dB return loss)

Topologies

LC tank (varactor): Varactor diode tunes a resonant LC circuit. Best phase noise for narrowband. SiGe, GaAs, or discrete implementations
Ring oscillator: Cascaded inverting stages. Wide tuning, easy CMOS integration. Higher phase noise. Used in PLLs on SoCs
YIG-tuned: Yttrium iron garnet sphere resonator. Ultra-wide bandwidth (octaves), excellent phase noise. Large, slow tuning. Used in test equipment and EW
DRO: Dielectric resonator oscillator. Fixed frequency, ultra-low phase noise. Used as stable LO references

Key Equations

VCO tuning:
f_out = f₀ + K_VCO·V_tune
K_VCO = tuning sensitivity (MHz/V)

Phase noise (Leeson):
L(f_m) ∝ (f₀/2Q)² × FkT/P_osc

Pushing/pulling:
Δf_push = K_push·ΔV_DD

Comparison

Type	Freq	K_VCO	PN @100k	Technology
LC-CMOS	1–10 GHz	50–200 MHz/V	−110 dBc/Hz	RFIC
LC-SiGe	5–40 GHz	100–500 MHz/V	−105 dBc/Hz	MMIC
Ring osc	0.1–5 GHz	1–5 GHz/V	−90 dBc/Hz	CMOS digital
YIG	2–20 GHz	20 MHz/mA	−130 dBc/Hz	Bench inst
Cavity	5–20 GHz	10–50 MHz/V	−120 dBc/Hz	Radar LO

Common Questions

Frequently Asked Questions

What is a VCO?

A voltage-controlled oscillator generates an RF signal whose frequency is set by a DC tuning voltage. Kvco (MHz/V) defines the sensitivity. VCOs are the core of PLL synthesizers and direct-tuned transmitters.

What VCO topologies exist?

LC tank (varactor-tuned): best phase noise. Ring oscillator: wide tuning, CMOS integration. YIG-tuned: octave bandwidth, excellent phase noise, large size. DRO: fixed-frequency ultra-low noise.

What is VCO phase noise?

Phase noise measures spectral purity in dBc/Hz at an offset from the carrier. Typical: -90 to -120 dBc/Hz at 100 kHz offset for LC VCOs. Determines receiver sensitivity and TX spectral purity.

Related Terms

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