How does a varactor tune the VCO frequency?

A varactor diode acts as a voltage-variable capacitor. When reverse-biased, its junction capacitance decreases as the bias voltage increases, following C = C0 / (1 + V/Vbi)^n, where C0 is the zero-bias capacitance, Vbi is the built-in potential (about 0.7V for silicon), and n is the grading coefficient (0.5 for abrupt junction, 1 for hyperabrupt). The varactor is connected across the LC tank circuit of the oscillator. As the control voltage changes the varactor capacitance, the tank resonant frequency f = 1/(2pi sqrt(LC)) shifts. A hyperabrupt varactor with C ratio of 10:1 can tune the frequency by a factor of sqrt(10) = 3.16, giving an octave-plus tuning range. The trade-off is that hyperabrupt varactors have lower Q than abrupt junction types, increasing phase noise.

What determines VCO phase noise?

VCO phase noise is governed by three factors: the loaded Q of the tank circuit (higher Q means lower noise), the noise figure of the active device (FET vs. bipolar), and the signal power in the tank. Leeson's equation shows that phase noise improves as Q squared and as signal power increases linearly. For an LC VCO with loaded Q of 20 (typical for a CMOS integrated VCO at 5 GHz), the phase noise at 1 MHz offset is typically minus 110 to minus 120 dBc/Hz. Improving Q to 40 (through inductor optimization and reduced varactor loss) improves phase noise by 6 dB. The VCO's free-running phase noise is relevant outside the PLL loop bandwidth; inside the loop bandwidth, the PLL suppresses the VCO phase noise by the loop gain.

What are VCO pushing and pulling?

Pushing is the frequency shift caused by changes in the supply voltage. If the supply varies by 0.1V and the pushing factor is 5 MHz/V, the frequency shifts by 500 kHz. Low pushing is achieved by using a regulated supply and cascode transistor topologies that isolate the tank from supply variations. Typical pushing specifications: 0.5 to 5 MHz/V. Pulling is the frequency shift caused by changes in the load impedance. If the VCO output sees a mismatched load (VSWR greater than 1), the reflected signal re-enters the oscillator and shifts its frequency. The pulling specification is typically given as the peak-to-peak frequency deviation when the load VSWR is 2:1 with all phases. Good buffer amplifier isolation (15 to 20 dB) between the VCO core and the output reduces pulling to acceptable levels.

Active Components

Voltage-Controlled Oscillator

VCO

A 5G base station synthesizer needs to generate any frequency from 3.3 to 3.8 GHz with sub-kHz accuracy. The VCO inside the PLL covers this range by converting a 0.5 to 4.5 V control voltage into a 3.1 to 4.0 GHz output: a tuning sensitivity of 225 MHz/V. The PLL feedback loop continuously adjusts this voltage to lock the VCO to the correct frequency. Free-running, the VCO drifts by hundreds of kHz per degree Celsius. Locked inside the PLL, the combined system achieves the reference oscillator's stability (ppb level) while maintaining the VCO's tuning speed (microsecond channel switching). The VCO provides the raw frequency agility; the PLL provides the accuracy.

Category: Active Components

Tuning: K_VCO (MHz/V)

Key Specs: Phase noise, tuning range, pushing

VCO Technologies by Frequency

VCO Type	Freq. Range	Tuning	PN at 1 MHz	Power	Technology
CMOS LC VCO	0.5 to 12 GHz	10 to 30%	−115 dBc/Hz	5 to 30 mW	Integrated SoC
SiGe LC VCO	1 to 40 GHz	10 to 25%	−120 dBc/Hz	20 to 100 mW	RFIC
GaAs VCO	2 to 50 GHz	20 to 40%	−110 dBc/Hz	50 to 200 mW	Discrete/module
Coaxial resonator VCO	0.5 to 6 GHz	5 to 15%	−135 dBc/Hz	50 to 150 mW	Module
SAW VCO	100 MHz to 2 GHz	0.1 to 0.5%	−145 dBc/Hz	10 to 50 mW	Narrowband
YIG VCO	2 to 20 GHz	Multi-octave	−125 dBc/Hz	500 mW to 2 W	Instruments, EW

Resonant frequency:
f₀ = 1 / (2π√(LC(V)))

Varactor capacitance vs. voltage:
C(V) = C₀ / (1 + V/V_bi)ⁿ
n = 0.5 (abrupt), n = 1 (hyperabrupt), C ratio up to 10:1

Tuning sensitivity:
K_VCO = Δf / ΔV (MHz/V or GHz/V)
Higher K_VCO = wider tuning but higher AM-to-PM noise conversion

Common Questions

Frequently Asked Questions

How does varactor tuning work?

Varactor junction capacitance varies with reverse bias: C = C₀/(1+V/V_bi)ⁿ. Connected across the LC tank, changing C shifts f = 1/(2π√LC). Hyperabrupt (n=1): 10:1 C ratio, octave tuning range. Trade-off: wider tuning = lower Q = worse phase noise.

What sets VCO phase noise?

Tank loaded Q (PN ∝ 1/Q²), device noise figure, and signal power. CMOS VCO at 5 GHz, Q=20: −115 dBc/Hz at 1 MHz. Doubling Q to 40: +6 dB improvement. Inside PLL bandwidth, loop gain suppresses VCO noise.

Pushing vs. pulling?

Pushing: frequency shift from supply voltage change (0.5 to 5 MHz/V). Fix: regulated supply, cascode topology. Pulling: shift from load mismatch (VSWR-dependent). Fix: buffer amplifier with 15 to 20 dB isolation between VCO core and output.

Related Terms

← Varactor VSWR →

Synthesizer Design

VCO Tuning Range Calculator

Enter varactor C_min/C_max, inductor value, and parasitic capacitance. Compute frequency range, K_VCO, and check if the tuning covers your target band with margin.

Design VCO Tuning