How do you design a voltage-controlled oscillator?
A voltage-controlled oscillator (VCO) changes frequency with a control voltage, usually by tuning a varactor in an LC tank: f = 1/(2π√(L·C(V))). Its tuning sensitivity KVCO (Hz/V or rad/s/V) enters the PLL loop gain. Wide tuning range with low KVCO is achieved with switched capacitor banks plus a fine varactor.[1][2]
What trade-offs govern VCO design?
- Higher tank Q lowers phase noise; inductor Q usually dominates on-chip.
- Large KVCO makes the VCO sensitive to noise on the control line.
- Varactors add loss and nonlinearity (AM-to-PM conversion).
- Higher signal swing improves phase noise until devices enter triode or breakdown.
Which topologies are used?
Cross-coupled NMOS or complementary (NMOS+PMOS) LC VCOs in ICs; Colpitts and Clapp in discrete designs; class-C and class-F VCOs for better phase noise efficiency; YIG-tuned oscillators for multi-octave instrument sources.[1]
How is a VCO characterized?
Frequency versus tuning voltage (and KVCO as its slope), phase noise at standard offsets, output power and harmonics, pushing, pulling, and figure of merit FOM = L(Δf) − 20·log10(f0/Δf) + 10·log10(PDC/1 mW) (lower is better).[1]
Frequently asked questions
What is KVCO?
The VCO's tuning slope, in Hz per volt.
Why use capacitor banks?
They give coarse tuning steps so the varactor covers only a small range, reducing KVCO.
What limits VCO phase noise?
Tank Q, signal swing, active device noise and noise up-conversion.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.
- “The Design of CMOS Radio-Frequency Integrated Circuits, 2nd ed.”. T. H. Lee, Cambridge University Press. 2004.