VCO
Understanding VCOs
VCOs are the most widely used tunable frequency source in RF systems. When combined with a phase-locked loop (PLL), a VCO generates a stable, low-phase-noise output at any frequency within its tuning range. The PLL compares the VCO output (divided down) to a crystal reference and adjusts the VCO control voltage to maintain frequency lock.
VCO Parameters
- Tuning range: The frequency span available by varying the control voltage. Can be narrow (10%) or wide (octave+).
- Tuning sensitivity (Kvco): MHz per volt of control voltage change. Higher sensitivity = wider tuning but more susceptible to noise.
- Phase noise: Spectral purity of the output. Determined by resonator Q, active device noise, and circuit design.
- Pushing: Frequency change due to supply voltage variation. Should be minimized with supply regulation.
- Pulling: Frequency change due to load impedance variation. Minimized by using a buffer amplifier or isolator.
where Kvco = tuning sensitivity (MHz/V)
PLL lock condition: f_out = N x f_ref
where N = divider ratio
Typical VCO phase noise:
GaAs/SiGe MMIC: -100 to -115 dBc/Hz at 10 kHz
Discrete GaAs FET: -110 to -125 dBc/Hz at 10 kHz
Frequently Asked Questions
What is a VCO?
A VCO generates an output signal whose frequency is controlled by a DC voltage. Varying the control voltage changes the capacitance of a varactor diode in the resonant circuit, which changes the oscillation frequency. VCOs are used in PLLs, synthesizers, and FM modulators.
How does a VCO work in a PLL?
In a PLL, the VCO output is divided down and compared to a crystal reference. The phase detector generates an error signal that adjusts the VCO control voltage to maintain frequency lock. This combines the VCO's wide tuning range with the crystal's low phase noise.
What determines VCO phase noise?
VCO phase noise depends on the resonator Q (higher Q = lower phase noise), the active device noise figure, the oscillation amplitude, and the tuning sensitivity. Leeson's model predicts phase noise from these parameters. A PLL further reduces VCO phase noise within the loop bandwidth.