VCO Tuning Sensitivity

Formula

Description

The VCO (voltage-controlled oscillator) tuning sensitivity or gain constant KVCO describes how much the oscillation frequency changes per volt of tuning voltage change. It is a critical parameter in PLL design because it directly affects the loop gain and stability. High KVCO allows wide tuning range but makes the VCO more sensitive to noise on the tuning voltage, degrading phase noise. Low KVCO gives better noise performance but limits tuning range. Modern PLL ICs often use multi-band VCOs with calibration to maintain moderate KVCO across a wide frequency range.

Variables

• KVCO — VCO tuning sensitivity (Hz/V)
• Δf — Frequency change (Hz)
• ΔV — Tuning voltage change (V)

Practical Notes

Typical KVCO values: LC VCO 10-100 MHz/V, ring oscillator 100-1000 MHz/V. The PLL loop gain is proportional to KVCO, so variations in KVCO across the tuning range affect loop dynamics. The total frequency range should require less than 80% of the available tuning voltage range to ensure lock over temperature and supply variations.