How do I design a low noise bias circuit for a VCO to minimize phase noise?
VCO Bias Noise
The VCO is a voltage-to-frequency converter. Any noise on the supply voltage or tuning voltage modulates the output frequency, creating phase noise sidebands. The supply pushing figure (Kpush) quantifies this sensitivity: Kpush = Δf/ΔV, typically 1-100 MHz/V. A VCO with Kpush = 10 MHz/V receiving 1 μV of supply noise has frequency modulation of 10 Hz, which appears as phase noise at the modulation frequency offset.
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
Frequently Asked Questions
How much does supply noise affect phase noise?
The contribution to phase noise from supply noise: L_supply(fm) = 20·log10(Kpush × Vn(fm) / fm), where Vn(fm) is the supply noise spectral density at offset fm. For Kpush = 10 MHz/V and Vn = 1 nV/√Hz at 10 kHz: L_supply = 20·log10(10e6 × 1e-9 / 1e4) = 20·log10(10^-6) = -120 dBc/Hz.
What about the tuning voltage?
Tuning voltage noise must be extremely low because Kvco is much larger than Kpush. For a VCO with Kvco = 100 MHz/V: the tuning voltage noise at 10 kHz offset must be below 0.1 nV/√Hz to achieve -130 dBc/Hz phase noise. This requires a very low-noise loop filter and charge pump.
Does grounding matter?
Yes. Ground noise from digital circuits coupling into the VCO bias creates spurs and broadband phase noise. Use separate ground planes for VCO/analog and digital sections, connected at a single point near the power supply. Star grounding minimizes ground-loop currents that couple noise into the VCO.