Power Supply Rejection
Understanding PSRR
Supply noise is one of the most insidious sources of RF performance degradation. Switching power supplies generate ripple at their switching frequency (100 kHz - 5 MHz), which can modulate oscillator frequency, amplifier gain, and signal phase if PSRR is insufficient.
PSRR by Circuit Type
- VCO/oscillator: Supply ripple modulates the active device's capacitances, causing FM modulation. Supply-induced sidebands appear as phase noise spurs. PSRR > 40 dB needed.
- LNA: Supply noise adds to the NF through the transconductance. PSRR > 30 dB to prevent NF degradation.
- PA: Supply ripple causes AM modulation of the output. PSRR > 30 dB for acceptably low spurious AM.
Improving PSRR
- Linear voltage regulator (LDO) before the RF circuit. 40-80 dB PSRR at low frequencies.
- Ferrite bead + capacitor filters on supply lines.
- Bypass/decoupling capacitors at multiple frequencies.
Frequently Asked Questions
What is PSRR?
PSRR measures how well a circuit rejects power supply noise from affecting its RF output. Higher PSRR = better rejection. Critical for oscillators (phase noise), LNAs (noise figure), and PAs (spurious AM).
How does supply noise affect phase noise?
Supply ripple modulates the VCO through bias-dependent capacitances and transconductance. Each supply frequency component creates phase noise sidebands at that offset from the carrier. A switching regulator at 1 MHz creates spurs at +/- 1 MHz from the carrier.
How do you improve PSRR?
LDO voltage regulators provide 40-80 dB PSRR. Ferrite beads with capacitors form low-pass pi-filters. Multiple bypass capacitors (100 pF, 10 nF, 100 nF) provide broadband decoupling. Separate supply domains for sensitive circuits.