Voltage Regulation
Understanding Voltage Regulation for RF
Every active RF component requires a stable DC supply. A VCO with 10 MHz/V pushing sensitivity converts 10 μV of supply noise into 100 Hz of frequency modulation, degrading phase noise. An LNA biased from a noisy supply exhibits gain fluctuations that appear as amplitude modulation on the received signal. A power amplifier with inadequate supply filtering produces intermodulation products at the switching regulator's fundamental and harmonics. The regulator is not an afterthought; it is as critical to system performance as the RF components themselves.
Two fundamental regulator topologies exist. Linear regulators use a series pass transistor that acts as a variable resistor, dropping the excess voltage as heat. They are inherently quiet (no switching) but waste power proportional to the voltage drop times the load current. Switching regulators use an inductor and a high-frequency switch to convert voltage levels efficiently (85 to 95%), but the switching action generates output ripple and conducted/radiated emissions. The standard RF design pattern uses a switching converter for bulk power conversion followed by an LDO for final voltage regulation and noise cleanup.
Regulation Specifications
ΔVout/ΔIload (mV/A or % over full load range)
Typical LDO: 0.01 to 0.1%
Line Regulation:
ΔVout/ΔVin (mV/V or % over input range)
Typical LDO: 0.01 to 0.05%
PSRR (Power Supply Rejection Ratio):
PSRR = 20 log(Vripple,in/Vripple,out) dB
Typical at 100 kHz: 40-60 dB; at 1 MHz: 20-40 dB
LDO Dropout Voltage:
Vdropout = Vin,min − Vout (typically 100-300 mV for modern LDOs)
Regulator Type Comparison for RF Applications
| Parameter | LDO Linear | Switching (Buck) | Switching + LDO Post-Reg |
|---|---|---|---|
| Output Noise | 4-10 μV RMS | 10-50 mV ripple | 4-10 μV RMS |
| Efficiency | Vout/Vin (linear) | 85-95% | 80-90% |
| Heat Dissipation | High (at large dropout) | Low | Moderate |
| PSRR at 1 MHz | 30-50 dB | N/A (source of noise) | 30-50 dB |
| Best For | VCO, LNA, mixer bias | Digital, PA drain (insensitive) | All RF stages (best practice) |
Frequently Asked Questions
Why are LDO regulators preferred for RF circuits over switching regulators?
LDOs use a linear pass transistor with no switching, producing output noise as low as 4 to 10 μV RMS. Switching regulators generate ripple of 10 to 50 mV plus high-frequency transients that couple into sensitive RF circuits as spurs. A VCO on a noisy supply shows degraded phase noise because supply variations modulate the varactor capacitance. The standard practice is a switching pre-regulator for efficiency, followed by an LDO for clean final regulation.
What is PSRR and why does it matter for RF design?
PSRR measures how well a regulator attenuates input ripple, in dB. An LDO with 60 dB PSRR at 1 MHz reduces 100 mV of ripple to 0.1 mV. PSRR degrades at higher frequencies: 80 dB at 100 Hz may fall to 30 dB at 1 MHz. For RF circuits powered by a switching pre-regulator, the LDO's PSRR must be adequate at the switcher's fundamental frequency and its harmonics to prevent coupling of switching noise into the RF signal path.
How does supply noise affect oscillator phase noise?
Supply noise modulates the varactor capacitance in a VCO, converting voltage noise directly to phase noise via the pushing figure (MHz/V). A VCO with 10 MHz/V pushing and 10 μV/√Hz supply noise at 100 kHz offset sees phase noise degradation of roughly −110 dBc/Hz at that offset. Ultra-low-noise LDOs (< 5 μV RMS) are mandatory for VCO supplies. Additional LC filtering or ferrite beads between the LDO output and VCO supply pin provide further attenuation above 10 MHz.