Voltage Regulator
Understanding Voltage Regulators for RF
Power supply quality is often overlooked in RF design but has direct impact on system performance. VCO phase noise, LNA noise figure, and PA linearity all depend on supply noise. The voltage regulator is the last line of defense against supply noise.
LDO vs DC-DC for RF
| Parameter | LDO | DC-DC (Switching) |
|---|---|---|
| Output noise | Very low (1-50 uV RMS) | High (10-100 mV ripple) |
| PSRR | 40-80 dB | N/A (source of noise) |
| Efficiency | V_out/V_in (often < 50%) | 85-95% |
| Heat | High (V_drop x I) | Low |
Best Practice
Use DC-DC converter for efficient bulk power conversion, followed by LDO for final regulation and noise filtering before sensitive RF circuits. The LDO cleans up the switching noise from the DC-DC output.
Frequently Asked Questions
Why use an LDO for RF circuits?
LDOs provide 40-80 dB PSRR, filtering switching noise from DC-DC converters. Their low output noise (< 10 nV/sqrt(Hz)) prevents supply noise from degrading VCO phase noise, LNA NF, and amplifier linearity.
Can I power an RF circuit directly from a switching regulator?
Generally not recommended. Switching regulators produce 10-100 mV of ripple at the switching frequency (100 kHz - 5 MHz). This ripple modulates VCO frequency (phase noise spurs), LNA bias (NF degradation), and PA output (spurious AM).
How do I select an LDO for RF?
Key specs: noise spectral density (< 10 nV/sqrt(Hz)), PSRR at your switching frequency (> 40 dB at 1 MHz), dropout voltage (< 200 mV), current capability, and thermal performance. Ultra-low-noise LDOs designed for RF include ADM7150, LT3042, and TPS7A47.