How does flicker noise affect oscillator phase noise?

In a free-running oscillator, the active device's flicker noise upconverts to the carrier frequency through the nonlinear mixing action of the oscillator. The Leeson model predicts three phase noise regions: (1) Close-in (1/f^3 region): dominated by upconverted flicker noise. L(fm) decreases at 30 dB/decade. Extends from the carrier to the flicker corner frequency divided by the resonator Q factor. (2) Mid-range (1/f^2 region): dominated by thermal noise amplified by the resonator's transfer function. L(fm) decreases at 20 dB/decade. (3) Far-out (flat): thermal noise floor, L = -174 + NF + 10*log(P_sig) - 3 dBc/Hz. The 1/f corner directly determines where the phase noise transitions from 1/f^3 to 1/f^2. Lower 1/f corner = lower close-in phase noise.

Why do different transistor technologies have different 1/f corners?

Flicker noise originates from carrier trapping and release at defects and interface states. The physical mechanisms vary by technology: SiGe HBT (1-10 kHz corner): bipolar transistors have very low flicker noise because current flows through the bulk crystal, away from interfaces. Few surface traps. Si CMOS (10-100 kHz): current flows at the Si-SiO2 interface, where interface traps are abundant. More traps = more flicker noise. GaAs pHEMT (100 kHz-1 MHz): surface states on III-V materials and defects in the buffer layers generate significant flicker noise. InP HEMT (50-500 kHz): similar to GaAs but slightly better. GaN HEMT (1-10 MHz): high defect density in epitaxial layers (threading dislocations, point defects) creates very high flicker noise. This is why GaN oscillators have poor close-in phase noise despite excellent power performance.

How do you reduce flicker noise impact?

Circuit techniques: (1) Use high-Q resonators: the resonator filters out close-in noise. Higher Q moves the 1/f^3 to 1/f^2 corner closer to the carrier. (2) Large signal operation: driving the transistor into compression reduces the flicker noise upconversion coefficient. (3) Feedback biasing: AC-coupled feedback can decorrelate the flicker noise from the oscillation. (4) Noise degeneration: source/emitter degeneration reduces the transconductance and thus the noise upconversion. Technology selection: use SiGe HBT for VCOs requiring best close-in phase noise. Use GaN only for power applications where far-out phase noise or output power is more important than close-in noise. For PLLs: the loop bandwidth can be set above the 1/f corner to suppress VCO flicker noise contribution.

Noise Mechanisms

Flicker Noise (1/f Noise)

/flik-er noyz/ (1/f)

Flicker noise has PSD proportional to 1/f. In oscillators, it upconverts to close-in phase noise with 1/f³ slope (Leeson model). The 1/f corner varies dramatically: SiGe HBT 1-10 kHz (best), Si CMOS 10-100 kHz, GaAs pHEMT 100 kHz-1 MHz, GaN HEMT 1-10 MHz (worst). Caused by carrier trapping at defects. Lower corner = better close-in phase noise.

PSD: S(f) = K/f

SiGe: 1-10 kHz corner

GaN: 1-10 MHz corner

Understanding Flicker Noise

Flicker noise is the hidden enemy of close-in phase noise. While thermal noise is flat and predictable, flicker noise rises as you approach DC, and when it enters an oscillator's nonlinear active device, it upconverts around the carrier as amplitude and phase modulation. The result: the close-in phase noise of an oscillator is often dominated by the active device's flicker noise, not thermal noise. Choosing the right transistor technology is therefore critical for VCO design.

Leeson Phase Noise Model

Flicker noise PSD:
S(f) = K_fI^a/(f^bC_oxWL)
a ≈ 2, b ≈ 1

Corner frequency:
f_c = where S_1/f = S_white

Hooge parameter:
S_V/V² = α_H/(f·N)

1/f Corner by Technology

Technology	1/f Corner	Close-in PN	Mechanism	Best For
SiGe HBT	1-10 kHz	Excellent	Bulk transport	VCOs, PLLs
Si CMOS	10-100 kHz	Good	Interface traps	Integrated VCOs
InP HBT	10-100 kHz	Good	Buffer defects	mmWave VCOs
GaAs pHEMT	100 kHz-1 MHz	Moderate	Surface states	LNAs, mixers
GaN HEMT	1-10 MHz	Poor	Threading dislocations	Power amps only

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Technology	f_c	Mechanism	PN impact	Application
SiGe HBT	1–5 kHz	Base recomb	Best	VCO/LO
Si BJT	5–50 kHz	Surface traps	Good	Precision osc
GaAs pHEMT	10–100 MHz	DX centers	Poor	LNA only
Si CMOS	0.5–10 MHz	Oxide traps	Moderate	Digital PLL
GaN HEMT	1–50 MHz	Buffer traps	Moderate	PA bias

Common Questions

Frequently Asked Questions

Phase noise impact?

Flicker noise upconverts in oscillators to 1/f^3 close-in phase noise. Leeson model: three regions (1/f^3, 1/f^2, flat). Lower 1/f corner = narrower 1/f^3 region = better close-in PN. SiGe HBT is best technology choice.

Technology differences?

SiGe HBT: 1-10 kHz (bulk transport, no surface). Si CMOS: 10-100 kHz (interface traps). GaN HEMT: 1-10 MHz (epitaxial defects). Use SiGe for VCOs; GaN only for power.

Reduction techniques?

High-Q resonator filters close-in noise. Large-signal operation reduces upconversion. Source degeneration. PLL loop BW above 1/f corner suppresses VCO flicker. Technology selection is most impactful.

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