How does phase noise limit receiver performance through reciprocal mixing?

When the LO in a receiver has phase noise, the noise sidebands mix with strong interfering signals and fold them into the IF passband. A strong interferer at offset f_offset from the desired signal mixes with the LO phase noise at that offset, creating in-band noise. The resulting noise floor equals the interferer power plus the LO phase noise at the offset frequency: N_reciprocal = P_interferer + L(f_offset) + 10 log10(BW). For a minus 20 dBm interferer 1 MHz away, an LO with minus 140 dBc/Hz phase noise at 1 MHz, and a 200 kHz channel bandwidth: the reciprocal mixing noise is minus 20 + minus 140 + 53 = minus 107 dBm. If this exceeds the thermal noise floor (minus 121 dBm for 200 kHz BW), it becomes the dominant noise source.

What does Leeson's model predict about phase noise?

Leeson's model shows that phase noise has three distinct regions as a function of offset frequency. Close to the carrier (inside the half-bandwidth of the resonator, f0/(2QL)), the phase noise drops at 30 dB per decade due to 1/f flicker noise upconversion. At intermediate offsets (beyond the resonator bandwidth but before the noise floor), it drops at 20 dB per decade. At far offsets, it flattens to a constant noise floor set by the active device's thermal noise and the oscillator's output power. Higher resonator Q pushes the 1/f^3 corner inward, which is why crystal oscillators (Q = 100,000) have extraordinarily low close-in phase noise compared to LC VCOs (Q = 10 to 30).

How does phase noise affect EVM in digital communications?

Phase noise causes the constellation points to smear tangentially around their ideal positions, contributing directly to EVM. The integrated phase noise over the signal bandwidth determines the EVM contribution: EVM_PN = sqrt(2 times integral of L(f) from f_low to f_high). For 256QAM with a 3.5 percent EVM budget, the phase noise allocation is typically less than 1 percent RMS, requiring integrated phase noise below minus 40 dBc over the signal bandwidth. For a 100 MHz 5G NR signal, the LO must achieve this integration from 100 Hz to 50 MHz offset, which demands a high-quality synthesizer with in-band phase noise below minus 95 dBc/Hz.

Signal Processing

Phase Noise

An ideal oscillator produces a single spectral line: all its energy at one frequency. A real oscillator spreads energy into noise sidebands that fall off with distance from the carrier. At 10 kHz offset from a 10 GHz VCO, the noise might be −100 dBc/Hz: one ten-billionth of the carrier power in each hertz of bandwidth. That sounds small until a strong interferer at that offset mixes with the noise sideband and folds directly into the receiver's IF passband. This reciprocal mixing mechanism means the LO's phase noise, not the LNA's noise figure, often sets the sensitivity limit in congested spectral environments.

Category: Signal Processing

Unit: dBc/Hz at offset

Key Model: Leeson's equation

The Noise Sidebands That Limit Your Receiver

Phase Noise by Oscillator Technology

Source	L(10 kHz)	L(100 kHz)	L(1 MHz)	Loaded Q	Application
OCXO (100 MHz)	−150 dBc/Hz	−170 dBc/Hz	−175 dBc/Hz	50,000+	Reference clock, test equipment
SAW oscillator (1 GHz)	−130 dBc/Hz	−150 dBc/Hz	−155 dBc/Hz	5,000+	Low-jitter clock
Dielectric resonator (10 GHz)	−115 dBc/Hz	−135 dBc/Hz	−155 dBc/Hz	2,000+	Radar LO, microwave link
LC VCO (MMIC, 5 GHz)	−95 dBc/Hz	−115 dBc/Hz	−140 dBc/Hz	10 to 30	Wideband synthesizer
Ring oscillator (CMOS)	−80 dBc/Hz	−100 dBc/Hz	−120 dBc/Hz	<5	Clock recovery, PLL CDR

Leeson's phase noise model:
L(f_m) = 10·log₁₀[(2FkT/P_s) × (1 + f₀/(2Q_Lf_m))² × (1 + f_c/f_m)]

where f_m = offset frequency, f₀ = carrier, Q_L = loaded Q,
F = device noise factor, P_s = signal power, f_c = 1/f corner

Three regions:
f_m < f_c: −30 dB/decade (1/f³, flicker upconversion)
f_c < f_m < f₀/(2Q_L): −20 dB/decade (1/f², white noise in loop)
f_m > f₀/(2Q_L): flat noise floor (−174 + NF + 10·log(1/P_s))

Common Questions

Frequently Asked Questions

How does phase noise limit receivers (reciprocal mixing)?

LO noise sidebands mix with strong interferers and fold into the IF passband. A −20 dBm interferer 1 MHz away with LO at −140 dBc/Hz creates −107 dBm in-band noise (200 kHz BW), which can exceed the thermal noise floor and become the dominant noise source.

What does Leeson's model predict?

Three slope regions: 1/f³ (−30 dB/dec) from flicker upconversion close-in, 1/f² (−20 dB/dec) at intermediate offsets, and a flat thermal floor far out. Higher Q pushes the 1/f³ corner closer to the carrier. Crystal Q of 100,000 vs. LC VCO Q of 20 explains their enormous phase noise difference.

Impact on EVM?

Phase noise smears constellation points tangentially. EVM_PN = √(2 × integrated L(f)). For 256QAM (3.5% EVM budget), integrated PN must be <−40 dBc over the signal BW. A 100 MHz 5G NR signal needs in-band PN below −95 dBc/Hz.

Related Terms

← Phase Modulation Phased Array →

Oscillator Design

Phase Noise Analysis Tool

Enter resonator Q, device NF, output power, and carrier frequency to predict phase noise using Leeson's model. Compare to measured data and compute integrated jitter.

Analyze Phase Noise