Phase Noise
Understanding Phase Noise
An ideal oscillator produces a pure sinusoid at a single frequency, appearing as a delta function in the frequency domain. Real oscillators have random phase and amplitude variations that spread the signal energy into a skirt around the carrier. This spectral spreading is phase noise.
Phase Noise Specification
Phase noise is specified as L(f) in dBc/Hz at a given offset frequency from the carrier. For example, -110 dBc/Hz at 10 kHz offset means the noise power in a 1 Hz bandwidth, 10 kHz away from the carrier, is 110 dB below the carrier power. Phase noise typically follows a slope of -20 dB/decade (1/f^2) far from the carrier, with steeper slopes (-30 dB/decade) close in due to flicker noise.
Impact on Systems
- Radar: Phase noise limits the ability to detect small targets in the presence of clutter. The phase noise of the transmitter local oscillator directly sets the clutter floor.
- Communications: Phase noise degrades EVM (Error Vector Magnitude) in high-order modulation schemes (64-QAM, 256-QAM). It also causes reciprocal mixing, where a strong adjacent channel mixes with the LO phase noise into the desired channel.
- Test equipment: The phase noise of VNA and spectrum analyzer LOs determines the dynamic range for close-in measurements.
Reducing Phase Noise
- High-Q resonators: Crystal oscillators, sapphire resonators, and optical references provide the lowest phase noise.
- Clean power supply: Power supply noise modulates the oscillator frequency.
- Phase-locked loops (PLLs): Lock a noisy VCO to a clean reference, cleaning up the close-in phase noise.
L(f) = P_noise(f) / P_carrier (dBc/Hz)
Integrated phase noise (jitter):
φ_rms = √(2 × ∫ L(f) df) (radians)
Timing jitter from phase noise:
t_jitter = φ_rms / (2π × f_carrier)
Leeson model (simplified):
L(f) = 10log10[(2FkT/P_s) × (1 + f0/(2Qf))²]
Phase Noise Comparison by Source Type
| Source Type | @ 10 kHz offset | @ 100 kHz offset | Application |
|---|---|---|---|
| Crystal oscillator (OCXO) | -145 dBc/Hz | -170 dBc/Hz | Reference standards |
| SAW oscillator | -130 dBc/Hz | -155 dBc/Hz | Radar, instruments |
| Dielectric resonator (DRO) | -120 dBc/Hz | -145 dBc/Hz | Microwave LOs |
| PLL synthesizer | -100 to -120 dBc/Hz | -120 to -140 dBc/Hz | General purpose |
| Free-running VCO | -80 to -100 dBc/Hz | -100 to -120 dBc/Hz | Tunable sources |
Frequently Asked Questions
What is phase noise in RF?
Phase noise is the random fluctuation in the phase of an oscillator output, measured as noise power density in dBc/Hz at a specified offset from the carrier frequency. It represents how spectrally pure the oscillator signal is. Lower phase noise (more negative dBc/Hz values) means a cleaner, more stable signal.
Why does phase noise matter?
Phase noise limits the performance of radar (target detection in clutter), communications (EVM degradation and adjacent channel interference), and test equipment (measurement dynamic range). In high-order modulation like 256-QAM, the LO phase noise directly limits achievable data rates.
How is phase noise measured?
Phase noise is measured using a spectrum analyzer (direct method) or a phase noise test system that uses cross-correlation to reduce the measurement floor. The signal is compared against a lower-noise reference. Results are displayed as L(f) in dBc/Hz vs. offset frequency from the carrier.