AN-001: Understanding Phase Noise

Application Note AN-001 · By KCCS Engineering Team · Updated 2026 · 9 min read

The Phase Noise Metric

Phase noise is the single most commonly quoted oscillator spec, and the most commonly misinterpreted. It measures how much energy the oscillator leaks into sidebands around the carrier, expressed as dBc/Hz at a specified offset frequency. A number like "-160 dBc/Hz @ 1 kHz" means that in a 1 Hz band 1 kHz away from the carrier, the noise power is 160 dB below the carrier power.

Lower is better. Each 10 dB improvement reduces sideband noise by a factor of 10. Going from -140 dBc/Hz to -170 dBc/Hz is a 1000× reduction in noise sidebands.

Why Offset Frequency Matters More Than the Headline Number

A datasheet quoting "-170 dBc/Hz" without telling you at what offset is meaningless. The same oscillator can read -110 dBc/Hz at 1 Hz offset, -150 dBc/Hz at 100 Hz, and -170 dBc/Hz at 1 kHz. The curve shape tells you far more than any single point.

Close-in offsets (1 Hz to 10 Hz) matter for radar and EW applications where nearby targets sit close to a strong clutter signal. Far-out offsets (10 kHz to 1 MHz) matter for wideband digital receivers and SerDes jitter.

Phase Noise vs Integrated Jitter

Phase noise is a frequency-domain spec. Jitter is the time-domain equivalent. To convert, you integrate the phase noise power over the offset range your system cares about. The relationship is:

Jitter (RMS) = sqrt(2 × 10^(L(f)/10) × df) / (2π f₀)

where L(f) is phase noise in dBc/Hz and f₀ is carrier frequency. In practice, datasheets that list both specs let you skip the math. A high-grade OCXO at 10 MHz typically integrates to less than 0.2 ps RMS jitter across 12 kHz to 20 MHz.

Why Two "170 dBc/Hz" Oscillators Are Not the Same

The measurement bandwidth, detector type and carrier frequency all affect the number. Two oscillators both quoted at -170 dBc/Hz @ 1 kHz may perform differently in your system if one is measured at 10 MHz and the other at 100 MHz, or if one uses a narrowband phase detector and the other a direct spectrum analyzer. Always ask for the full phase noise plot, not just the headline number.