Phase Noise to RMS Jitter Calculator

Enter a phase-noise profile and an integration band, and get RMS jitter — plus which decade of offset is producing it, which is the part that tells you what to fix.

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Phase noise and jitter are the same measurement

A phase-noise plot and a jitter number describe the same imperfection in different domains. L(f) is the noise power in a 1 Hz bandwidth at an offset from the carrier, in dBc/Hz. Jitter is what you get when you integrate that across the band your system cares about.

The integration band is not a detail — it is the specification. The 12 kHz–20 MHz band shown by default is the telecom convention; PCI Express, Ethernet and JESD204 all specify different ones, and a part that passes in one band can fail in another with identical hardware. Always quote the band with the number.

Jitter scales inversely with carrier frequency

The same phase-noise profile produces half as much jitter at twice the carrier frequency, because a given phase error occupies a smaller fraction of a shorter period. This has two consequences that catch people out:

• Comparing oscillators on jitter alone is meaningless unless they are at the same frequency and integrated over the same band. A 156 MHz part will look better than a 25 MHz part with identical phase noise.
• Multiplying a clock up does not make jitter worse in the way people expect. A PLL multiplying by N raises the phase noise by 20 log N in dBc/Hz, and the carrier by N — so the jitter is roughly unchanged. What actually degrades it is the PLL's own noise inside the loop bandwidth.

The per-decade table is the useful part

Jitter is an integral, so it is dominated by wherever the area is — and that is rarely where the plot looks worst. The table gives each segment's share, because knowing that 70% of your jitter comes from the 10–100 kHz decade tells you to look at the PLL loop bandwidth and its supply, while a far-out contribution points at the output buffer and the power rail feeding it.

Rough map: close-in offsets are the reference and the PLL's loop; the mid region is loop bandwidth and VCO; far out is the buffer noise floor. Supply noise appears as spurs, which this integral treats as broadband and therefore underestimates — a real spur needs adding separately as a deterministic term.

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