AN-005: CSAC vs Rubidium vs OCXO

Application Note AN-005 · By KCCS Engineering Team · Updated 2026 · 8 min read

The Four Timing Reference Options

When an oven OCXO is not stable enough, the next step is an atomic reference. There are three grades above high-end OCXO: chip-scale atomic clock (CSAC), coherent population trapping (CPT) clock, and traditional rubidium. Each occupies a different tradeoff between power, size, stability and cost.

High-End OCXO — The Baseline

An ultra-high stability OCXO (KOT/KOS series) delivers ±0.01 ppb over temperature and ADEV around 1E-13 at 1 s. It needs 100–600 mW, takes a few minutes to warm up, and costs hundreds of dollars. It is the right answer for any application where temperature is controlled and holdover is measured in hours, not days.

CSAC — Chip-Scale Atomic Clock

A CSAC (like KCSA35) uses a miniature rubidium vapor cell on a silicon chip. It consumes only 130 mW — low enough for battery-powered operation — and delivers holdover around 5 μs over 24 hours, with aging below 1E-10 per month. It starts up in under a minute and fits in a 35×35 mm package.

The tradeoff: phase noise is higher than a good OCXO (around -140 dBc/Hz at 1 kHz), and short-term ADEV is worse. CSAC is a holdover solution, not a low-noise reference.

CPT Clock — Mid-Grade Atomic

A CPT-based clock (KCPTA50) uses coherent population trapping in a rubidium cell. It consumes around 3 W, delivers holdover under 1 μs over 24 hours across a wide temperature range (-40 to +80 °C), and has temperature stability around ±5E-11. It fits between a CSAC and a traditional rubidium: better holdover than CSAC, much lower power and smaller than rubidium.

Rubidium — The Laboratory Standard

A rubidium oscillator (KRA series) uses a rubidium gas cell and a physics package to lock the crystal to the Rb atomic resonance. It delivers aging around 5E-11 per month, phase noise comparable to a high-grade OCXO, and 20+ year service life. The tradeoff is power (5–20 W), warm-up time (5–15 minutes) and cost (thousands of dollars).

Decision Criteria

Use OCXO if: temperature is controlled, holdover window is hours, and you need the lowest phase noise.

Use CSAC if: GNSS is unreliable, battery operation is required, and you need sub-microsecond holdover over days.

Use CPT if: wide temperature range and outdoor deployment matter more than absolute power, and you want better holdover than CSAC without rubidium cost.

Use Rubidium if: this is a lab reference, a broadcast studio, or a long-haul telecom node that needs the best long-term stability regardless of power and size.