SA.45s Chip-Scale Atomic Clock
SA.45s CSAC User Guide Revision D
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Figure 18 • Frequency Stability (Allan Deviation) Versus Averaging Time (τ)
When disciplining, the stability of the output of the CSAC (combined clock) at any averaging time reflects
the noise properties of the dominant (most noisy) source. For example, if disciplining the CSAC to a GPS
source, which is noisier than the CSAC for averaging times τ < 5000 seconds, the disciplining time
constant should be set to τ > 5000 seconds so that the (superior) CSAC stability dominates for τ < 5000
seconds and the (superior) GPS stability dominates for τ > 5000 seconds. On the other hand, consider
the case where the CSAC is disciplined to a high-performance cesium clock, which is more stable than
the CSAC on all time scales. The noise is dominated by the phase meter for τ < 2 seconds and by the
CSAC for τ > 2 seconds. In this case, the disciplining time constant could be set to τ = 2 seconds for
optimal performance.
The following graph shows an example of a CSAC, which is disciplined to a superior reference (in this
case a hydrogen maser) with a time constant of 20 seconds. For this measurement, the CSAC was
deliberately mis-tuned in both frequency (y) and phase ( ) prior to the measurement, by
φ
y = 100 × 10 and = 50 ns
–10
φ
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