Operation
2019 Microchip Technology Inc.
DS50002938A-page 25
3.4.2
Selection of Disciplining Time Constant, “Tau”
Algorithm performance will be predicated on the selection of the disciplining time con-
stant Tau. When possible, it is advisable to choose a Tau that corresponds to the “least
noisy” averaging time of your 1PPS input source. Generally, a longer Tau is required
for a noisy 1PPS input reference (such as a simple GPS receiver); shorter Tau is
acceptable for stable lab-grade instruments (such as a Cesium Beam Tube). Further-
more, shortening the Tau may be necessary to quickly adjust the MAC if its inherent
stability is under influence of external environmental conditions (such as g-forces, rapid
temperature changes, etc.).
FIGURE 3-2:
Sample ADEV Curves.
shows the performance of several frequency references. For optimum Dis-
ciplining performance when disciplining to a GNSS reference, a time constant should
be selected where the MAC and GNSS ADEV curves intersect (
τ
= 25,000 seconds, in
this example). In this way, the resulting performance of the Disciplined-MAC will have
short and mid-term stability of a free-running MAC, combined with the long-term stabil-
ity of GNSS.
Note:
The above set of curves is very generic and a designer should inde-
pendently quantify the stability of their own GNSS source. Usually, ADEV
information for GNSS receivers is not published because the stability will
vary according to a variety of environmental conditions; the designer will
have to generate this plot on their own. A rough approximation would be to
measure the 1s ADEV performance of a GNSS and extrapolate the curve
proportional to 1/
√
(Tau).
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