SA.45s Chip-Scale Atomic Clock
SA.45s CSAC User Guide Revision D
12
3.3.9
1PPS Disciplining
A high-resolution phase meter is implemented within the CSAC for improved synchronization (<100 ns)
as well as for frequency calibration of the CSAC. The phase meter measures the time difference between
the internal CSAC 1PPS (pin 10) and the externally applied reference 1PPS (pin 9). The phase meter
measures the relative phase between the CSAC and the reference once per second with a resolution of
450 ps.
Based on the measurements of the phase meter, the disciplining algorithms adjust the CSAC's
microwave synthesizer through the digital tuning value (
, observed from the telemetry string).
steer
The algorithm will simultaneously steer both the phase and frequency to that of the external reference,
ultimately achieving accuracies of <5 ns and 5 × 10 , respectively. Note that, unlike the
and
–13
!FA
!FD
commands, there is no software limitation upon the value of steer.
Disciplining can be enabled/disabled through bit 4 (0x0010) in the mode register (see
). The time constant of the steering algorithm is user selectable
Operating Modes (M)" (see page 24)
"Set 1PPS Disciplining Time Constant (D)" (see page 25)
Note: Both mode setting and time constant are non-volatile, that is, preserved across power cycles.
Prior to the onset of steering, the disciplining algorithms first perform an initialization sequence in which
the variables of the steering algorithm are reset to defaults and a 1PPS synchronization operation (see
) is executed to bring the 1PPS output within 100 ns of the
"1PPS Synchronization (S)" (see page 25)
reference, thereby avoiding large frequency excursions. Initialization is performed when Disciplining is
first enabled in the mode register and, in the case where Disciplining is already enabled after the CSAC
achieves frequency lock (BITE = 0, status = 0).
In the event that the 1PPS reference is removed from pin 9 while Disciplining, the CSAC remains in
holdover and preserves the most recent steering value. If the 1PPS reference subsequently reappears,
Disciplining continues where it left off, without reinitializing. The notable exception to this is the case in
which the CSAC 1PPS has drifted significantly in phase (>1 μs) from the reference 1PPS during the
outage. In this case a synchronization is performed, though the Disciplining variables are not
reinitialized.
If it is necessary to force re-initialization of the disciplining variables, perhaps because the reference
source is subsequently deemed untrustworthy and subsequently recovers, this can be accomplished by
disabling and re-enabling Disciplining in the mode register (see
"Set/Clear Operating Modes (M)" (see
).
When Disciplining is enabled, the most recent phase meter measurement, rounded to the nearest
nanosecond, is reported in the standard telemetry (see
"Telemetry (6 and ^)" (see page 20)
of the reported value reflects the measurement of (1PPS_EXT–1PPS_CSAC), that is, if the CSAC 1PPS
rising edge occurs after the external 1PPS rising edge, then the sign is negative.
The status of Disciplining is indicated by the
parameter in the telemetry. DiscOK = 0 upon
DiscOK
startup. DiscOK = 1 when magnitude of phase measurement is less than phase threshold (see
) for two time constants of duration.
Phase Threshold for Discipline Status OK Check (m)" (see page 27)
DiscOK = 2 when in holdover (disciplining enabled but no 1PPS present).
Note: Automatic Synchronization mode (see
"Automatic Synchronization" (see page 11)
Disciplining mode are mutually exclusive. Enabling either in the mode register disables the other.
In CSACdemo, enabling/disabling Disciplining and setting the discipline time constant are both
accomplished on the 1PPS panel, accessible from the View menu (See
modify the discipline time constant, enter the new value in the field (10–10000) and click
.
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