SA.45s Chip-Scale Atomic Clock
SA.45s CSAC User Guide Revision D
23
3.4.3.2
Frequency Adjustment (F)
The output frequency of the CSAC may be adjusted (steered) through RS232. The internal resolution of
the fractional frequency correction is approximately 1 × 10 . The correction is entered in integer units
–12
of ×10 . The maximum allowed correction in a single
command is ±20000000 (2 × 10 ). The
–15
!FD
–8
maximum allowed correction in a single
command is approximately ±2000000000 (2 × 10 ).
!FA
–6
Corrections may be applied as either Absolute or Relative, depending on the first character following the
, that is
or
for absolute or relative (delta) respectively. In the case of absolute steering, the
!F
!FA
!FD
value of the Steer register is replaced with the new value. In the case of relative (delta) steering, the
new value is summed with the existing value in the Steer register, that is, two relative corrections of
–10000 result in a total offset of –2 × 10 . The current steering value is reported in the
field of
–11
steer
the telemetry in units of ×10 .
–12
The format for the Adjust frequency command is
!FYXXXXX[CRLF]
where Y is either A or D and XXXXX is the new correction in ×10 .
–15
Example (apply absolute tuning correction of –1.23 × 10 ):
–10
Command:
!FA-123000[CRLF]
Unit response:
Steer = -123[CRLF]
Example (apply delta tuning correction of –1.23 × 10 ):
–10
Command:
!FD-123000[CRLF]
Unit response:
Steer = –246[CRLF]
Example (report current value of
):
steer
Command:
!F?[CRLF]
Unit response:
Steer = –246[CRLF]
Note: That the single-character is a shortcut for
.
F
!F?[CRLF]
The contents of the Steer register are volatile, that is, the Steer is reset to 0 when power is cycled to the
CSAC. In many cases, it is desirable to preserve the steer upon power-down (for example, calibration of
the CSAC). This is accomplished by sending a Frequency Latch command to the CSAC, which updates the
internal calibration (stored in non-volatile memory) according to the current value of the Steer register
and resets Steer to zero. Note that the Latch command is only valid when the CSAC is locked (Status = 0).
Example:
Command:
!FL[CRLF]
Unit response:
Steer Latched [CRLF] Steer = 0[CRLF]
Once a Latch command is issued, the previously latched value cannot be recovered. There is no
command to restore “factory defaults”.
Note: The frequency steering command (
) is recommended for real-time disciplining of CSACs, but
!F
the value should not be latched (
) on every steer due to the physical limit on the number of times
!FL
the non-volatile memory may be written before damage (20,000). For example, if an
command was
!FL
applied to the CSAC, accompanying a steer (
), at a rate of 1/sec, the CSAC may experience
!F
operational failure within 8 hours.
