SA.45s Chip-Scale Atomic Clock
SA.45s CSAC User Guide Revision D
8
Note: Driving a 50 Ω line at 13 dBm consumes nearly as much power as the CSAC itself. If a high-level
(high-power) output driver is required, a driver circuit must be implemented external to the CSAC, such
as the one implemented on the Evaluation Board (see
).
"Notes on the Evaluation Board" (see page 37)
The RF output appears on pin 12 after the CSAC is powered ON and is always present, regardless of the
lock status. When the CSAC is out of lock (BITE = 1, status ≠ 0), the output frequency is provided by the
free-running TCXO, which has frequency accuracy specification of ±20 × 10 and temperature sensitivity
–6
of ≈ ±30 × 10 /°C. Typically, the unlocked frequency accuracy during acquisition is significantly better
–9
than this (<1 × 10 ) as the CSAC memorizes its last-known-good tuning voltage and restores this voltage
–8
upon power-up and/or subsequent recovery from loss-of-lock.
3.3.5
What to Expect During First Power-Up
All CSACs have their output frequency re-centered prior to shipment. However, the shipment conditions
will affect the absolute frequency offset when received by the end-user (temperature excursions,
vibration, duration of transit, and so on). Therefore, some unknown frequency offset should be
expected when the CSAC is first powered on by the user. Offsets may be corrected, as explained in the
following section.
3.3.6
Frequency Steering
Note: CSACdemo is a graphical interface used to communicate and control a CSAC. To display the
functionality of CSAC, screen shots of CSACdemo are included in the sections that follow. For more
information on CSACdemo, see
.
"CSACdemo Operation" (see page 31)
For external steering and/or calibration, the CSAC internal microwave synthesizer may be adjusted by
the user through the RS232
). Steering
!F
"Frequency Adjustment (F)" (see page 23)
values are entered in (integer) units of 1 × 10 , though the resolution realized by the CSAC hardware is
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approximately 1 × 10 . Steering commands may be entered as either absolute steers (
) or as
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!FA
relative steers (
). In the case of an absolute steer, the contents of the steer register are replaced
!FD
with the new value. In the case of a relative steer, the new value is summed with the existing value in
the steer register. The maximum steer that can be entered in a single
command is ±2 × 10
!FD
–8
(±20000000 × 10 ). The maximum steer that can be entered in a single
command is approximately
–15
!FA
±2 × 10 (±2000000000 × 10 ). If a larger value is sent to the CSAC, the maximum allowed steer is
–6
–15
applied.
Caution: It is recommended to limit steering commands to <±2 × 10 increments, to allow the CSAC to
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slowly adjust. A larger steer may cause the CSAC to unlock. Do not allow steering to accumulate beyond
the stated datasheet limit.
To overcome the software limit (±2 × 10 ) imposed by the
command, the latch command (
)
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!FD
!FL
will need to be employed (for more information, see
).
"Frequency Calibration" (see page 9)
Ultimately, the CSAC steering capability is physically limited by the TCXO tuning voltage. An
will trigger when this voltage is breached. Generally, steering the CSAC far from its nominal
output frequency is undesirable as it will make disciplining difficult, therefore increasing the likelihood
of unlock.
Note: Steering commands may be entered during acquisition (Status ≠ 0) but will not take effect until
lock is achieved.
Frequency steering is volatile. Upon reboot, the CSAC returns to its nominal (calibrated) frequency
setting. To update the non-volatile calibration, use the Frequency Latch command (
).
The current steering value appears in the telemetry string as
.
Steer
Note: Steer reports the actual hardware steering, in units of ×10 , even though the software registers
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maintain resolution of ×10 , so that many small relative corrections may be applied. As a result, the
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reported value may appear to disagree with the applied correction by one unit or so due to roundoff
error. An example is provided in
.
"Frequency Adjustment (F)" (see page 23)
To apply a frequency correction from the main panel of CSACdemo, select relative or absolute from the
