SA.45s Chip-Scale Atomic Clock
SA.45s CSAC User Guide Revision D
14
Figure 8 • CSACdemo Time-of-Day Panel
The raw CSAC TOD value is shown in the lower field of the panel (here 1260881710). The upper display
of the TOD panel realizes the timekeeping convention of the C programming language (in UNIX and
Microsoft Windows), which counts time in seconds from midnight on January 1, 1970. Upon clicking
, it sets the CSAC time according to the host PC's TOD counter (either local time or UTC depending
Send
on the setting of the pull-down menu to the left of the
). The and buttons for hours and seconds
Send
+
-
adjustment will increment or decrement the CSAC TOD by ±3600 or ±1 second respectively.
3.3.11
Analog Tuning
To enable analog frequency tuning for implementation in legacy (quartz crystal) applications, the
frequency of the CSAC can be tuned with an external voltage applied to pin 1. This functionality can be
enabled/disabled through a bit in the mode register (see
"Set/Clear Operating Modes (M)" (see page 24
to the microwave synthesizer at a rate of once per second, that is, the maximum tuning rate is 1 Hz.
When analog tuning is enabled, the voltage applied at pin 1 and the resultant steering are reported in
the standard telemetry stream (see
"Telemetry (6 and ^)" (see page 20)
range is 0 VDC–2.5 VDC, which corresponds to a full scale tuning range of 4.4 ×10 . Nominal zero-
–8
correction tuning occurs at a tuning input voltage of 1250 mV. The fractional frequency correction, for a
given applied voltage, is given by the following equation.
Δf/f = (V
– 1250 mV) × 1.77× 10 /mV
tune
–11
Note: This formula is accurate for the standard SA.45s CSAC, operating at 10.0 MHz output frequency.
Consult the datasheet for tuning curves of CSACs at alternate frequencies. The tuning input pin is
nominally biased at ≈1250 mV, that is, approximately zero correction.
Note: Bias voltage may vary due to component variations and/or exhibit temperature sensitivity.
Therefore, analog tuning should not be enabled unless the functionality is necessary and the analog
tuning input pin is connected to a low noise, low impedance voltage source. For non-legacy applications,
it is recommended that this feature remain disabled, and that corrections be applied through the digital
communications interface (see
) to avoid degradation of the CSAC
"Frequency Steering" (see page 8)
short-term stability due to voltage noise applied to the tuning pin.
Analog tuning can be enabled/disabled and monitored from the CSACdemo application from the Analog
Tuning… panel (accessible from the View menu), as shown in the following screen shot.
