Microsemi SA.45s, User Manual

SA.45s Chip-Scale Atomic Clock

SA.45s CSAC User Guide Revision D

13

3.3.9.1

Cable Length Compensation

The zero point of disciplining can be adjusted to accommodate cable and other instrumentation delays 
(or advances) which impact the arrival time of the 1PPS at the CSAC 1PPS input pin. The compensation 
value can optionally be stored in the CSAC non-volatile RAM for one-time calibration.

The maximum compensation adjustment is ±100 ns, with resolution of 100 ps. The compensation value 
is entered into the CSAC as a signed integer in units of 100 ps, where positive sign indicates phase 
advancement of the input 1PPS. For example, if there is 45 ns of delay (approximately 33 feet of RG-58 
coaxial cable) between the on-time point and the CSAC 1PPS input then the compensation value would 
be +450.

Note: Cable length compensation can also be employed to correct for dynamic known errors in the 1PPS 
reference provided, for example, from an external measurement system. For this reason, upon 
application the compensation is subsequently applied to the previous 1PPS measurement.

Note: Compensation is implemented in the disciplining algorithm, not in the phase measurement itself. 
The phase measurement, as reported through telemetry, reports the actual phase measurement, that is, 
if the CSAC is disciplined with +50 ns of compensation, the phase meter reports –50 ns of phase error.

Compensation is set with the 

 command (see 

!DC

"Set 1PPS Disciplining Cable Length Compensation 

).

(DC)" (see page 27)

3.3.10

Time-of-Day

The CSAC maintains time-of-day (TOD) as a 32-bit unsigned integer, which is incremented synchronously 
with the rising edge of the 1PPS output. Until set otherwise, TOD begins counting from zero when the 
CSAC is powered on.

TOD is retrieved from the CSAC over RS232 with the 

 command (see 

!T?

Time-of-Day (TOD) (see page 

). When the 

 command is received, the CSAC waits for the next rising edge of 1PPS before 

26)

!T?

replying with the TOD of the current epoch, that is, if the command is received during epoch N, then the 
reply N+1 appears immediately following the next 1PPS. This strategy provides the host system with 
minimum ambiguity in interpreting the response.

TOD can be set with the 

 command through the RS232 interface. The 

 command includes provision 

!T

!T

both for setting an absolute number or for a differential (±) adjustment of the present TOD. An example 
is provided in the 

. To avoid ambiguity in setting the TOD, it is 

Time-of-Day section (see page 26)

recommended that the host system wait for 1PPS and transmit the setting/adjustment immediately 
thereafter.

The CSACdemo program shows TOD on the Time Of Day panel, accessed from the View menu, as shown 
in the following screen shot.