15254-201.docx
9
Rev. C, 07/12/2011
disciplining to an external primary reference to remove frequency offsets due to long-term
aging.
Operation
Connect the 1 PPS source to the 1 PPS IN on the rear panel of the 8040C/8040CLN. The
8040C/8040CLN’s Symmetricom Synchronization Adaptive Algorithm (SSAA) qualifies
the 1 PPS input reference by detecting 256 valid 1 PPS input pulses and determining
the number of outliers based on the time constant. An outlier is detected when the
absolute time difference between the input 1 PPS and its expected time is greater than 1
microsecond. Once the SSAA detects two outliers (two bad 1 PPS pulses) or no input 1
PPS, the algorithm places the 8040C/8040CLN into flywheel (holdover) mode. The
flywheel mode provides for Rubidium short-term and long-term stability without the
benefit of an external reference. The 8040C/8040CLN remains in flywheel mode until 256
“good” 1 PPS input pulses are detected. Once the number of outliers is less than 2, the unit
disciplines to the external reference. This implementation was designed to support
applications where the reference input is a GPS receiver without serial
communications between the receiver and the 8040C/8040CLN. There are two modes
of operation for the 1 PPS input: manual and automatic.
Manual Control
The manual mode is beneficial to applications where the quality of 1 PPS is worse than
50 nanoseconds, or applications where the noise profile is well known and a
deterministic solution yielding more control to the system designer is desired. The
manual mode requires the user to input two parameters, which are tau (or time constant)
and dampening factor. Tau is expressed in seconds and determines the PLL time
constant for following a step in phase for the reference. The range of tau is 5 to 100,000
seconds. Tau values outside of this range put the unit in automatic disciplining mode.
The dampening factor determines the relative response time and ringing in response to
each step. There are no limitations for the dampening factor value; however, values
between 0.5 and 2 are strongly recommended.
Automatic Control
The automatic mode requires no user inputs to the SA.22C Rubidium oscillator.
Automatic mode is adaptive and changes the SSAA time constant as changes in the
1 PPS reference are detected. The automatic mode is optimized for a 1 PPS input with up to
50 nanoseconds RMS of noise such as from a GPS timing receiver. Since short-term
jitter of 50 nanoseconds is typically present on 1 PPS from a GPS reference, the
automatic mode is generally suitable for most applications.
Configuring Outputs
The 8040C/8040CLN has the unique capability of providing flexible output
configurations. To change the output configuration of your 8040C/8040CLN:
1. Unplug the 8040C/8040CLN from the AC or DC power source and remove the
top cover by removing the Phillips head screws around the perimeter of the unit.
2. Locate the main PCB inside the 8040C/8040CLN as indicated in Figure 4 and
identify the dipswitches shown in Figure 5.
Each dipswitch configures the output
format on its corresponding BNC
(e.g., SW
1
configures the output format on
BNC
1
).
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