Theory Of Operations; Generation Of Timing Signals; Startup Sequence - Symmetricom ATS-6511A User Manual

4 Theory of Operations

The ATS-6511's function is the generation of timing signals extracted from the proprietary time
code contained in the fiber cable connected to an upstream 4372A installed in an ATS-6501.
4.1

Generation of Timing Signals

An OCXO serves as the low noise disciplined reference for the ATS-6511. The frequency of the
low noise OCXO is measured against the frequency from the fiber signal and the low noise
OCXO is steered to be on frequency. The phase adjustments made to the OCXO allow it to be
aligned with UTC(USNO), via the ATS-6501 which is the source of GPS time and is used as a
long term frequency reference. In doing this, the ATS-6511 exhibits the short-term frequency
stability characteristics of the OCXO while exhibiting the long-term characteristics of GPS. The
OCXO provides low phase noise performance for the 10 MHz outputs.
The 10MHz output provided by the OCXO serves as the time base for the ATS-6511. All timing
signals (e.g., 1 PPS, IRIG-B, 10 MHz) are derived from this reference and thus exhibit the same
frequency and phase characteristics. The 100 MHz TCXO which is the clock for the main FPGA
is divided by 10 and phase locked to the OCXO 10 MHz output. The rear panel 10 MHz plug in
card has a D/A converter which is used along with a sine look up table to provide a 10 MHz
signal from the 100 MHz internal clock. This D/A output is then filtered and distributed to the
four outputs of the plug in card. The FPGA which is clocked by the 100 MHz TCXO provides 1
PPS and serial time codes which are used by the 1 PPS/DC IRIG and AM IRIG cards to produce
1 PPS and IRIG time codes.
The 2 channel Low Noise Synthesizer (LNS) contains the low noise OCXO. A 1 PPS signal is
generated in the LNS which is used to measure the fiber optic 1 PPS time of arrival. This
measurement is used by the software PLL to steer the OCXO frequency to keep the ATS-6511 1
PPS on time with the ATS-6501. Commands are sent by the CPU to the Low Noise Synthesizer
clock to adjust the analog control voltage to the OCXO as a means of affecting this steering.
The PPS signal from the fiber is used in aligning both the IRIG-B signal and the CPU clock to
UTC(USNO). At startup this PPS is used to align the internally generated 1 PPS and IRIG-B
signal with the correct 10MHz clock cycle. From that point on the 1 PPS and IRIG-B signals are
kept on time by using the OCXO 10MHz signal as its reference. The 1 PPS signal from the fiber
generates an interrupt to the ARM 9 processor which serves as an indicator of the top-of-second.
This is used by the CPU along with EPOC messages received from the fiber to set the CPU time
to UTC(USNO).
When both fibers inputs are no longer available the ATS 6511B and C provides superior holdover
performance frequency locking the OCXO to the internal Rubidium or external reference input.
4.2

Startup Sequence

A very specific sequence exists for starting each of the internal devices within the ATS-6511.
This sequence exists to ensure that the unit generates the appropriate timing outputs and that they
are all in sync with each other. Startup time of the ATS-6511 varies from 5 to 15 minutes
depending upon how long the unit has been powered off but the timing outputs are not enabled
until 1 PPS and 10 MHz are accurate.
ATS-6511A/B/C Users Guide
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DOC 6511_Release K