DOC 4380A
4380A O & M Manual
58
098-00723-000
– Jul 2017 Revision 4.0
SyncSystem 4380A O&M Manual
4
Theory of Operations
Two distinct tasks are performed by the 4380A: collection of GPS measurement data or the deterministic
alignment to an external source and the generation of timing signals. Because the generation of timing
signals requires that events (e.g., clock steering) occur at very specific times, operational precedence is
always given to timing tasks. As a result, any operations concerning the collection and storage of GPS data
are postponed until all timing operations have been completed.
4.1 Generation of Timing Signals
A rubidium standard (when installed) serves as the short term and holdover with no GPS frequency
reference for the 4380A. The frequency of the Rubidium is measured and the low noise OCXO is steered
to be on frequency. The frequency adjustments made to the OCXO allow it to be aligned with
UTC(USNO) which is the source of GPS time and is used as a long term frequency reference. In doing
this, the 4380A exhibits the short-term frequency stability characteristics of the Rubidium 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 4380A. 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.
4.2 GPS
The GPS receiver locks its internal reference to the 10MHz signal provided by the OCXO frequency
reference. A 1 PPS signal is then generated by the receiver which has a fixed phase relationship to the
frequency reference. Measurements of this PPS are made to determine its offset to UTC(USNO) and this
information is passed to the CPU via a serial port. These measurements are used in generating a timescale
that estimates the clock offset from UTC(USNO). The CPU slowly adjusts the frequency of the OCXO to
align the PPS to be coincident with UTC(USNO), Depending on the type of frequency reference, this
adjustment period varies from a day (rubidium) to several months (external cesium), although rough
alignment to within 10 ns occurs on the order of a few hours in either case. 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.
4.3 Output Signals
The PPS signal from the GPS receiver/reference source is used to align 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 10 MHz 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 GPS receiver 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 messages received from the GPS receiver to set the CPU time to UTC(USNO). In doing
this, the CPU can serve as a stratum-1 network time protocol (NTP) server and provide time to other
computers and systems on the network. The OCXO control voltage is polled every LNS time constant in
order to weed out transients due to steering allowing the system to accurately determine if the OCXO
control voltage is at a minimum or maximum value. The averaging of this control voltage provides more
stable steering over time.
Summary of Contents for SyncSystem 4380A
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