EndRun Meridian II, Руководство пользователя

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M e r i d i a n I I U s e r M a n u a l 124
C H A P T E R T E N
125 M e r i d i a n I I U s e r M a n u a l
R E A R - P A N E L I / O
Theory of Operation
The 5071A Primary Frequency Standard is the “gold” standard of commercial Cesium frequency
standards. Their release in the early 1990’s introduced a quantum leap in performance over previous
devices, which is to this day unsurpassed. As such, they are the dominant contributors to the global
ensemble of clocks that make up the UTC time scale. International timing laboratories equipped with
exotic reference frequency standards and capable of measuring the characteristics of the 5071A have
verified their performance and published their results, so they are well known.
As with all free-running clocks, they do not maintain perfect synchronization with another indepen-
dent timescale, such as UTC(USNO). Error in initial frequency calibration along with the ran-
dom, divergent effect of flicker frequency modulation inevitably results in the phase drifting away
from UTC. In order to maintain long term synchronization with the UTC(USNO) timescale, the
5071A must be steered . The obvious way to do this is to measure the phase of the 5071A versus
UTC(USNO) via GPS and adjust the frequency of the 5071A based on the measurements. (In this
respect, the 5071A is no different than the other optional oscillators which might be installed inside
the chassis of the Meridian II.)
The GPS receiver module inside of the Meridian II has the ability to phaselock its internal oscillator
to an external oscillator. This provides the most accurate and stable means of measuring the phase
of the 5071A versus GPS and ultimately UTC(USNO). The 5071A has the ability to be steered with
very fine resolution, about 6.3 parts in 10
15
. By combining these two features, a UTC(USNO) time
and frequency reference with extraordinary performance may be realized, with the excellent medium
to long term stability of the the 5071A and the longer term accuracy and stability of the UTC(USNO)
or GPS timescale. The 3
rd
order phase lock algorithm reduces the long term Allan deviation of
the 5071A from flicker frequency modulation to white phase modulation relative to the selected
UTC(USNO) or GPS reference.
Because the 5071A stability is so good, a poorly designed GPS timing receiver and frequency control
algorithm could easily degrade the normal performance of the 5071A. The Meridian II GPS receiver,
when equipped with its optional Real Time Ionospheric Corrections (RTIC) algorithm and a precisely
determined antenna position, provides GPS timing measurement accuracy and stability that does not
degrade the stability of the 5071A while implementing a tight-enough control of the 5071A to realize
the long term accuracy of the reference timescale, either UTC(USNO) or GPS. The control loop
parameter “sweet spot” to achieve this was determined from the published measured performance of
the 5071A along with the measured performance of the Meridian II GPS receiver equipped with the
RTIC option at NIST in May, 2016.
Installation
Setup of the 5071A for use with the 5071A Cesium Control Module is simple:
1. The 5071A should be powered on and the RS-232 serial port settings configured via the front
panel CONFIG->SERIAL menu:
Baud rate: 9600
Data bits: 8
Parity: none
Stop bits: 1