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The SAW and DRO tuning,
87
MHz
PLL,
UP
clock PLL, +/-12 volt supply, and
5 volt supply values should be relatively constant
from initial startup. All may
vary by 4- 0.1 volts during
normal operation,
The
t e m p e r a t u r e value is the internal temperature
of the standard. The value should
typically range from
35
to
45
degrees Celsius and should remain constant. It will
change as the standards environment changes,
CLOCK FAILURES
The recording
of
the parameters once per hour, every hour, and retaining the
information in permanent data files has
a
great benefit for detecting and analyzing
failures. It
also
allows for the prediction of some future failures.
U s u a l l y ,
analysis
of the historical parameters may only indicate
that
a failure will occur, but not
when. USNO has experienced a few problems
with
the standards and often looking
at the parameters has pointed directly to the needed repair. By taking hourly
readings of the parameters, one can watch
a
standard fail
and
prevent a related
system disaster.
An
example of
a
minor problem that can be seen
in
the parameters is a significant
change i n the standard's environmental t e m p e r a t u r e .
A change
in
the environmental
temperature
can show
up as an unusually large change
in
the electron multiplier
voltage (see Figures 1, 2, and 3),
More serious problems are seen when an electronic card inside the standard fails.
USNO has
had
some problems with the A2 and
A6
cards failing and has
documented the parameters showing the indications of
a
failure.
A
failure in the
A2
CBT controller card shows up as
a
significant change
i n
only
one parameter. The electron multiplier voltage will jump
m o r e
than 10
volts.
This
greater
than
10-volt change will occur over a 12- to %hour period (see Figure 4 ) .
Although a step in
this parameter is not always
an
indication of
an
A2
card failure,
the change is
an indication that it is probable. No parameter
changes
seem to
foretell this problem.
A
failure in the
A6
servo card causes the clock
to
lose frequency lock and shows
up
as
a change
i n
most of the parameters. The failure is
most
notable in the
oscillator control,
rf
amplitudes, and CBT parameters. The oscillator control will
jump more
than
5 percent when the failure occurs
(see Figure
5).
The rf
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