Fluke PM6681 - Page 50

Preventive
Maintenance
Calibration
To
maintain performance of PM
6681
we recommend that
you
calibrate your instrument
every
year, or more often, if
greater
time base accuracy is
required. Calibration
should
be
performed with traceable
references and
instruments at
a certified calibration
laboratory. Contact your
local Fluke
service
center for calibration.
To
know the
present status of your
instrument, test your
ti-
mer/counter from
time
to time. The
test
can be made accor-
ding to the
information in
Chapter
2,
Performance
Check.
Oscillators
The frequency of the
reference crystal
oscillator
is the
main parameter
affecting accuracy in
a counter. The fre-
quency is
affected by external
conditions like the
ambient
temperature
and supply
voltage, but also by aging.
When
recalibrating, the
reference crystal
oscillator
is compensa-
ted only for
deviation in
frequency due to aging.
•
Some
important points:
-
The high
stability oscillators
have
been built into an
oven
in
order to
keep the oscillator
temperature as stable
as
possible.
Continuous operation is
also important for
sta-
bility. After
a power
interruption,
the
oscillator
restarts at
a slightly
different frequency.
It will then, as
time
goes
on,
age at an
equal rate.
-
The
stability
indicated for the
oscillators
is valid within a
temperature
range of 0
to 50°C, with a
reference
tempe-
rature
of 23°C.
If the
timer/counter
is
used in a room
temperature
of
20 to 30°C, the
temperature
stability
of
a
TCXO
or
OCXO
will
be
increased by a
factor of 3.
-
The
temperature
stability
indicated
for
TCXO and
stand-
ard
oscillators
are
mainly
dependent on the
ambient tem-
perature.
When
operating
there is always a
temperature
increase
inside
the
counter
which
will influence
the oscil-
lator.
•
Recaiibration
intervais
The Mean
Time
Between
ReCalibration,
MTBRC,
is defi-
ned as:
MTBRC
=
{Acceptable
error)
-
(Temperature stabiiity
)
(Aging
)
MTBRC can be
calculated when
the
total
acceptable
error
and the
oscillator
specifications
are known.
The total
acceptable
error is defined
as:
(PeviatiorDf
reference
frequency)
{Nominal
frequencyreference
)
(Acceptableerror)
='
Example:
-
A user can
accept
a maximum of
3 Hz deviation on
the
10 MHz
frequency of the
oscillator.
This
results
in:
{Acceptableerrot
)=
—
x
= S^i^lO"^
10
*
10
®
The aging and
temperature factors
can be
selected from
the
table
on page
5-3.
The
value of the aging
factor
is correctly
selected from the
table when the
calculation
of MTBRC
results in 1 to 30
days (use /24h),
1
to 12 months (use
/month) or
over
1
year (use
/year) (not, e.g., 43 days or
17 months or
0.8
ye-
ars).
Example:
—
The user has the same
requirements
as
in the example
above.
The counter has a
PM
9690
oscillator.
-
Look up
information about PM
9690 in the
table
on
page
5-3.
The results will be
the
following:
Relative
Frequency deviation
caused
by:
•
Ambient
temperature
deviation
(within 0 to 50°C;
reference
point at
23°C): Less than
1.5
*
10“®
•
Aging/year:
Less than 1
*
1
0“^
“
Use the MTBRC
formula with
the
above
values. This gi-
ves
a MTBRC of maximum:
(3*1
0~^)-(1
.5*10"°
°)
1
=
5
=
10
'
r7
:
2.9
year
See
also Figure 5-1, Figure
5-2,
and Figure
5-3.
5-2
Preventive
Maintenance