Fluke PM6685 - Preventive Maintenance; Calibration

Preventive Maintenance
Calibration
To maintain the performance of your counter 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 labora
-
tory. Contact your local Fluke representative for calibration.
To know the present status of your instrument, test your timer/coun
-
ter from time to time. The test can be made according to the informa
-
tion in Chapter 2, Performance Check.
Oscillators
The frequency of the reference crystal oscillator is the main parame
-
ter affecting accuracy in a counter. The frequency is affected by ex
-
ternal conditions like the ambient temperature and supply voltage,
but also by aging. When recalibrating, the reference crystal oscillator
is compensated only for deviation in frequency due to aging.
n
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 stability. 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 tem
-
perature range of 0 to +50 °C, with a reference temperature of
+23 °C. If the timer/counter is used in a room temperature of
20 to 30 °C, the temperature stability of an OCXO will be in
-
creased by a factor of 3.
–
The temperature stability indicated for the standard oscillator is
mainly dependent on the ambient temperature. When the coun
-
ter is operating there is always an internal temperature increase
that will influence the oscillator.
n
Recalibration intervals
The Mean Time Between ReCalibration, MTBRC, is defined as:
MTBRC
Acceptable error Temperature stability
Agi
=
−()( )
(
ng)
MTBRC can be calculated when the total acceptable error and the
oscillator specifications are known.
The total acceptable error is defined as:
()
()
(
Acceptable error
Deviation of reference frequency
=
Nominal reference frequency)
5-2 Preventive Maintenance
Model PM6685 PM6685R
Option:
Timebase type:
Standard
UCXO
PM9691
OCXO
PM9692
OCXO Rubidium
Total uncertainty,
for operating temperature
0°Cto50°C,at2
s (95%) confidence interval:
- 1 month after calibration
- 3 months after calibration
- 1 year after calibration
- 2 years after calibration
< 1.2 x 10
-5
<1.2x10
-5
<1.2x10
-5
<1.5x10
-5
<3x10
-8
<4x10
-8
<1x10
-7
<2x10
-7
<8x10
-9
<1.2x10
-8
<2.5x10
-8
<5x10
-8
<4x10
-10
<4x10
-10
<4x10
-10 *
<6x10
-10 *
Typical total uncertainty, for operating
temperature 20°C to 26°C, at 2
s (95%) confi
-
dence interval:
- 1 month after calibration
- 3 months after calibration
- 1 year after calibration
- 2 years after calibration
<4x10
-6
<4x10
-6
<7x10
-6
<1.2x10
-5
<3x10
-8
<4x10
-8
<1x10
-7
<2x10
-7
<8x10
-9
<1.2x10
-5
<2.5x10
-8
<5x10
-8
<1x10
-10
<2x10
-10
<2.5x10
-10 *
<5x10
-10 *
*
After 1
st
year of operation. For 1
st
year add: < 3
x
10
-10
Table 5-1 Stability of timebase oscillators.