Example:
–
A user can accept a maximum of 3 Hz deviation on the
10 MHz frequency of the oscillator. This results in:
()Acceptable error =
×
=×
−
3
10 10
310
6
7
The aging and temperature factors can be selected from the table on
page 5-2.
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 years).
Example:
–
The user has the same requirements as in the example above.
The counter has a PM9691 oscillator.
–
Look up information about PM9691 in the table on page 5-2.
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
3*10
–8
–
Aging/year: Less than 1.5 * 10
–7
–
Use the MTBRC formula with the above values. This gives a
MTBRC of maximum:
310 310
15 10
18
78
7
×−×
×
=
−−
−
.
. year
NOTE: When recalibrating, the reference crystal oscillator will be
compensated only for frequency deviation caused by aging.
When to Replace the Fan
(PM6685R only )
To maintain the high reliability of a counter used in
around-the-clock’ applications, you must replace the fan every sec
-
ond year. For part time and low ambient temperature use, you can ex
-
tend this service interval to 6-10 years or more. Additional informa
-
tion can be found in Chapter 9, Appendix.
Preventive Maintenance 5-3
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