Chapter 2
Device Overview
©
National Instruments
2-11
NI 660x User Manual
Measuring OCXO Stable Frequency Deviation
When yo
u
power the NI 6608 device, the OCXO req
u
ires adeq
u
ate
warm-
u
p time to reach stable freq
u
ency. Five min
u
tes is adeq
u
ate warm-
u
p
time for a power-off d
u
ration of less than one ho
u
r, with maxim
u
m
deviation within 20 ppb, or parts per billion, while fo
u
r ho
u
rs of operation
is adeq
u
ate for a power-off d
u
ration of
u
p to 90 days.
Note
For best performance, minimize power-off periods for the OCXO.
The OCXO is calibrated to within 0.1 Hz of 10.000000 MHz prior to
shipment. Table 2-2 shows additional change in stable freq
u
ency that
occ
u
rs over time. A change in stable freq
u
ency of approximately 45 ppb
occ
u
rs after the first year of normal
u
se.
For example, if the OCXO has a perfect stable freq
u
ency of 10 MHz after
warm-
u
p, after the first 10 days of operation, the stable freq
u
ency drifts
11.25 ppb. D
u
ring the next 50 days of operation, this freq
u
ency will drift
an additional 11.25 ppb, th
u
s making the total drift ca
u
sed by aging to be
22.5 ppb. After 365 days, drift will be 45 ppb. If yo
u
calibrate the OCXO
after 365 days of operation to restore the stable freq
u
ency to a perfect
10 MHz, the drift d
u
ring the first 10 days following calibration
(days 366-375) will now be 5.63 ppb—the stable freq
u
ency in this case will
Table 2-2.
Change in Stable Frequency over Time
Days of Operation
Additional Change in
Stable Frequency (ppb)
0-10
11.25
11-60
11.25
61-200
11.25
201-365
11.25
366-375
5.63
376-425
5.63
426-565
5.63
566-730
5.63
731-740
2.82
741-790
2.82
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