Pendulum GPS-12, User Manual

Page: 36 / 62

Calibration
Calibration and adjustment in the traditional
sense are not necessary as long as the instru-
ment is operating in disciplined mode. Then the
internal rubidium timebase is continuously
monitored and forced to follow the cesium
clocks of the navigational satellites. These
clocks are traceable to, for instance, NIST and
USNO. Thus the normal aging characteristics
of the internal timebase will be concealed.
However, if the device is operating in hold-over
mode for long periods, the aging characteristics
are no longer concealed. Then you may need to
reset the timebase from time to time by letting
the device operate in disciplined mode for at
least 48 hours. See also next paragraph.
n
Calibration Intervals
Normally it is quite adequate to check the fre-
quency deviation between one of the reference
outputs and a corresponding, independent, ce-
sium-controlled reference source once a year,
using the procedure described below. Shorter
intervals may be necessary if the main operat-
ing mode is
hold-over , and the allowed devia-
tion is less than the specified annual aging. See
the timebase specifications to collect the data
for these calculations.
n
Equipment
–
The DUT (Device Under Test).
–
Timer/Counter Pendulum CNT-90.
–
A GPS-controlled reference frequency
source capable of generating both 2.048
MHz and alternatively 1, 5 or 10 MHz at
an amplitude between 0.1 and 5 V
rms
in
50
W
. It should also have a traceable cali-
bration history. The Pendulum GPS-89
with Option 72 is apt to the task.
–
BNC cables of suitable lenghts.
Note: There are several ways to perform
this checkup, depending on available
equipment. The setup and procedure
described in the next paragraphs are
based on the list above.
n
Setup
–
Connect the reference source (1, 5 or
10 MHz) to the
EXT REF input on the
counter.
–
Connect one of the 2.048 MHz outputs on
the front panel of the DUT to
Input A on
the counter.
–
Connect the 2.048 MHz reference source
to
Input B on the counter.
–
Find a good antenna position, and connect
the antenna to the DUT. Power up the
DUT and the reference source at least 24
hours before starting the verification pro-
cedure. Make sure the GPS contact is es-
tablished and the system is working within
15 minutes after the DUT was switched
on.
–
Power up the timer/counter at least one
hour before starting the verification proce-
dure.
n
Procedure
Note: Each key depression is not described
here. See the Operators Manual for
the timer/counter if you need more
information.
–
Recall the default settings of the counter.
–
Set the trigger level to Man 0.0 V on both
Input A and Input B .
–
Set the measuring time to 10 s.
–
Check that the frequency is 2.048 MHz ±
0.01 Hz.
–
Change measurement function to Time In-
terval A to B .
6-2 Calibration
Preventive Maintenance