GFS2102™ OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company
Page 170
Figure 108.
Time Calibration window
7.5.3.3
LCM TIME ADJUSTMENT
7.5.3.4
EQUIPMENT REQUIRED
•
Reference Frequency Counter. Almost any calibrated frequency counter
available will meet the uncertainty requirements.
•
BNC cable connectors.
7.5.3.5
OPERATION
n
Remove the LCM top cover. Connect the BNC cable to the frequency
counter. Connect the ground/shield test clip to the loop marked TP5 (GND)
on the LCM driver board and connect the signal test clip to the loop marked
TP1 (CNTR). Set the input impedance on the frequency counter to 50 ohms.
o
Note the frequency being read by the counter. It should be 1000 Hz,
± 0.035 Hz. If the frequency counter is having a difficult time locking on to
the frequency, in GFS Tools use
[Tools]
,
[Remote Communications]
to set
up communications with the LCM and send the command
CNTR:TEST=1
to
the LCM. The LCM will now output a square wave at 500 Hz, ± 0.0175 Hz
that the counter should have no trouble locking on to. Once the power on
the LCM is cycled the frequency will return to its default state.
p
If the frequency is not within the ± 0.035 Hz limit, it can be changed in
0.035 Hz (or 0.0175 @ 500 Hz) increments by changing the LCM timer pre-
scale coefficient in steps of 1: an increase of 1 lowers the frequency by
0.035 Hz (0.0175 Hz), and a decrease raises the frequency by the same
amount. In GFS Tools software, use
[Setup]
,
[Calibration]
to read all of the
user level coefficients from the LCM. Change the value of the LCM timer
pre-scale coefficient on the
Time/Mass
tab (see
Figure 109
) to bring the
frequency within the limits and select
OK
.
q
Cycle the power on the LCM so the coefficient takes effect and/or to reset
the LCM frequency to its default state. Repeat steps 2 thru 4 if necessary.
Replace the top cover of the LCM.