Campbell Scientific CR7 Measurement and Control System - Manual (Page 172)

APPENDIX D. CALIBRATION PROCEDURES
D-2
D.2 CLOCK CALIBRATION
PROCEDURE
The 700X control module contains 3, 4, or 5
cards. The CPU card has one blue connector
with a ribbon cable connecting it to the 9 pin
SERIAL I/O port on the front of the CR7. The
clock circuitry resides on this card.
The frequency of the crystal exhibits a parabolic
response to temperature. The frequency
maximum occurs at room temperature and
drops off slowly at hotter or colder
temperatures. When the CR7 leaves the
factory it is calibrated to be 20 ppm fast. If the
CR7 is placed in a controlled environment with
the temperature close to room temperature, it is
better to set the crystal frequency exactly on.
SUGGESTED INSTRUMENTS:
Digital frequency counter
STD Bus extender card
PROCEDURE:
1. Remove the CPU card from the control
module. Insert an STD Bus extender card
into the emptied slot. Plug the CPU card
into the extender card. This is the best way
to gain access to the adjustment points.
However, if an "extender card" cannot be
obtained, remove all the other cards and
move the CPU card into the back most slot
for better, though still difficult, accessibility.
2. Set up the frequency counter to measure
period and connect to pin 3 of the
Integrated Circuit shown at Location B16 on
Figure D.2-2. Connect the ground lead to
the negative (flat) side of the 10 µF
capacitor at location F21. Adjust the
variable capacitor at location E21 on the
same figure for a period of 49,999.00
microseconds (20 ppm fast) or 50,000.00
microseconds (exactly on).
3. Lock the variable capacitor into position with
a dab of finger nail polish.