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WARNING: Never interrupt the protective ground-
ing intentionally. Any interruption of the
protective ground connection inside or out-
side the instrument, or disconnection of
the protective ground terminal is likely to
make the instrument dangerous.
Connecting External
Reference
If you wish to use an external 10 MHz reference frequency
source, connect it via a BNC-cable to the EXT REF INPUT
on the rear panel of the Frequency Counter.
When the Frequency Counter starts measuring, it automat-
ically detects the external reference and begins to use it.
The EXT REF indicator on the display is switched on.
Installing Options
Introduction
The options ordered at the same time as the Frequency
Counter are normally factory-installed. Other options can
be fitted when needed.
The options fit inside the Frequency Counter, but not all at
the same time: The HF-input, the high stability-oscillator
and either of the GPIB-interface or the Battery-unit can be
installed in one and the same Frequency Counter.
Calibrating the MTCXO
The MTCXO Time-base can easily be recalibrated to any
10 MHz reference. To maintain the accuracy of the
MTCXO, use a reference with an accuracy of 3*10
The PM 9691 oven-enclosed oscillator used in Fluke
counters version /.5. meet this requirement, if calibrated.
Preparations
If you remove the cover when counter has been switched
on, the temperature of the MTCXO will rapidly drop about
10°C. Since the MTCXO must have a stable temperature
when calibrated you must wait an hour between removing
the cover and calibrating.
If the counter has been switched off more than three
hours, you can calibreate it directly.
PM 6669 - OPERATORS MANUAL
INSTALLATION
Removing the Cover
WARNING: When you remove the cover you will ex-
pose live parts and accessible terminals
which can be dangerous to life.
Loosen the two screws
using a Pozidrive No. 1 screwdriver
Figure 5
–
Make sure that the power cable is disconnected.
WARNING: Although the power switch is in the off
position, the line voltage is present on the
printed circuit board.
–
Loosen the two screws in the rear feet.
–
Grip around the front panel and pull the Frequency
Counter out of the cover.
Calibration Procedure
-8
.
–
Remove the cover from the counter.
–
Allow the MTCXO to adapt the new ambient tempera-
ture. (See 'Preparations'.)
–
Connect the 10 MHz reference to Input-A.
–
Switch ON the counter.
–
Adjust the sensitivity control so that the counter counts
properly.
–
Hold down the CALIB-button, on the main printed-circuit
board in the counter, and press the Reset-button.
Loosen These Screws to Remove Cover.
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