Model 8640B TM 9-4935-601-14-7&P
PERFORMANCE TESTS
4-44. INTERNAL REFERENCE DRIFT RATE (STABILITY) TEST (Cont'd)
PEAK DEVIATION......................................................................................5 kHz
PEAK DEVIATION Vernier........................................................................Full cw
RANGE...............................................................................................0.5 -1 MHz
OUTPUT LEVEL....................................................................................+19 dBm
RF ON/OFF ....................................................................................................ON
2. Set variable voltage transformer to nominal voltage set on generator's line power module (i.e., 100, 120, 220, or 240
Vac). Set temperature controlled chamber for 25°C. Allow equipment to warm up for two hours.
3. Set frequency counter so that it's using its internal reference oscillator. Set counter to read frequency directly (i.e., not
divided down). Use a ls gate time so that last three digits span from 000 to 999 Hz.
4. Calibrate the recorder for a zero to full-scale reading that corresponds to a 000 to 999 Hz reading of the frequency
counter's last three digits.
5. To check drift rate as a function of time and line voltage, now set counter so that it's using the quartz oscillator. Set
the counter's time base control for a 10s gate time.
NOTE
The above procedure sets the counter's actual gate time to 100s because the reference being
used is 1 MHz instead of the 10 MHz reference the counter normally uses. This means that the
counter's reading must be divided by 10 to find the actual frequency of the signal being
measured (i.e., the recorder's calibration is 0.00 to 9.99 Hz full scale).
6. Record the generator's internal reference frequency for one hour. The frequency change in one hour should be <0.27
Hz (<0.05 ppm 1 digit counter ambiguity).
Time: 0.27 Hz
NOTE
Any change in line voltage or chamber temperature could make the instrument's drift rate vs time
appear to be out of specification.
7. Set variable voltage transformer 5% above the nominal voltage set on generator's line power module (e.g., if nominal
line voltage is 120 Vac, set transformer for 126 Vac). Then note the frequency (the counter's indication must be
divided by 10).
8. Set variable voltage transformer 10% below nominal line voltage (e.g., for a nominal 120 Vac, set transformer for 108
Vac), then note the reference frequency. The frequency change from the reading noted in step 7 should be <0.52 Hz
(< 0.1 ppm +1 digit counter ambiguity).
Voltage: 0.52 Hz
4-74
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