HP 10811A, Operating & Service Manual

Page: 28 / 73

Model 10811A/B
20
Table 4-2. Operational Verification Procedure (Continued)
A COUPLING·······································································DC
B COUPLING·······································································DC
A SLOPE··············································································
+
B SLOPE ·············································································
+
CHECK/COM/SEP ······························································SEP
e. Adjust the 10811A/B to read ≈ 9.5 Hz (reading must be less than 10 Hz).
NOTE
The display on the 5345A should be changing only in
the .001 Hz or .0001 Hz digits. This represents
frequency deviations of parts in 10
11
and 10
12
,
respectively.
f. Type the program from Figure 4-1 into the 9835A calculator. (See paragraph 4-13 for use
of other controllers and counters.)
g. Press "RUN". When the program asks for the number of samples, enter "100" and press
CONTINUE.
h. Allow 2 to 3 minutes for the program to finish. Verify a short-term stability of less than 5 X
10
-12
.
NOTE
Measurement of short-term stability is somewhat difficult and exacting.
If the 10811A/B fails this test, be sure no signal sources other than the
test reference are operating near the measurement system, as these
can cause interference with the measurement. Other sources of error
are vi brat ion, nearby electrical equipment, poor shielding, or motors
that can radiate signals into the 10811A/B. The failure to connect the
HC input (to ground) can cause poor frequency stability as can a noisy
voltage being used for the EFC control. If another oscillator is available
(known to be good), verify the accuracy of the measurement system.
This could save considerable troubleshooting time.
Two other possibilities external to the oscillator are the oven and
oscillator power supplies. These must be stable in order for the circuits
to function properly. See
Table 1-1 , Specifications, and paragraph 2-6
for power supply noise requirements.
5. Long-Term Stability
NOTE
The 10811A/B will typically take 24 to 48 hours to reach its specified
aging rate after storage or shipment. In some cases, if extreme
environmental conditions were encountered during storage, the
10811A/B could take up to 1-week to achieve its specified aging rate.
a. Connect reference frequency standard (multiple or submultiple of 10 MHz) to the EXTERNAL
SYNC INPUT of the oscilloscope.
b. Connect the output of the 10811A/B to CHANNEL A INPUT. Adjust sweep speed to 0.1 μ s/div.
c. Set oscilloscope to EXTERNAL TRIGGER and adjust it so that its sweep is synchronized with
the reference frequency. The oscilloscope pattern will probably be moving.
d. Carefully adjust oscillator frequency control so that pattern on oscilloscope stops. Use a higher
sweep speed on oscilloscope for better resolution.
e. When the oscillator is adjusted as close as possible to frequency, measure the frequency as
described in paragraph 5-13 .
f. Allow the oscillator to remain undisturbed for 48 hours, then again measure the frequency. The
difference between the frequency measured in step e and that measured in this step is the aging
rate per 48 hours and should be less than 1 X 10
-9
. The measurement is allowed to run for 48
hours so that the frequency offset measured will be well in excess of the system resolution. In
many cases the daily aging rate of <5 X 10
-10
per day can be verified in 24 hours.