HP 10811A Operating & Service Manual page 68

8-99. TIME DOMAIN STABILITY (SHORT-TERM STABILITY) OUT OF SPECIFICATION. Mea-
surement of time domain 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
vibration, 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.
8-100. 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
paragraph 2-6 for power supply noise requirements.
8-101. When troubleshooting this condition, carefully monitor the output waveform. Check for distortion
or intermittent distortion, small amplitude variations (there should be none), or spurious
oscillations on the output signal. Use a spectrum analyzer for this test (see
These symptoms can be more readily traced than small frequency fluctuations.
8-102. Almost any of the circuits can cause poor short-term stability. However, the most probable cause
is crystal Y1. Other possible causes are the oven controller circuit (keeps changing temperature)
or defective frequency determining components (CR1, C1, C3, C8, or instability in the 5.7V
supply). Q1 and Q2 are also good suspects. Instability can also be caused by the AGC circuit, but
this is usually visible as output amplitude variations or instability on the output waveform.
8-103. To check oven stability, allow the 10811A/B to operate normally away from drafts or sudden
temperature changes. Allow the unit to warm up for at least 1-hour then connect a sensitive
recorder or digital voltmeter to the OVEN MONITOR OUTPUT. Once the oven has stabilized, the
monitor voltage should not vary more than 100-200 µV when measured over 3- to 4-minute
periods. Causes of oven instability are temperature sensing elements RT1, U2, U3, and
associated components.
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Model 10811A/B
Table 1-1, Specifications, and
Table 4-2, step 3).