3. Theory of Operation
Functional Block Descriptions
135
Operating Firmware
The firmware is stored in two ROMs on the A3 board. It provides six
operating modes for the instrument: Power On, Warm Up, Normal
Operation, Warning, Standby, and Fatal Error. This subsection describes
the instrument's behavior in each of these modes.
Power On Mode: When power is applied, the 5071A starts operation in the
Power On mode. The firmware checks the functions of the A3 board and
determines if all other assemblies and modules are present (Power-on Self-
tests). If all tests at power-on pass, the instrument changes automatically to
the Warm Up mode. If one or more of the power-on A3 self tests fail, the
instrument will not be able to function, and the mode changes automatically
to Fatal Error. An error message will be logged and can be used, with
observations of LEDs on the top of the A3 board, to diagnose the fault.
Warm Up Mode: At power-on, power is applied to A19 (VCXO oven) and
A17 (CBT Ion Pump). If the Ion pump current is correct at the beginning of
Warm-up mode, then the CBT Cesium oven and Electron Multiplier voltages
are turned on to their set-points. After the CBT and VCXO ovens have
reached the correct temperatures, the firmware adjusts the electron multiplier
so that a threshold CBT output is achieved. The tuning voltage of the VCXO
is then swept to find the peak of the Cesium resonance pattern.
The firmware then steps through various levels of RF attenuation to obtain
optimum power for the microwave probe signal. Next, the electron multiplier
is adjusted to obtain the desired CBT output current. After these settings
have been logged in memory, the C-field is set to the proper value. When
these adjustments are complete the operating mode changes automatically
to Normal Operation (the green continuous operation light blinks), and the
message āLock Completedā is logged.
If CBT ion-pump current is excessive after 45 minutes, an error will be logged
and the instrument goes to Fatal Error mode. If the instrument is unable to
find a setpoint during the warm-up sequence, the mode will go to Fatal Error,
and an error message will be logged. If something is wrong with the
instrument, it is usually detected during the lock-up procedure.
Normal Operation Mode: In the Normal Operation mode, five separate
servo loops operate continuously, regulating the CBT Oven temperature, the
CBT peak output current, the VCXO frequency value, the microwave power
level, and the CBT C-Field value. The microwave probe signal fed to the CBT
is frequency and amplitude modulated to produce appropriate error signals
(feedback) when convolved with the transfer function of the Cesium
resonance.
The Cesium oven servo operates independently. Oven temperature is read
about four times a second and the oven heater voltage is adjusted to achieve
the desired set-point.
The four remaining servo loops operate synchronously with the microwave-
probe-signal modulation sequence. The CBT output signal current is digitized
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