Symmetricom 5071A, Assembly And Service Manual

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3. Theory of Operation
Functional Block Descriptions
136
by a voltage-to-frequency converter and counter (VFC on A6). The resulting
sampled data is processed by the servo algorithm to generate correction
signals for the D-to-A converters that generate the VCXO EFC signal (DAC
on A6), the microwave probe signal amplitude modulation, the C-Field
current, the analog signal gain, and the CBT Electron Multiplier voltage.
Because the microwave probe signal undergoes frequency and amplitude
modulation during normal operation, frequency and power measurements are
difficult to obtain. This is why most of the testing must be done in Fatal Error
mode.
During normal operation, a diagnostic program (known as the “Health
Monitor”) operates continuously. This program checks operating parameters
and digital-state variables to detect anomalies or errors.
If an error condition is detected that is not fatal, a warning is logged, and the
operating mode goes to “Warning”.
If a persistent error condition prevents the feedback loops from operating
normally, the operating mode goes to Fatal Error, and an error code is sent to
the log. The error code will contain information about which health monitor
errors were associated with the actual failure (see chapter 2, sections 1.1.1,
1.2.1, 1.3.1, and 1.4.1. for decoding information.).
Warning Mode: The warning mode occurs when the health monitor detects
a possible problem or internal standby-battery operation. The instrument can
still run as a frequency standard, but the user is warned of a possible
problem or future problem. The Continuous Operation LED flashes, and the
Warning LED is lit. After a transient warning, the instrument can return to
normal operation by entering “UTIL, Reset” from the front panel, or by via
SCPI commands.
Fatal Error Mode: The Fatal Error mode occurs when the 5071A encounters
an unrecoverable error condition. An error message is logged, and the CBT
is shut down except for the ion-pump supply. It is not possible to exit Fatal
Error mode without cycling instrument power or pressing the reset button on
the A3 board.
It is useful to diagnose instrument problems in the Fatal Error mode because
all servo loop activity and microwave probe signal modulation is halted. In the
Fatal Error mode the RF power input to the CBT is maximized and the
synthesizer is set to give an unmodulated frequency of 9192.631770 MHz.
Standby Mode: The Standby mode is a user-selected mode which can be
selected during the Warm Up or Normal Operation modes. This mode can be
used to run diagnostic self tests or to store the 5071A while maintaining the
CBT for ready use. The ion pump remains on, preserving the CBT vacuum.
The instrument can be addressed using SCPI commands in all operating
modes, although some instructions are mode-specific.