Agilent Technologies 8753ES Option 011 Service Manual page 362

Theory of Operation
Power Supply Theory
Shutdown Circuit
The shutdown circuit is triggered by overcurrent, overvoltage, undervoltage, or
overtemperature. It protects the instrument by causing the regulated voltage supplies to
be shut down. It also sends status messages to the A9 CPU to trigger warning messages on
the analyzer display. The voltages that are not shut down are the +5 VD and +5 VCPU
digital supplies from the preregulator, the fan supplies, the probe power supplies, and the
display supplies. The shutdown circuit can be disabled momentarily for troubleshooting
purposes by using a jumper to connect the SDIS line (A8TP4) to ground.
Variable Fan Circuit and Air Flow Detector
The fan power is derived directly from the +18 V and −18 V supplies from the A15
preregulator. The fan is not fused, so that it will continue to provide airflow and cooling
when the instrument is otherwise disabled. If overheating occurs, the main instrument
supplies are shut down and the fan runs at full speed. An overtemperature status message
is sent to the A9 CPU to initiate a warning message on the analyzer display. The fan also
runs at full speed if the air flow detector senses a low output of air from the fan. (Full
speed is normal at initial power on.)
Display Power
The A8 assembly supplies +5 VCPU to the A19 GSP through the motherboard. The GSP
converts a portion of the +5 VCPU to 3.3 V to drive the display and LVDS (low voltage
differential signaling) logic. The A19 GSP also controls and supplies power to the A27
backlight inverter. The voltages generated by the inverter are then routed to the display.
Display power is not connected to the protective shutdown circuitry so that the A18 display
assemblies can operate during troubleshooting when other supplies do not work.
If blanking pulses from the A19 GSP are not present, then +3.3 V will
NOTE
not be sent to the display.
Probe Power
The +18 V and −18 V supplies are post regulated to +15 V and −12.6 V to provide a power
source at the front panel for an external RF probe or millimeter module.
12-8 Chapter 12