Keysight N9912A Service Manual page 51

Theory of Operation
Measurement Group
The instrument model number and serial number are stored on the A5 system
board.
System Monitor (SM) Microcontroller
This is the heart of the analyzer. It is active whenever there is DC power
available. The microcontroller has ultra-low power dissipation. It is sometimes
referred to as the "MSP 430".
The following is a list of some of the functions performed by the SM.
— It monitors the On/Off button on the front panel and responds
appropriately.
— It controls the on/off functions of the various power supplies in the analyzer.
— It drives the On/Off button LED.
— It reads battery pack information via the Smart Batt Bus.
— It communicates with the battery charger IC via the Smart Batt Bus.
— It interfaces to the Battery Charger and Manager circuit.
— It interfaces to the Serial Bus [B] for communication with the temperature
sensors and ID EPROM's on the printed circuit boards.
— It communicates to the A6 SOM board CPU via a serial port, UART.
— It stores the instrument model number and serial number.
Battery Charger and Manager
This circuitry controls charging of the battery based on a battery charger IC.
This IC is powered by a digital output on the System Monitor. The System
Monitor has the capability to reset the IC, if required, by recycling the power to
the IC.
This circuitry also controls the source of the Unreg DC: Batt or Ext DC. It reads
the battery pack information via the Smart Batt Bus. Batt Neg is connected to
the A5 ground plane.
There is an input inrush current control circuit on the Ext DC input that limits
inrush current to 4 amps.
Analog Power Supplies and Load Switching
Seven different analog supply voltages are generated from the Unreg DC. All of
these are used on the A4 RF board and some are used on the A5 system board.
Some sections of the circuitry on the A4 RF board may be switched to low
power modes to conserve power and minimize heat. This is accomplished
through load switching. See
more information.
The System Monitor controls the on/off functions of the analog power supplies.
All are on or off together except for power sequencing.
Keysight N9912A Service Guide
"Power Management Circuitry" on page 5-10 for
5-11