A5 System Board - Keysight N9923A Service Manual

Theory of Operation
Measurement Group

A5 System Board

The A5 system board controls all of the functions of the analyzer and digitizes
the RF IF signals (NA-R, NA-A, and NA-B) from the A4 RF board.
The A6 SOM (System on Module) board is mounted directly on the A5 system
board.
All external connections for the analyzer come through the A5 system board
except for the PORT 2 and PORT 1 ports.
The A5 system board also controls the power switching and power
management.
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
sensor 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.
5-10 Keysight N9923A Service Guide