A5 System Board - Keysight N9912A Service Manual

Theory of Operation RF Analyzer
Measurement Group N9912A

A5 System Board

The A5 system board controls all of the functions of the analyzer and digitizes the RF IF signal 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 RF IN and RF
OUT 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 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 "Power Management Circuitry" on
page 5-10 for more information.
The System Monitor controls the on/off functions of the analog power supplies. All are on or off together
5-12 Service Guide N9912-90003