5 Theory of Operation
134 Keysight 34970A/34972A Service Guide
The 5 volt earth referenced supply (+5V_ER) is produced by rectifier CR101,
CR102, CR103, CR104, and regulator U101. This supply is earth referenced by the
screw which mounts the PC board to the instrument chassis.
The unswitched +5V_ER supplies U305, U303, U320, and U302. The rear-panel
interfaces (GPIB, RS-232, Alarms, and external triggers) are powered from the
switched +5V_BP supply to ensure that when power is turned off no voltages are
present at the interfaces.
The +5V_ER supply is switched by Q101A and Q101B to create the +5V_BP
(backplane) and fan power supplies.
Undervoltage sensor U102 provides the earth reference controller reset at initial
power on.
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