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Chapter 5 Theory of Operation
On-Board Power Supplies
5
On-Board Power Supplies
See “A1 Power Supply Schematic” on page 204.
The on-board power supply controls the on/off state of the power supply
and conditions the main supplies for use by the analog and digital circuits.
Over-temperature protection is also provided.
The main supply provides a +12 V standby power supply that is used by
the power on/off circuitry. The electronic power switch is controlled by
the PWR_SWITCH* line. This line is grounded when the front-panel
power switch is pushed and turns on Q2004 through R2026.
Pressing the power switch turns on Q2004, and C2043 and C2047 begin
to charge up. Depending upon the state of relay K2001, R2025 will be in
parallel with either R2023 or R2024, so one of the capacitors will charge
much faster than the other. The charged capacitor turns on either Q2006
or Q2008 and energizes the coil of K2001, changing the relay to the
opposite state. Repeatedly pushing the power switch toggles the relay
from one state to the other. In the ON state, PWR_ON* is grounded
through Q2009, turning on the main power supply.
Q2009 can turn off the main supplies if an over-temperature condition is
sensed by U2006, which is powered by the +12 V standby power supply.
U2006 has two trip points for over-temperature. The first trip point is set
at approximately 85 °C and is asserted by OUT 1. This is a warning to
the microprocessor and this condition can be read via GPIB in the status
byte. The second trip point is set at approximately 90 °C and is an actual
over-temperature condition that asserts OUT 2. This turns off Q2009
and shuts off the main supplies.
The actual temperature sensed by U2006 can be read by the microprocessor
through the MEAS_TEMP signal. This reading is used during the
calibration and adjustment process.
Secondary logic supplies are derived from the main power supply’s +5 V
(VCC). Switching regulator U2004 provides the +3.3 V supply (VDD) for
the synthesis IC and waveform memory. U2005 provides the –2.1 V ECL
termination supply (VTT).
The +12 V earth-referenced supply, +12V_ER, is reduced to +5 V by
voltage regulator U2003. This is the earth-referenced logic power supply
(+5_ER).
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