Option
-130
High-Stability
Reference
130-1.
INTRODUCTION
Option
-130
the High-Stability Reference,
with
the rear panel REF INT/EXT switch set
to INT, configures the Generator’s
reference to be supplied
by
the High-Stability
Reference,
130-2.
OPERATION
The
High-Stability Reference consists
of
the
Auxiliary Power Supply (A3A2),
and an
Ovened Oscillator ( Y
1
).
The Auxiliary
Power
Supply is
mounted inside
the Generator
on
the rear panel,
and
the Ovened Oscillator is mounted inside the instrument on the side
rail. Only the Auxiliary Power Supply is field repairable, and it is described here. The
Auxiliary Power Supply is also available under the Module Exchange program.
130-3.
CIRCUIT DESCRIPTION
The Auxiliary Power Supply is connected
directly to the line power on the
fuse/filter/ line-voltage
selector
assembly to supply power to the Ovened Oscillator even
when
the Generator POWER switch is off. The
Auxiliary
Power Supply PCB (A3A2A1)
includes a linear-regulated supply and an
automatic line-voltage selector circuit.
The linear-regulated supply consists of
a
diode
rectifier bridge CRl, filter capacitor Cl,
voltage regulator
U
\
,
and
associated resistors R2, R3, and R4. The circuit associated with
CR3,
Q1
and U2, provides automatic line voltage
selection between two line-voltage
ranges. This is implemented
by
configuring
the rectifier circuit as a bridge rectifier for the
lower line voltages
or
as a center-tapped, full-wave rectifier for the higher line voltages.
At low line voltages (less than approximately 150V
ac), transistor
Q1
is conducting,
thus
grounding the minus terminal of rectifier CRl and causing diode CR3 to be reverse
biased. This causes the full secondary voltage of T1 to
be rectified
by
the
bridge
rectifier,
CRl.
When the line voltage is greater than 1 80V ac, (there is approximately 30V hysteresis), the
comparator U2 turns off transistor Ql. Diode CR3 becomes forward biased, and the
transformer center tap is effectively grounded. The voltage applied
to
the rectifier CRl is
then half the secondary voltage.
The comparator
U2
input voltages
are set by resistors Rl, R6, R9, and zener diode CR4.
U2
controls the
base of transistor Ql. The comparator switching point
is
set between the
low and high line voltages, with sufficient hysteresis
to accommodate variations
in input
loading. At very low line
voltages,
the resistor diode combination RIO and CR5, from the
37V output of the main power supply, augment
the Auxiliary Power Supply.
130-1
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