3-1. INTRODUCTION
3-2. The
theory of
operation for the Model 893
AC-DC
Differential
Voltmeter is contained in this section of the
manual. A block
diagram analysis is used to functionally
describe
the overall operation of the
instrument which is
then followed by a detailed
circuit description
of
each
particular block
section. The
functional block diagram is
illustrated in Figure
3-1
and the schematic diagram is
contained at the rear of
the manual.
3-3.
BLOCK DIAGRAM ANALYSIS
3-4. INTRODUCTION
3-5, The Model 893
circuitry
is
comprised
of a refer-
ence
supply,
a
Kelvin-Variey divider,
a
dc input divider,
an ac converter, a
null detector,
a
meter, and optional
recorder output. This circuitry is interconnected by
various switching
arrangements when performing
the
desired ac
or
dc
conventional or differential voltage
measurements.
3-6. CONVENTIONAL TVM MODE
3-7.
When the
instrument
is
used as a conventional
transistorized dc voltmeter,
the circuitry is connected
as
illustrated in Figure
3-1.
The input divider
is con-
nected across
the input terminals of the
instrument to
provide a constant
input impedance of 100
megohms on
all
raises.
With a
full-scale input voltage applied to the
inpiri: of the instrument,
the selected tap of the input divid-
er provides a dc ii^ut voltage of ±
one millivolt to the in-
put of the null detector
which
drives
the front panel meter.
Gain of the null
detector is precisely controlled by the
negative feedback network and
provides
a
full-scale meter
deflection for any full-scale input.
Positive
or
negative
voltage
measurements are facilitated by reversing the
meter terminals
through the contacts of the
MODE
switch.
3-8.
By placing the
MODE
switch of
F^re
3-1
to the
AC position, the
instrument
circuitry is
connected
as
a
conventional transistorized ac voltmeter. The
resistor
R12 and
the
ac
converter are connected across the biput
terminals of the instrument and provide a
constant input
impedance of one megohm on all ac voltage
ranges. The
ac
converter
is
an operational amplifier driving a full-
wave
detector whose overall gain is controlled by
the neg-
ative feedback network selected with the
RANGE switch.
A
full-scale
input
voltage
at the input terminals of the
instrument results in an ac
converter output voltage
of
one volt dc which
is applied
to
the input of the null detec-
tor. The null detector, whose gain is
precisely con-
trolled by
the negative feedback network, drives
the front
panel meter which indicates
the magnitude of tlie measured
ac voltage.
3-9. DIFFERENTIAL NULL MODE
3-10.
When the instrument
is used
as a dc
differential
voltmeter, the
MODE
and
NULL SENS switches of Fig-
ure
3-1
are
placed to their respective
±DC
and
the desired
full-scale meter sensitivity positions.
On this mode of
operation, the NULL SENS
switch selects
a
suitable re-
sistance
value from
the
input divider
resistors to deter-
mine the meter
full-scale sensitivity and
also applies
the Kelvin-Variey divider
output to the common
input of
the
null detector. The dc input voltage
applied to
the
instrument is then compared by
the null detector
to the
Kelvin-Varley divider
output voltage and any
resulting
difference used to drive the meter.
Any right or
left
meter deflections correspond to an input
voltage above or
below
the Kelvin-Varley divider output
voltage.
The
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