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Model
41 2A
SECTION IV
THEORY
OF
OPERATION
4-1. GENERAL.
4-2.
The k p - Model
412AIAR
uses a chopper-stabilized
amplifier circuit to produce a meter indication proportional
to the voltage, current, or resistance being measured.
4-3. BLOCK DIAGRAM ANALYSIS.
4 4 .
Figure
4-1
is a block diagram of the Model
412AIAR.
The following paragraphs describe operation of the various
circuits.
4.5. RANGE AND FUNCTION SELECTION.
4-6. The Voltage Divider. Current Shunts, and Resistance
Standards circuits provide a 0.9 mV output for full-scale
input in all ranges and functions. Full scale is defined as a
reading of
1
on the top scale of the meter for all ranges.
Simplified schematics of the switching circuits for all ranges
and functions are shown in Figures
5-8
through
5-10,
47. FILTERS.
4-8.
input filtering is provided to attenuate any ac
components present in the input signal. The second fdter
section filters the Demodulator output.
RANOC
SELtCllOW
r
Section
1V
4-9. MODULATOR, AMPLIFIER,
DEMODULATOR,
AN0 CHOPPER.
4-10. A
light-beam chopper driven by a synchronous motor
sets the Modulator/Demodulator frequency at
516
power
line frequency. The Modulator output is an ac signal
proportional
to
the Model
412A
input. The amplifier gain is
stabilized at 1
,111
by the feedback network, resulting in an
output of
1
. O volt for a full-scale input. The Demodulator is
synchronized with the Modulator
so
that the amplified ac
signal is converted t o a steady dc, which
is
applied
to
the
Cathode Follower. This voltage is the same polarity as the
input signal.
4-11. CATHODE FOLLOWER A N D POLARITY
SELECTION.
4-12.The Cathode Follower drives the Meter and the
DC
Amplifier Output. Polarity selection provides upscale
meter reading for either polarity input. The Polarity switch
is disabled in the
OHMS
function.
I
Figure
4-1.
Block Diagram
00
129-5
4-1
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