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Model 427A
Section IV
SECTION
IV
THEORY
OF
4-1. GENERAL.
4-2.
The Model 427A measuresac voltage, dc voltage,
and
resistance.
It is battery
operated,
but with Op-
tion 01, can be powered by line voltage.
Figure 4-1
shows a simplified block diagram of the 427A.
4-3.
The Battery Regulator
(А1) regulates the bat-
tery output and provides +6. 7 and -6. 7 volt bias sup-
ply to the amplifiers.
The 427A uses two amplifiers,
the DC Amplifier
(А1) and the AC Metering Circuit
(A2).
The former
is a high input impedance
unity
gainamplifier used toamplify de and resistance inputs.
It also serves as å preamplifier for ac signals.
The
AC
Metering
Circuit
amplifies
ac signals from
the
preamplifier,
converts them to dc signals proportional
to the average ac, and
connects
them
to the meter.
The
meter
is calibrated
to display
the rms value of
the ac input.
4-4,
The DC Offset Adjust (A2) compensates
for
leakage current from the de amplifier, and DC CAL and
OHMS CALareresistive circuits used for calibration.
4-5.
DC
OPERATION.
4-6.
Figure 4-2 shows Ше Model 427A in the DC Mode
of Operation.
The ас input is first applied to the DC
Range Attenuator
where it is attenuated by 10dB for
each stepoftheattenuator.
Thedc signal from the at-
tenuator
goes to the DC Filter, and the filter rejects
any ac superimposed noise that may be present on the
input.
The dc output of the filter goes to the DC Am-
рііНег (А1) and then to the meter.
The DC Amplifier
matches
the high impedance of the attenuator to the
low impedance
of the meter.
The DC CAL circuits
ATTENUATOR
Figure 4-2.
01791-2
OPERATION
are
resistive
circuits
in series with the meter used
to adjust
the meter
current
to calibrate
the lower
ranges.
4-7.
OHMS
OPERATION.
4-8.
Figure 4-3 15 a block diagram of the Model 427A
in the OHMS Mode of Operation.
With the input open,
Ra and Rp forma voltage divider.
The voltage across
R,, causes full scale current toflow through the meter.
The OHMS CAL circuit adjusts the meter current for an
indication of » with the input open.
When R, is equal
tothe parallel combination of Ва and Rp, the total re-
Bistance from
the OHMS
terminal
to ground will be
half the parallel combination of R4 and Ву, the voltage
into the amplifier will be halved,
and the meter indi-
cation will be half scale.
The Model 427A is designed
во that thefull RANGE
setting will be displayed in the
center of the scale.
For example,
10 Поп the X10
range is a center scale reading.
4-9.
AC
OPERATION.
4-10. Figure 4-4 shows a block diagram of the 427A
in the AC Mode of Operation,
The input signal goes to
the AC Range Attenuator.
On the 1 volt range and be-
low, the signal is not affected by the AC Range Attenuator;
but on all the higher ranges,
the signal is attenuated
by 504В,
Capacitor C3adjusts the frequency response
of the attenuator with а 3 volt 100 КН?
input.
Тһе
signal from
the AC Range Attenuator
goes through
the DC Amplifier to the AC
Post Attenuator where it
is attenuated by 10 dB for each step of the RANGE
selector.
The DC
Amplifier
matches the low impe-
dance of the Post Attenuator to the high impedance of
the Range Attenuator,
acting ав а preamplifier.
ос
ос
FILTER
AMPLIFIER
FEEDBACK
DC Operation
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