Fluke 893A - 3-33. AC CONVERTER

oaoA
sistance
value
is
in
series
with the remaining ten lOOK
ohm parts of the
first
divider and presents
a
constant
load
resistance
of 1. IM
ohms to the reference supply.
There
are eleven equal voltage steps available from
the first divider
which
can be selected by
the rotary
switch
S6.
Since the reference supply output voltage
is either
1.
1,
11,
110,
or
1,
100 volts dc, the rotary
switch
S6
is
calibrated
in 0.
1,
1,
10,
or 100 volt in-
crements.
3-31.
The
second,
third,
and
fourth
order
divisions
of the reference supply
voltage
are provided by
the
corresponding resistive dividers R13
through
R23, R24
through
R34,
and
potentiometer
R37.
The
second and
third
order
dividers function in
a
manner similar to the
first
order
divider with their associated switches
S7
and S8 performing
the desired connections. Each of
these dividers are composed of
11
equal value resistors,
thus providing
ten equal divisions of the preceding di-
vider
output voltage. Potentiometer
R37
comprises
the fourth order divider
which operates
on
the same
principle as the
preceding
dividers
but
provides a
con-
tinuously variable
output voltage at its
wiper.
Re-
sistors R35 and R36 provide
the
necessary
trimming
resistance for
the fourth order divider.
3-32. The
voltage
appearing at
the
wiper
of R37
is the
output voltage of
the Kelvin-
Varley divider. This
volt-
age
directly corresponds
to
the readout dials digit value
for
the particular reference supply voltage being used.
The RANGE switch S1AF2 selects the appropriate ref-
erence supply
voltage
of
1.
1, 11,
110,
or 1100 volts
dc corresponding
to the
1, 10,
lOO, or 1000
volt
ranges
of the instrument. The NULL switch S3AF applies
the
Kelvin-
Varley
output voltage to the null detector common
on the various null ranges of the instrument. When-
ever
the instrumait
is operated on the TVM mode, S3AF
disconnects the output of the
Kelvin-Varley
divider from
null detector common
and
connects the low input com-
mon
to null detector common.
Reversal
of the Kelvin-
Varley
output
voltage
polarity is provided
by
the MODE
switches S4AF1 and S4AF2.
3-33.
AC CONVERTER
3-34. GENERAL. 'The
AC Converter clianges measured
ac voltages to dc voltages that
can be
measured with
the dc portion of the instrument. Circuitry of the AC Con-
verter consists of
a
transconductance aii^lifier coirprised
of
Ql
through QlO and the feedback networks
selected with
the RANGE switch, a symmetrical half-wave
detector CR3
and CR4, and the power supply CR5 and
CR6. A
block
diagram of these circuits Is illustrated in Figure
3-3.
3-35. BLOCK DIAGRAM DESCRIPTION.
AH qperatlng
voltages for
the AC Converter are produced by
the half-
wave rectifiers CR5 and
CR6 and tteir associated filters.
A
3
kHz signal derived from
the Reference Inverter As-
senJ^ly
is
used
to
drive tte half-wave rectifiers,
hiput ac
voltages applied
to
the AC Converter are amplified by
the
transconductance airplifier coriprised erf Ql
through
QlO.
The resulting
transconductance amplifier output current
flows through the
symmetrical half-wave detectors wiiich
in turn produce a full-scale
output
voltage erf
one volt dc.
This
dc
voltage is then applied to the dc
measurement
circxiitry of the instrument for measurement.
A voltage
proportional to the
transconductance amplifier output
current is fed back to the anplifier input
througli a feed-
back network selected with the RANGE switch to establish
a
high
degree of gain stability
and
provide ac
rar^u^ for
the instrument.
3-36. CmCUIT DESCRIPITON. The
iiput circuit erf the
AC
Converter is conposed of R12 and
a
capaetty conpen-
sator which maintains
a
high input
inpedance erf
cxie
meg-
ohm shunted
by less
than
20
pf for all ac ranges. Over-
load
prctection
for the
AC
Converter is provided by diodes
CRl
and
CR2. The
input stage of
Ql
is a field effect
transistor
(FET)
utilized for its high input inpedance and
low noise features. The common-emitter stages
of
Q2,
3-5