Fluke 893A - Page 25

ay3A
Figure 2-5. BRIDGE DETECTOR
-
FLOATING
SUPPLY
Gm
')
Rjj
»
Unknown resistance
=
Null detector input resistance
of 10
megohms
for NULL SENS
settings of
. 01. Null detec-
tor
input resistance is
100
megohms on
all
other
settings.
Ej.q
=
Readout dial
voltage
E-n
=
Meter deflection in volts (Proportional to se-
lected NULL SENS).
2-37. OPERATION AS A NULL DETECTOR
2-38. The Model
893,
because of its high input im-
pedance
on the dc differential mode, can be used
to
monitor small
voltage differences in bridge circuits,
potentiometers, and
other measurii^ devices.
In
most
of these applications, the
circuits
are adjusted
for
a
zero deflection or null on the Model 893.
Equipment
connections for two types of null detector configurations
are
illustrated in Figure
2-5
and
2-6.
To operate the
instrument
as
a null detector, perform the
Preliminary
Operatii^ Procedures described in paragraphs
2-9
and
2-10,
and proceed as follows:
a.
Select the
desired equipment applications illustrated
in Figure
2-5
and
2-6
and
make the appropriate
connections.
b.
Hace
the Model 893 MODE and NULL SENS
switches
to
the +DC and desired null detector
sensitivity
Figure
2-6.
BRIDGE
DETECTOR
-FLOATING
NULL DETECTOR
positions,
respectively. Adjust the circuit beir^
measured for a zero
or null deflection
on
the Model
893.
2-39. MEASUREMENT OF DBM
2-40.
'Ihe
Model
893,
when
operated
on
either the TVM
or
Differential AC mode, can be used as an
output
meter^ For this
application, the outpiri: to be measured
must be terminated
into
a
600 ohm load and the Model
893 INPUT connected
across the 600 ohm
load.
The
Model
893 is then operated on
the
TVM or
Differential
AC mode and
the resulting deflection or Readout
dial
indication
converted to DBM using the information con-
tained in
Figure
2-7.
For DBM measurements above
one volt ac,
multiply the voltages listed in Figure
2-7
by
ten for
each
range and
then
add
20 DBM for each
range.
INDICATED VOLTAGE
DBM LEVEL FOR
1
VOLT
RANGE
(1
mw into 600G)
.0975
-18
. 1227
-16
.
1545
-14
.
1946
-12
.
2450
-10
.3088
-8
.3897
-6
.4887
-4
.
6150
-2
.7746 0
.9752
+2
Figure
2-7.
1 VOLT RANGE DBM CONVERSION TABLE
2-7