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OPERATING PROCEDURE
bridges. Line B is the dc resistance at a given voltage,
Edc.
Another value, line C, is the incremental value
using a low-level ac signal superimposed on a dc bias
(refer to paragraph 3.1.3).
c.
Thermal Voltages.
If the two .connections to
the unknown are not at the same temperature, a small dc
thermocouple voltage is induced that can cause an error
in dc measurements. The error varies with the applied dc
level.
Land C .measurements:
f
2
f
±0.001%
(ik'd '
±0.1% D 1 kc
Rand G Measurements:
±0.002% (2-)
2,
±O.OOOOOI
(...L)
4
1 kC'
1 kC'
2.5 AC MEASUREMENTS WITH EXTERNAL GENERA·
TOR.
2.5.1
PROCEDURE.
The procedure for making meas-
urements with an external generator is the same as that
with the internal 1-kc oscillator except for the following:
a.
Connect the external oscillator to the instn!-
ment as described in paragraph 2.5.3.
(Note that the
GEN LEV control does not control the level of an ex-
ternally applied signal.)
b·.
Set the function
switch
to EXT AC (this con-
nects the EXT GEN terminals to the bridge input trans-
former and switches the detector to a flat frequency
characteristic).
c. Multiply the D and Q readings by the following
factors to oetermine the value at the test frequency, f.
These extra terms and the total error are shown
diagrammatically in Figures 1-5 and 1-6.
In order to
achieve this accuracy,
it
is necessary to correct for the
effect of the residual impedances of the terminals· and
connecting leads, which become more important at higher
frequencies (refer to paragraph 2.5.7).
The percent D or Q error
is
5% for Land C meas-
urements at any frequency, but the fixed error term be-
comes
0.0005 1
tc
or 0.0005, whichever is larger. For
R s and G p measurements the Q accuracy is ±2% ±0.0005
fl1 kc.
For large applied voltages, a somewhat larger
Q error may be caused by saturation of the phase-com-
pensating inductor. This error may be as large as 0.005
fl1 kc.
2.5.3 CONNECTION OF EXTERNAL GENERATOR. In
most cases when an external generator is used it should
be connected to the EXT GEN terminals.
In this con-
nection, the external generator
is
connected directly to
the internal bridge transformer when the function switch
is in the EXT AC position, and the low generator ter-
minal
is
connected to the bridge chassis (which should
be grounded; refer to paragraph 2.1.2). A second ground
connection to the generator should be avoided.
If the external generator can be overdriven when
connected to a low-impedance load,
it is
generally de-
sirable to place a resistor in series with the ungrounded
generator connection to the bridge. This resistor should
be large enough to prevent distortion even when the
bridge input is short-circuited. The bridge input imped-
ance at the EXT GEN terminals is a minimum ot" 30 ohms
(resistive) at 1 kc when the bridge
is
set to measure a
short circuit on the UNKNOWN terminals. This is shunt-
ed by the inductance of the primary of the bridge tranS-
former, which
is
approximately 0.25 henry.
In some cases where more input power
is
required,
particularly in measurements of low impedance, a match-
ing transformer between generator and bridge is useful.
This
transformer need not be shielded.
When the desired bridge voltage is higher than can
be applied by the internal bridge transformer, the genera-
tor can be connected directly in the bridge circuit by
fl1 kc
1 kc/f
fl1 kc
fl1 kc
~
-fl-.
k-
Multiplying Factor
LOWD
HIGH D
Q
Q
LOW Q
HIGH Q
Bridge
F/U:t;;
I/(
elf-
<l-C3~ 11
.
~-~-
If the presence of a nonlinear unknown causes distortion
in the detector, the best meter null may not give the cor-
rect value.
Also, excess noise may limit the null ob-
tainable.
Earphones (connected to the DET OUT ter-
minal) are helpful in distinguishing a null at the funda-
mental frequency, or an external selective amplifier,
such as the Type 1232-A Tuned Amplifier and Null De-
tector, can be used.
In extreme cases, distortion or
noise could have enough amplitude to overdrive the in-
ternal detector when the function switch is at EXT AC
and could thus give erroneous readings on a selective
detector connected to the DET OUT terminals. In such
cases, the external detector should be connected from
the LOW UNKNOWN terminal to panel ground.
2.5.2 ACCURACY. The accuracy of measurements made
with
an external generator
is
the same as that with the
internal oscillator except that the following frequency-
dependent terms are added to the specifications:
19
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