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~ __ T_Y_P_E_16_0_8-_A_I_M_P_E_D_A_N_C_E_B_R_1 D_G_E
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capacitance from the floating bridge or power supply to
ground, which can cause an error if it is pJaced across a'
bridge arm. Second, there is generally cap~citive coupl-
ing between the floating bridge dc supply and the ac
line, which causes hum pickup in the detector, resulting
in a residual deflection.
If the dc supply is self-powered, it should be left
floating and spaced away from any ground, and the
bridge should be grounded.
If the dc supply is line-
operated, it will probably have more capacitance to
ground and to the power line than has the bridge, and
therefore the supply should be grounded and the bridge
ungrounded. To disconnect the bridge from ground, open
the link between the rear terminal labeled 3RD WIRE
GROUND and the adjacent CHASSIS terminal. The 3RD
WIRE GROUND terminal will be grounded if a three-wire
power cord is used and should be grounded externally if
a
two-wire cord is used.
When the bridge is floating, there is approximately
300 pf between the case and the 3RD WIRE GROUND in-
ternally.
External capacitance from the case
to
ground
will increase this total value somewhat.
If the BIAS
terminals or the UNKNOWN terminal not marked LOW is
grounded, this capacitance will be placed across the
standard capacitor for capacitance measurements, across
the fixed standard resistor, R, for conductance measure-
ments, and across the CGRL adjustment for resistance
and inductance measurements.
The error due to this
capacitance
can be computed from the equations of
Table 2-6. For 300 pf, the main errors are a 0.2% error
in capacitance measurements, a Q error of - 0.013 for Gp
measurements, a maximum
Q
error of
+
0.013 for R s
measur.emenJ:s (dependent upon the CGRL counter set-
ting) and a maximum D
~
error of - 0.013 for inductance
measurements (dependent upon the CGRL control set-
ting).
If the bridge is grounded at the LOW UNKNOWN
terminal, this capacitance is placed across the detector
where it causes no error.
The residual deflection caused by hum pickup can
seriously limit the accuracy obtainable, particularly if
the detector is not selective as it is when an external
geqerator is used (refer to paragraph 3.1. 4).
The hum
pickup will be about the same when either UNKNOWN
terminal or the BIAS terminals are grounded when low
impedances are measured, but can be much worse when
high impedances are measured and the LOW UNKNOWN
terminal is grounded.
Earphones may be helpful in de-
tecting the null of the fundamental in the presence of
hum.
In extreme cases, the bridge can be disconnected
from the power line and a battery-operated selective de-
tector, such as the Type 1232-A Tuned Amplifier and
Null Detector, can be used to avoid all internal hum
pickup.
26
3.2
MEASUREMENTS ON SHIELDED THREE- TER-
MINAL COMPONENTS.
When the unknown component is shielded, and the
shield is not tied to either unknown terminal, a three-
terminal component is formed (see Figure 3-5). The im-
pedance, Z, of the component itself is the direct imped-
ance of the three-terminal system. To measure the direct
impedance, connect the shield (third terminal) to the
bridge chassis, using any grounded terminal or a ground
lug held by the screw directly below the UNKNOWN ter-
minals. Connect the UNKNOWN terminal with the larger
capacitance to ground to the LOW UNKNOWN terminal,
because capacitance from the other UNKNOWN terminal
to ground may cause an error if it is large enough. See
Table 2-6 and paragraph 2.4.5.1.
Often the shield of an inductor is not connected to
either terminal. When the inductance and frequency are
so low that stray capacitance across the inductor causes
negligible error, the shield should be connected to the
LOW UNKNOWN terminal.
When the inductance (or fre-
quency) is high, the effective inductance is increased
because of the shunting capacitance. The error is
+
100
(u;2L x C x )% (refer to paragraph 2.4.2.2). To avoid an in-
ductance error, the shield may be tied to the panel of the
bridge. The inductor terminal that has the large capaci-
tance to the shield should be tied to the LOW UNKNOWN
terminal.
A Q error results from the capacitance from
the other UNKNOWN terminal to the shield (Cb in Fig-
ure 2-3) but a better measurement
of Lx is possible
(this connection does not affect the winding capacitance
itself).
3.3 REMOTE MEASUREMENTS.
Because of the small effect of capacitance to
ground, particularly for capacitance measurements (refer
to paragraph 2.4.5.1), the unknown may be placed some
distance from the bridge. At least one of the connecting
leads should be shielded
to
avoid the errors due to ca-
pacitance between the leads shunting the unknown. The
shielded lead should be connected to the LOWUNKNOWN
terminal and its shield tied to the bridge chassis. The
other lead may also be shielded, but this will increase
the capacitance to ground, causing an error (see Table
2-6 and paragraph 2.4.5.1).
When low-impedance meas-
Figure 3-5. Shielded three-terminal
impedance.
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