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~ __ T_YP_E_1_6_08_-A_1M_P_E_DA_N_C_E_B_R_I_D_G_E
_
TABLE 2·6
CORRECTION TERMS FOR ERRORS
CAUSED BY CAPACITANCE TO GROUND (C
b )
(Add or subtract from measured value as indicated:)
C t
=
0.15
fLf,
R
t
= 6.67k.o
R n
=
0.667 x (centade reading*)
C s
CxCb
(l-D x
2)
C
p
CxCb
+ - - -
+ - - -
C
t
Cr
D
Cb
Dx(l +D x
2)
D
Cb
---
D x
(low)
C t
(high)
C t
G
p
NO ERROR
R s
+ wCbR n RxQx
Q
+ wCbRt
Q
- wCbRn
(cap)
(ind)
L s
+wCbRnLx
+wCbRnLx
Qx
L
p
Q x
Q
-wCbRn Qx
2
Q
- wC b R nQx
2
(low)
(high)
*omitting decimal point; e.g., for a centade reading of 10.000,
R n = 6670.0·
a 0.1% error.
Note also that for the other bridges, Cb
causes an error in Q only, except when low-Q inductors
or high-Q resistors are measured.
Measurements made with the unknown grounded are
discussed in paragraph 3.5 and measurements on three-
terminal, shielded components are discussed in para-
graph 3.2.
2..4.5.2
Voltage on Unknown. The voltage applied to the
bridge is approximately 1 volt with a source impedance
of 50 ohms when the GEN LEV control is fully on. The
actual ac voltage on the unknown can be calculated with
the aid of Table 2-7 and the circuit diagram of Figure
1-2, or it can be measured with a high-impedance volt-
meter (which should be removed when high-impedance
measurements are made in order to avoid shunting the
unknown).
2.4.5.3
AC Sensitivity.
The generator-bridge-detector
system is sensitive enough to balance the bridge to the
stated accuracy specifications.
However, there are
cases where additional sensitivity may be useful, such
16
as measuring accurate D or Q when the main CGRL ad-
justment is at the low end of its range or when the sig-
nal level on the unknown must be set at some low level.
In these cases an external detector following the inter-
nal detector may be of use.
The Type 1232-A Tuned
Amplifier and Null Detector is recommended.
It should
be connected to the DET OUT terminals.
When very low impedances are measured, there
may be enough inductive hum pickup to limit the sharp-
ness of the null. This is caused primarily by harmonics
of the power-line frequency that are close enough to the
tuned frequency to pass through the selective detector.
In some cases a small "beating" on the meter may be
noticed; this is a beat between harmonics of the oscil-
lator and line.
An oscilloscope connected to the DET
OUT terminals may be used to advantage in such cases.
If the oscilloscope is set to synchronize with the power
line, the voltage at the line frequency and its harmonic
will be a fixed display pattern and the bridge output sig-
nal will be a time-varying display. The final bridge bal-
ance adjustments should be made to remove any time-
varying component from the oscilloscope display.
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