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__ T_Y_P_E_l_60_8-_A_1 M_P_E_D_A_N_C_E_B_R_I_D_G_E
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SECTION
4
PRINCIPLES
OF
OPERATION
4.1
BRIDGE CIRCUITS.
Figure 1-2 shows the
SiX
bridge circuits used in
the Type 1608-A Impedance Bridge as well as the bal-
ance equations. These six bridges completely cover the
passive half of the complex impedance plane as shown
in Figure 4-1. There i·s considerable overlap between the
D and Q ranges of the various bridges, allowing the meas-
urement of series or parallel C or L over a wide range.
L s and C p can each be measured over a full 90 degrees.
The D coverage extends down to 0.02 (Q to 50), and at
D's below 0.02 L p
=
L s and C s
=
C p to 0.04%, and at
high D's or low Q's, the unknown can be measured as a
resistance or conductance and L s and C p can be calcu-
lated from R or G and Q.
Both ac and dc measurements
can be made on the Rand G bridges.
The coaxial CGRL balancing controls consist of
a 114-position detented switch and a continuously ad-
justable vernier, wire-wound rheostat.
The switch in-
troduces in the variable bridge arm fixed steps of resist-
ance proportional to the first three digits of the indicat-
ing counter that it drives.
This adjustment is called a
"centade," because it is similar to a decade-resistance
unit but with approximately 100 positions.
It uses pre-
cision wire-wound resistors (details of its operation are
discussed in paragraph 4.4).
The vernier sets the last
two digits of the counter and adds resistance propor-
tional to this reading in series with the resistance of the
centade.
The ratio-arm resistors, which range from 1 ohm to
1 megohm, are all General Radio precision wire-wound
resistors.
The two ganged DQ adjustments are wire-
wound rheostats with a 40-db logarithmic range.
4.2
BRIDGE SOURCES AND DETECTORS.
There are three dc sources of approximately 3.5,
35 and 350 volts open-circuit that are connected to the
bridge for dc resistance and conductance measurements
according to the schedule of Table 2-1.
Resistors in
series with these sources limit the power supplied to the
bridge to less thim 1/2 watt to avoid damaging the in-
ternal bridge resistors or the unknown.
The dc detector is a panel meter, with a sensitiv-
ity of 1 fLa/mm near zero and a shaped characteristic to
facilitate balancing.
Its resistance is approximately
500 ohms.
A more sensitive null indicator can be con-
nected if desired, through connectors on the rear panel
(refer to paragraph 2.3.4).
The ac generator is a l-kc, two-stage, transistor
RC oscillator.
This drives a 3-to-1-stepdown shielded
bridge transformer, with a maximum output of approxi-
mately 1 volt behind 50 ohms.
The GEN LEV control
adjusts the voltage to the primary of the transformer.
The ac detector is a high-gain transistor ampli-
fier with a twin-Tin a feedback loop for selectivity at
1 kc.
The DET SENS control on the input adjusts the
gain. The range switch causes the gain to be increased
The standard capacitor is specially constructed
for low temperature coefficient. Most of its capacitance
is that of a General Radio silvered-mica unit, which has
a positive temperature coefficient of a,pproximately 35
ppm.
A small, stabilized, polystyrene capacitor is in
parallel with the mica unit
to
reduce the over-all temper-
ature coefficient.
D=I (0=1)
0=1.2 (0=0.83)
~------+-------~R
"
0=0.02
Figure 4-1. DQ coverage chart.
28
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