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PRINCIP_LES
OF
OPERATION
2R
-- ...
~--
... ---
....
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sudden bridge unbalances that would momentarily de-
flect the panel meter.
A binary scheme, using only
seven resistors, could be switched to give 128 fixed
values, but there would be many places over the range
where two switching operations would have to occur
at exactly the same moment to avoid a large transi-
ent.
Or, 114 precision wire-wound resistors could be
connected in series on a simple selector switch with a
shorting rotor (as in a decade box) to give the desired
operation, but would be quite expensive.
To effect a
114-position decade-type switch (which we call a "cen-
tade") using fewer resistors, a scheme using three
rotor contacts is llsed.
R
R
I
DETENTED POSITIONS
4 -
I
3
I
2
R
~iN'v--'--O
TERMINAL
Figure 4-2. Diagram showing centade
operation.
R
TERMINAL
The operation of the centade is best explained by
an examination of Figure 4-2.
Briefly, fixed values in
between the values of the series-resistor chain are ob-
tained first by the shunting of one
s~ries
resistor with
two resistors that will reduce it to 1/3 of its value. In
the next step, one shunting resistor is removed, increas-
ing the resistance to 2/3. In the third step, the series
resistor is un shunted giving its full value, and the
shunting resistors are moved into position to shunt the
next series resistor on subsequent steps.
With this scheme, the number of series resistors
is reduced to one third of 114 and two resistors are add-
ed to the rotor.
This idea could be extended to reduce
the number of resistors even further, but the number of
resistors saved for each additional rotor contact be-
comes smailer, and the mechanical design becomes more
complex.
4.4 CENTADE OPERATION.
The adjustment for the first three digits of the
counter used as the CGRL readout places in the bridge
circuit 114 precise steps of resistance.
These steps
increase or decrease continuously with no discontinuity
in the switching, other than the increase Qr decrease
from one fixed value to the other, in order to avoid
4.3 BRIDGE SWITCHING.
The FULL-SCALE RANGE switch (Sl) changes
the ratio-arm resistor of the bridge. Two separate rotors
are used so that a clockwise rotation will increase the
size of the unit for all six bridges. Both ends of the re-
sistors are switched out, and the unused resistors are
grounded to reduce stray capacitance. The range switch
also positions the decimal point on the main readout, de-
termines which dc supply will be used for dc G or R
measurements and where the supply and meter will be
connected to the bridge, and increases the ac gain on the
extreme bridge ranges.
The BRIDGE SELECTOR switch (S2) switches
the internal bridge components to form the six bridges of
Figure 1-2. It also connects the appropriate set of rotors
for the range switch, determines which type of unit is
illuminated above the main readout, indicates the correct
D or Q scale or type of resistance Q, and permits dc to
be applied to the bridge only when it is in the G or R
positions.
The function switch (S3) connects the appropriate
generator and detector for internal and external ac and
dc measurements.
In the EXT DC position the EXT
GEN terminals are connected directly to the bridge, and
in the EXT AC position they are connected directly to
the primary of the bridge transformer.
All switches used in the bridge have solid silver
contacts, and double contacts are used on the range
switch for low contact resistance.
on the two extreme bridge ranges, and a compression cir-
cuit is used to reduce the necessity for constant read-
justment
of the DET SENS control during balance.
This amplifier drives the panel meter and has an aux-
iliary DET OUT connection.
In the EXT DC position of the function switch, the
EXT GEN terminals are connected across the vertical
diagonal of the bridge (see Figure 1-3) (with no series
resistor), and the internal dc detector is in place. When
EXT AC is used, the EXT GEN terminals are connected
directly to the bridge transformer (see Figure 1-3) and
the twin-T is removed from the detector to give it a flat
frequency characteristic.
When other plug-in frequency modules replace the
1-kc module supplied, the selective circuits for the os-
cillator and detector are changed to produce the desired
signal frequency and to provide selective amplification
at
that frequency (refer to paragraph 2.4.4).
29
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