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5.2 1650 BRIDGE.
T h i s section d i s c u s s e s some d e t a i l s of the
construction and frequency compensation of the 1650
Bridge.
The variable bridge components are General
Radio precision wire-wound rheostats.
The CGRL
rheostat u s e s a mechanical justifying mechanism for
high accuracy, and the DQ rheostat h a s a 54-dB loga-
rithmic range.
T h e standard capacitor is a General
Radio Precision Polystyrene Capacitor and the r e s i s -
tors are 0.1%, low temperature coefficient, metal-film
resistors except for the 1-ohm ratio arm that u s e s a %%
precision General Radio wire-wound card.
5.2.1
BRIDGE SWITCHING.
The CGRL MULTIPLIER switch (S101) s e l e c t s
the bridge range by switching in various ratio-arm
resistors.
Clockwise roiation of t h i s two-rotor switch
increases the multiplier value for the G, R, L , and C
bridges. Both e n d s of t h e range resistor are switched
out s o that the unused r e s i s t o r s c a n be grounded to
reduce capacitance a c r o s s t h i s arm.
Double, solid-
silver contacts ensure low switch r e s i s t a n c e and long
switch life.
T h e CGRL PARAMETER switch (S102) s e l e c t s
the bridge circuits.
T h e actions of t h i s switch are
such that it (1) s e l e c t s the correct rotors of S l O l and
grounds one of the unused rotors, (2) s e l e c t s the correct
standard arm, and (3) reverses the bottom two arms of
the.bridge t o form the L and R or C and G bridges.
T h e function switch s e t s up the correct internal
source and detector circuits for the desired operation.
When t h i s switch i s in either of the two external posi-
tions, the EXT GEN terminals, used for externally
applied a c or d c , are connected in a s the bridge source.
5.2.2
COMPENSATION TECHNIQUES.
T o achieve the required D-Q accuracy over such
wide ranges, several compensating schemes are used.
The components used for t h i s purpose are listed below,
with brief description of their functions.
Component
designations refer t o the schematic wiring diagrams
(Figures
6-6
and 6-9).
C2:
T h i s capacitor corrects for the p h a s e shift
caused by stray capacitance a c r o s s the CGRL rheostat
(RN). T h i s capacitor forms a three-terminal T-network
with the two parts of the rheostat to produce a n effec-
tive inductance t o balance out the stray capacitance.
C3:
T h i s capacitor compensates for the induc-
tance of the 1 - 0 ratio arm (R5).
C4:
T h i s capacitor compensates for the induc-
tance of the 10-n ratio arm (R6).
C 7 and C8: T h e s e capacitors correct the phase
angle of the DQ potentiometer (RT) to compensate for
the inductance of the winding.
Without compensation,
t h i s inductance would c a u s e an error in C, and L p a t
high frequencies, and in C p and L, when the unknown
h a s a very low Q or high D.
C9:
A sixth compensating capacitor consisting
of two turns of grounded wire about the 1-MO resistor
R13 compensates for the stray capacitance a c r o s s R13.
5.2.3
BRIDGE SOURCES AND DETECTORS.
The d c bridge supply is taken from the four
internal D c e l l s , which supply about
6
V limited by a
1 0 0 - 0 resistor t o a maximum of 6 0 mA. The d c indica-
tor on the panel h a s a sensitivity of 2 ,uA/mm near
zero, a r e s i s t a n c e of 75-0, and a shaped characteristic.
T h e a c source i s a 1-kHz transistor RC Wien-
bridge oscillator.
The output voltage is about 1 V at
the secondary of t h e 4-to-1 step-down transformer. The
OSC L E V E L control adjusts output voltage by ad-
justing the voltage a c r o s s the transformer primary.
The a c detector is a four-transistor,variable-gain
amplifier, which u s e s a twin-T RC filter t o obtain
selectivity when on the AC INTERNAL 1 kHz position.
T h i s amplifier drives the panel meter t o provide a visual
a c null indication, and the output from the amplifier i s
supplied t o the s i d e panel D E T phone jack.
T h e a c oscillator and detector combined draw
approximately 10 mA from the internal 6-V battery.
5.2.4 ORTHONULL.
Orthonull i s a mechanical device that improves
the bridge balance convergence when low Q inductors
or high D capacitors are measured. Ordinarily, balances
with s u c h components are tedious and often impossible
due t o the "sliding null" resulting from the interde-
pendence of the two adjustments. Rapid balances are
possible with Orthonull, which d o e s not affect electrical
balance but which d o e s help avoid f a l s e nulls, improv-
ing bridge accuracy for low Q measurements.
T h e bridge output voltage for the L, (Maxwell)
bridge can be expressed, a s in the equation of Figure
5-7.
Eo
R,+jo
L,-(U
+
j o ~ , ~ , c , )
-=
RT
El,
DENOMINATOR
Figure 5 -7.
Loci of RN and RT adiustments on Z plane.
P R I N C I P L E S O F OPERATION 5-5
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