GENERAL RADIO COMPANY 1650-B Instruction Manual page 37

IDEALIZED BALANCING LOCI 0 = 1/2
Figure 5
-
8. Loci of 'sliding nullm balance.
W e will assume that the denominator i s more or l e s s
constant in the region of the null.
The numerator is
the difference between t h e unknown impedance R,
+
jwL, and what c a n be called the "bridge impedance."
The bridge output is proportional t o t h i s difference,
which is the distance between them on the complex
plane. T o balance t h e bridge, the bridge impedance is
varied by adjustment of RN (the CGRL d i a l ) and RT
(the DQ d i a l ) until i t equals the unknown impedance.
An adjustment of R T varies only the r e a l part of the
bridge impedance, whereas a n adjustment of R N varies
both parts, and is therefore a multiplier of the bridge
impedance. Thus, adjustment of R T moves the bridge
impedance horizontally on the complex plane, while
adjustment of R N moves i t radially (Figure 5-7). Each
control is adjusted for a minimum voltage.
When X > > R (i.e., Q is high) t h e s e two adjust-
ments are almost orthogonal, and rapid convergence is
possible. When Q is low, however, the adjustment
becomes more parallel and convergence is slow, caus-
ing a "sliding null", a s shown in Figure 5-8, where
Q =
%.
With smaller Q's, convergence is even slower.
The Orthonull device makes the two adjustments
orthogonal by nonreciprocally ganging RN and RT.
From the equation it is apparent that if RN/RT re-
mained constant a s RN was varied, only t h e imaginary
part of t h e bridge impedance would change. But when
R T is adjusted, RN must not move t o vary only the
r e a l part. The solution is a simple mechanism t o
permit nonreciprocal action. Both the inherent differ-
ence in friction of the two rheostats and the pulley
ratio favor torque transmission in the desired direction.
T h e ratio RN/RT must be constant for variation
in R N for any initial settings of RN and RT, s i n c e RT
may be moved independently of RN. T h i s requires
rheostats with exponential characteristics (and loga-
rithmic dials). The DQ rheostat is a 54-dB exponential
potentiometer with the correct initial r e s i s t a n c e (R3)
added when the L, and C p bridges are used. The
CGRL rheostat is exponential in the d i a l range from 1
t o 11, and linear below 1. Thus, for correct Orthonull
action, the CGRL d i a l must be in the range above 1.
T h e Orthonull mechanism is shown in Figure 6-2.
The clutch action i s between the wire and the free
pulley driven by the wire belt. The clutch is disengaged
by the switch on the panel s o that normal operation i s
possibIe for high-Q (low-D) components.
PRINCIPLES O F OPERATION 5-6