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Section
5
- Principles of Operation
5.1 GENERAL.
5.1.1
NULL METHODS.
Null methods h a v e long been recognized a s the
most p r e c i s e and convenient way t o measure a l l types
of impedance
-
r e s i s t i v e and reactive, inductive and
capacitive, from low frequencies t o uhf. Most null-type
instruments a r e evolved from t h e century-old Wheat-
stone bridge, s t i l l the fundamental circuit for meas-
uring d c r e s i s t a n c e .
Other nu11 c i r c u i t s , s u c h a s the
admittance meter and transfer-function bridge, have been
developed by G e n e r a l R a d i o t o meet t h e d i v e r s e require-
ments of modern measurement. In a l l , General Radio
produces bridges covering virtually the entire field of
impedance measurement. Some of t h e s e bridges include
built-in generator and detector and are t h u s complete,
self-contained measurement systems. Others a r e avail-
able i n combination with various GR oscillators and
detectors, a s complete a s s e m b l i e s .
5.1.2 DC BRIDGES.
T h e Wheatstone bridge measures a n unknown
r e s i s t a n c e , R,,
i n terms of calibrated s t a n d a r d s of
r e s i s t a n c e connected a s shown i n Figure 5-1.
T h e
relation i s :
5.1.3 AC BRIDGES.
T h e Wheatstone bridge circuit i s eas.ily adapted
t o a c measurement.
With complex impedances, two
balance conditions must be s a t i s f i e d , one for the r e s i s -
tive component and one for t h e reactive component.
At balance:
Equation (2) e x p r e s s e s t h e unknown in terms of
impedance components; equation (3) e x p r e s s e s the
admittance. T o s a t i s f y t h e s e equations, a t l e a s t one of
the three arms
1,
2, or
3
must b e complex.
T h e reactance Xx c a n be measured in terms of a
similar r e a c t a n c e in a n a d j a c e n t arm (Figure 5-2) or
an unlike r e a c t a n c e in the opposite arm (Figure 5-3).
The complex arm required t o s a t i s f y the balance
conditions of equation (2) or (3) is a combination of a
F i g u r e
5-1.
T h e
Figures
5-2
and
5-3.
Circuits for copaci-
which i s s a t i s f i e d when the voltage a c r o s s t h e d e t e c t o r
general Wheotstone
tance bridges i n which l i k e reactances (left)
terminals i s zero.
bridge circuit.
or unlike reactances (right) ore compared.
P R I N C I P L E S O F O P E R A T I O N
5-1
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