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3.5 MEASUREMENT O F AC RESISTANCE OR CON-
DUCTANCE WITH REACTANCE.
The ac resistance and conductance bridges of
the 1650-B are very useful for making incremental
measurements of nonlinear components like Thyrite@
varistors or diodes, and for measuring input and output
impedances of field-effect transistors or transistor am-
plifiers, gyrators, impedance scalers, etc. For exam-
ple, a negative impedance converter was being used to
cancel some positive resistance in one arm of a bridge
circuit, but the bridge was not balancing properly.
A
resistor was put in series with the negative impedance
converter and the input impedance was measured.
It
was determined that the negative resistance had an
inductive component, discussed below, which calcula-
tions showed to be the result of phase shift in the
operational amplifier (Figure 3-5).
Rin
=
input resistance
: :
feedback resistors
R2
' )
A (S).
%s
A(S)= operational am-
piifiar open loop
R r a - R
5
IN
F
R
~
transfer function
I
I
K
=
gain at
DC
16508-3
S
=
complex frequency,
RI
i0
7
= time constant
Figure 3
-
5.
Operationol amplifier.
If the null is not sharp, i.e., sensitive t o
a
small
change in the CGRL d i a l position, the "resistance" is
either capacitive or inductive.
A capacitive resis-
tance i s measured by connecting an external capaci-
tance decade box (CD)from the HIGH UNKNOWN post
on the bridge t o the c a s e (Figure
3-6).
An inductive
measurement is made by connecting an external capaci-
tance decade box between the OPP ARM banana jack
on the bridge and the c a s e (Figure 3-7).
Measurements can be made in terms of conduc-
tance, also.
The conductance bridge has Ra and Rb
DECADE
CD
F i g u r e 3
-
6.
C i r c u i t f o r
F i g u r e 3
-
7.
C i r c u i t for
m e a s u r i n g c a p a c i t i v e
m e a s u r i n g
i n d u c t i v e
resistors.
resistors.
interchanged, causing the balance formulas t o be a s
follows
:
(Refer to paragraph 1.4 for term definitions.)
Monitor the active circuit's output voltage with
an oscilloscope, and keep the 1650-B Bridge oscilla-
tor level reduced a s much a s possible to keep the de-
vice under test from saturating.
NOTE
A sliding null can occur if the unknown
i s
inductive because Ra appears in the
null equations of both Rx and
L,.
3.6 MEASUREMENT O F TRANSDUCERS.
The small residuals, careful frequency compen-
sation, Orthonull, and relatively high DQ resolution
of the 1650-B Bridge facilitate impedance analysis of
transducers up to 100 kHz.
Microphones, vibration
pickoffs and ultrasonic transducers can be analyzed1S2
by the use of the 1650-B Bridge t o plot their impedance
versils frequency curves.
Useful accessories for this work are a recording
wave analyzer (GR Type 1910 Recording Wave Analyz-
er) and a frequency counter (GR Type
1191
Counter).
Mechanical resonances are usually high Q, and since
it takes time t o balance the bridge for every point,
one would like t o know where the interesting regions
are.
The best procedure is t o balance the bridge
every 1 kHz or so, and sweep up and down about the
frequency with the wave analyzer in the tracking gen-
erator mode.
The measurements should be made with
an 80-dB potentiometer in the level recorder.
When
bumps occur in the plot of null voltage versus fre-
quency, a region of interest i s indicated and can be
analyzed by balancing the bridge a t this frequency.
The wave analyzer is a l s o very useful a s a tuned null
detector a t frequencies above 20 kHz.
The GR Type
1232 Tuned Amplifier and Null Detector can be used
below 20 kHz for a detector, and with the addition of
the GR Type 1232-Pl R F Mixer, frequencies greater
than 20 kHz can be analyzed.
The input impedance
of the wave analyzer i s high enough s o that it can be
connected between the LOW UNKNOWN terminal and
l ~ r e d e r i c k V. Hunt, Electroacoustics, Harvard Monographs
in Applied Science Number
5, 1954,
Harvard University P r e s s
(New York: John Wiley
&
Sons, Inc.)
2 ~ . E. Kinsler and A. R. Frey, Fundamentals of Acoustics,
John Wiley
&
Sons, Inc.,
1962.
S P E C I A L MEASUREMENTS
3-5
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