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Figure
3
-
8.
T y p i c a l impedance-vs-frequency response of an
Figure
3
-
9.
T y p i c a l impedance-vs-frequency response of a
ultrasonic transducer.
piezoelectric microphone.
I I
10
9
8
7
the c a s e , but the rf mixer will have to be driven from
.the DET output jack.
The frequency counter is necessary t o obtain
high frequency resolution and accuracy.
Conversion
from C and D to real and imaginary parts of impedance
are most conveniently done by a short computer pro-
gram. Typical impedance curves are shown in Figures
3-8 and 3-9.
Consult paragraph 4.12 for measurement
procedures and accuracy above 20 kHz.
3.7 RESONANT FREQUENCY OF TUNED CIRCUITS.
33.5
33.7
7
-
l o
REAL PART (kHz)
16508.6
4 6
The resonant frequency of a series or parallel
tuned circuit may be found by means of an external
variable-frequency oscillator and the a c resistance
bridge.
The external oscillator is connected a s des-
cribed in paragraph 2.4, and the tuned circuit i s con-
nected to the UNKNOWN terminal.
The frequency and the CGRL dial are then varied
for the best null attainable.
The bridge indicates, a t
balance, the effective series resistance of a series
tuned circuit or the effective parallel resistance of a
parallel tuned circuit, while the oscillator indicates
the resonant frequency.
three-terminal system.
T o measure the direct capaci-
tance of a three-terminal system, connect the third
terminal to the panel of the instrument, using any
grounded panel terminal or a ground lug with screw
just below the UNKNOWN terminals. The capacitances
t o the shield have negligible effect a s long a s one of
them is reasonably small (paragraph 4.6).
Often the shield of an inductor is not connected
t o either terminal. When the inductance and frequency
are low s o that stray capacitance across the inductor
causes negligible error, the shield should be connected
to the UNKNOWN terminal marked LOW. When the in-
ductance (or frequency) i s high, the effective induc-
tance is increased because of the shunting capacitance.
The error is +lo0 ( w 2 ~ , c x ) % (paragraph 4.4). To avoid
an inductance error, the shield may be tied to the panel
of the bridge. The inductor terminal that has the larger
capacitance to the shield should be tied t o the LOW
bridge terminal. A
Q
error results from the capacitance
from the other UNKNOWN terminal t o the shield but a
better measurement of Lx is possible.
(This connec-
tion does not affect the winding capacitance itself.)
::
20
25
3.9 REMOTE MEASUREMENTS.
3.8 SHIELDED THREE-TERMINAL COMPONENTS.
Due t o the small effect of stray capacitance to
FREQUENCY
ground,
particularly
for capacitance measurements
When the unknown is shielded and the shield is
(paragraph 4.6), the unknown may be placed some dis-
not tied to either unknown terminal, a three-terminal
tance away from the bridge. If at least one of the con-
component is formed (Figure 3-10). The impedance Z
necting leads is shielded, the capacitance between
of the component itself is the direct impedance of the
the leads is avoided.
The shielded lead should be
IN kHz
.
SHIELD
connected to the LOW UNKNOWN terminal, and the
@$-f
a
bridge should be grounded.
The other lead may also
be shielded, a t the cost of increased capacitance to
ground. When low impedance measurements are made,
16CB.I
the effect of the lead resistance and inductance should
Figure
3
-
10.
Shielded three-terminal impedance.
be considered (paragraph 4.10).
S P E C I A L M E A S U R E M E N T S
3-6
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