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Operation
-Section
2
2
.1
GENERAL.
The manner in which the 1417 should be used depends
on
many factors:
the
accuracy required, the
test
frequency,
the
applied voltage,
the
capacitance values required, and
the
instrument with which
it
is
used. The operation
is
very sim·
pie in many applications,
but
can become more complex,
particularly when
the
highest accuracy
is
required. It
is
strongly suggested
that
the
theory and calibration techniques
used
be
thoroughly understood
to
avoid possible errors.
Because different instruments measure under different
conditions and have different accuracy specifications, speci·
fie instructions are given in Section 3 for each
GR
instrument
that
measures high-valued capacitance. These refer
to
the
following paragraphs which describe procedures and pre·
cautions
that
should be used
to
obtain accurate calibrations
for
the
1417, so
that
it may be used
to
make accurate
cal
i·
brations
of
measuring instruments.
2.2 CONNECTIONS TO INSTRUMENTS.
2.2.1 Four-Terminal Connections.
The
connections
to
4-terminal measuring instruments are
made
to
the
Hand
L CURRENT terminals and H
a-td
L
POTENTIAL terminals. Usually
the
terminals
of
the
meas·
uring instrument use a similar notation (although + may be
used instead
of
Hand-
instead
of
L)
and, in genera
l,
these
terminals are connected
to
the
corresponding terminals
of
the
1417,
but
not
always. Section
3,
which describes
the
calibration
of
specific GR instruments, gives
the
proper
connections
for
each instrument.
The 1417 also has terminals
that
tie
directly
to
its case.
It
is
not
usually necessary
to
make any connection
to
this
terminal. However,
if
strong fields are present, grounding the
case may give
better
results. There
is
no direct connection
between
the
case and
the
internal circuitry.
2.2.2
Terminal Impedances.
The
1417
has impedance in series with each
of
its termi-
nals as shown
in
Figure 4-4
and
Table 4-2. These have
no
effect
on
a perfect 4-terminal measurement. However,
no instrument makes such a perfect measurement. so
that
connections should
be
made in such a way
that
the
larger
terminal impedances are connected
to
the bridge
or
meter
terminals
that
are most immune
to
such errors. This
is
why
the
various instruments described
in
Section 3 have different
connections. There
is
a best way
to
connect
to
each unit.
2.2.3
Connecting Leads.
The connecting leads used should
be
short
and use reas-
onably heavy wire
to
avoid increasing these terminal impe·
dances. However, in many cases test lead sets are supplied
for particular instruments. If these lead sets are used also in
the
application
of
the
instrument,
it
is
desirable
to
use
them
for
the
calibration as
wel
l, so
that
the
calibration
is
made
on
the instrument
as
it wi
ll
be used. If special lead sets are
used
it
must be remembered
that
they
can
affect
the
cali-
bration.
2.2.4
Mutual Inductance In Leads
While a gqod four-terminal measurement
can-
be immune
to
.self-inductance in
the
le
ads, mutual inductance between
the
current leads and the potential leads can still cause an
error,
an
error which
is
very
important
at
very high capaci-
tance. The equivale
nt
circuit
of
figure 4-3 gives
the
formula
c
Cmeas = 2
1
±w
CM
The mutual inductance can be positive
or
negative. This
error
is
critical
at
very high capacitance, even
at
100
Hz,
and much more critical
at
1 kHz.
For
example 2.5
nH
g
ives
0.1%
error
when measuring 1
Fat
100Hz
or
when measuring
10
mF
at
1 kHz. The inductance
of
2 coaxial, circular,
single-
turn
coils
both
of
1
-e
m radius, 1 em apart, is about
5 nH;
it
is
obvious
that
great care
must
be
taken
when w2 C
is
large.
Mutual inductance between
the
current
circuit
and the
potential circuit occurs in the 1417, the
connect
i
ng
leads,
and inside
the
measuring instrument. The mutual inductance
inside
the
1417
is
usually negligible compared
to
that
of
the
leads.
At
the
highest capacitance values (100
mF
and 1 F)
it
is
approx 1
nH
.
At
lower values
it
causes negligible error
at
120
Hz
and less
than
.02% at 1 kHz.
The sections following, on the calibration
of
specific in·
struments, note
the
effect,
if
any,
of
the
mutual inductances
of
these instruments and their lead sets.
2.3 ACCURACY.
2.3.1 Accuracy
of
the
1417.
With no calibration,
the
accuracy
of
the
1417
depends
on
many factors:
the
accuracy
of
the
initial adjustmen
t,
drift in
the
standardcapac
it
or,i
ts
temperature coeff
ici
ent,
the
signal level applied. and mutual inductance in i
ts
connec·
tions. Under reasonab
le
conditio
ns
(see para
2.3
.3),
its
un-
calibrated
or
direct-reading accuracy
is
0.25
%.
The ultimate corrected accuracy
is
much
better
. Measure·
ments have been made
to
±.01
%.
This
corrected accuracy
is
possible because
the
inductive voltage dividers used
to
scale
the
capacitance
va
lues (see Section 4) are extremely precise.
This scaling accuracy is
the
RATIO ACCURACY given
in
the
specifications.
Th
is is
the accuracy of
the
ratio bet·
ween
the
capacitance v
al
ue
at
any setting
to
the
value at
1
fJF,
assuming
that
the
si
gnal level
on
the
internal standard
remains constant,
or
that
the
change caused by a changing
3
