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SECTION III
Model 4277A
voltage across DUT
Figure 3-9. Four Terminal Pair Measurement Principle.
3-47.
MEASUREMENT
OF
GROUNDED
SAMPLES
3-48. Samples which have one terminal (except
GROUND terminal) grounded to earth cannot
normally be
measured
by
the
4277A.
Such
measurement conditions are, for example, the
distributed
capacitance
measurement
of
a
coaxial cable with a grounded shield conductor
or the input/output impedance measurement of a
single
ended
amplifier.
When
a
one-side-grounded
sample
is
connected
for
measurement,
the
4277A
may
display
a
measurement
error
message
or
incorrect
measurement results. This is because the bridge
section
cannot
achieve a balance
with
any
measurement
terminal
grounded
and,
additionally, any grounding modifies the four
terminal pair measurement architecture (other
than
an
internal
connection
of
the
shield
conductor to instrument chassis at one point).
Note
If one terminal is grounded, a signal
current
of
equal
magnitude
(an
operating condition of the four terminal
pair configuration
measurement)
will
not
flow
in
the
inner
and
outer
conductors of the measurement cable.
3-49. SELECTION OF TEST CABLE LENGTH
3-50. The propagation signal in a transmission
line will develop a change in phase between two
points on the line as illustrated in Figure 3-10.
The difference in phase corresponds to the ratio
of the distance between the two points to the
wavelength
of
the
propagating
signal.
Consequently, owing to their length, test cables
used to connect a sample to the UNKNOWN
terminals
will
cause
a
phase
shift
and
a
propagation loss of the test signal. For example,
the wavelength of a IMHz test signal is 300
meters which is 300 times as long as the Im
standard test cables. Here, the phase of the test
signal at the end of the test cable will be shifted
by about 1.2 degrees (360 ® f 300) in reference
to the phase at the other end of the cable. Since
the effect of test cables on measurements and
the resultant
measurement error increase in
proportion to the test frequency, cable length
must be taken into consideration when making
high
frequency
measurements.
The
CABLE
LENGTH switch must be selected so as to
provide
the
correct
phase
compensation
for
measurements made with the Im standard test
cables or for a test fixture attached directly to
the UNKNOWN terminals.
When standard test
cables (Im or 2m) are used, the CABLE LENGTH
switch
must
be set
to the
Im
position to
minimize additional measurement errors. The 0
position
is for
direct
attachment
type
test
hxtures.
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