Correlating Impedance Measurements Among
Different Types of Measurement Instruments
Product Note 4291-4
HP4291B RF Impedance/
Material Analyzer
Introduction
As the technology of impedance
measuring instruments advances,
problems correlating
measurements between
instruments, instrument
configurations, and even
instrument platforms become
common. The factors affecting the
correlation of impedance
measurement values and the
attention to be paid for achieving
higher measurement correlation
are studied by making an example
of HP 4291B RF impedance/
material analyzer and HP 4191A
RF impedance analyzer in this
product note.
1
Factors affecting
correlation
It is desirable to get the same
results when a device is measured
by any instrument. However, there
is always some difference
between measurement
instruments. The fundamental
difference is a result of the
instrument's specified accuracy.
The measurement accuracy is
specified at the calibration plane
of the instrument. This plane is
defined by calibration standards,
and, in the case of the HP 4291B
and the HP 4191A, is defined at the
7mm (APC-7) test port. If the
device under test (DUT) is
measured at the calibration plane
(requires an insertable device),
the measurement values are only
affected by the specified
measurement accuracy, thus there
is no correlation problem among
different instruments (refer
Figure 1).
Since most DUTs are nonÂinsertable (do not directly attach
to the 7mm connector) at the
calibration plain, a test fixture is
usually used to contact the device.
There are various error factors
associated with fixtured
measurements, caused by the
existence of the test fixture (refer
Figure 2).
The factors affecting the
correlation are discussed as
follows.
1. Performing compensation
When a fixtured measurement is
made without compensating for
the electrical characteristics of
the test fixture, the measurement
result reflects the characteristics
of the DUT as well as the
characteristics of the test fixture
(fixture parasitics). Several
compensation functions are used
to remove the error caused by a
test fixture. Since different
compensation functions are
sometimes provided with different
instruments, the correlation of
measurement values will be
affected.
Figure 1. Measuring DUT at Calibration Plane
Figure 2. Factors to Affect Correlation Problem
calibration plane and the DUT.
This leads to an impedance
measurement error when
measuring phase. Residual
impedance and phase shift error
can be compensated with OPEN/
SHORT compensation and
electrical length compensation.
The HP 4291B provides both
OPEN/SHORT/(LOAD)
Figure 3. Residual Error Model of
Test Fixture
compensation and electrical
length compensation functions. All
error factors mentioned above can
Figure 3 shows the error model
caused by a test fixture. There are
2 types of error, one is residual
impedance error and the other one
is electrical length error. The
residual impedance which
includes small resistance,
inductance, conductance and
capacitance makes the
measurement value inaccurate.
The electrical length error occurs
due to a phase shift of the test
signal along the
50Ω transmission line between the
be removed effectively. As a
result, the real characteristics of a
device can be measured, since the
device can be equivalently moved
to the calibration plane (Figure 4).
(Remarks: Strictly speaking, one
factor still remains. This factor is
regarded as the time fluctuation of
compensated values).
The HP 4191A provides only the
electrical length compensation
function (Figure 4(b)). Since the
measurement value includes the
impedance of the DUT and the test
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