New Technologies for Wide Impedance
Range Measurements to 1.8 GHz
Product Note 4291-1
HP 4291B RF Impedance/
Material Analyzer
Introduction
With the current trends in the
communications and data
processing industries requiring
higher performance, smaller
physical size, lower cost, and
higher reliability, accurate and
efficient electronic component
characterization is an increasingly
important part of product design.
Many of these advanced products
are operating at frequency in the
RF range.
Older low-frequency or indirect
methods of component evaluation
are often not able to provide the
high quality impedance parameter
information required for
HP 4291B RF Impedance/Material Analyzer with SMD fixtures
understanding component
performance under actual
operating conditions in the RF
range. This product note
describes new impedance
measurement techniques and
innovations contained in the
HP 4291B RF Impedance/
Material Analyzer that allow
accurate and efficient direct
impedance measurement and
analysis from 1 MHz to 1.8 GHz.
Five general topics will be
discussed:
Limitations of traditional
methods of impedance analysis.
Extending the frequency range
and impedance magnitude
range, while maintaining high
accuracy.
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Making high accuracy Quality
Factor (Q) and Dissipation Factor
(D) measurements at RF.
Increasing test flexibility by
extending the device-under-test
(DUT) location 1.8 meters from
the instrument while maintaining
accuracy.
Eliminating fixture errors
critical issues for accurate RF
measurements.
Limitations of traditional
impedance measurement
solutions
Most conventional measuring
instruments such as traditional
LCR meters and impedance
analyzers are limited to lower
frequency analysis where the
4-terminal pair method can
provide very high accuracy. For
higher frequencies a vector
network analysis approach is
often used by first measuring the
reflection coefficient, then
calculating impedance values.
This indirect method may be
useful for impedance values near
50 Ω, but can be inaccurate for
impedance values significantly
higher or lower than 50 Ω. In
addition, calibration and removal
of test fixturing errors can be a
tedious procedure and in some
cases, not possible. The issue of
fixturing at RF can be a major
source of error and expense. In
general, impedance measurements
in the RF range in the past have
been very difficult and often
yield widely varying results with
questionable accuracy. For
today's RF component or circuit
designer, a new solution is
needed to provide the accuracy
and test efficiency required for
complete RF impedance
characterization of circuit
elements.
Hewlett-Packard now offers a
new analyzer dedicated to the
direct measurement of
impedance and material
parameters from 1 MHz to 1.8 GHz
that overcomes many of the
limitations of previous approaches.
The HP 4291B provides highly
accurate RF impedance
measurements and offers a
family of surface mount device
(SMD) test fixtures. With a color
display and powerful firmware,
up to 15 impedance parameters
as well as equivalent circuit
models and more are easily
measured and displayed. The
following topics describe this
new analyzer's capabilities and
measurement technology.
Figure 1. Ranges of impedance
measurement (Accuracy of
measurement: 10%)
The HP 4291B achieves its
accuracy over these wide
impedance ranges using tow new
techniques:
Direct impedance using
the RF I-V method
The analyzer uses a new method
to measurement impedance; the
RF I-V method. Figure 2 shows
the basic principles of the RF I-V
method and reflection coefficient
method (conventional method
using a vector network analyzer).
and reflection coefficient
methods, the impedance is given
by ratios of the readings of the
two voltmeters. Therefore one
would expect that the accuracy
of both methods would be similar.
However, the reflection
coefficient method magnifies the
measurement error when
converting reflection coefficient
to impedance. As impedance
goes away from 50 Ω, Figure 3
shows that a small changes in
reflection coefficient value
produces a large change in
impedance. In other words, a
small error in the reflection
coefficient leads to a large error
in impedance. (For example, for
an impedance of 2 kΩ, a 1 % error
of reflection coefficient results
in a 24 % error in impedance.)
Figure 3. Relationship between
impedance and reflection coefficient
Expanding the range of
impedance magnitudes
As shown in Figure 1, the
HP 4291B provides an
exceptionally wide range of
impedance measurements. It is
the ideal instrument for measuring
very small capacitance (1 pF)
and inductance (1 nH) values in
the RF range.
Figure 2. I-V method and reflection
coefficient impedance method
As can be seen from the equations
in Figure 2, the RF I-V method
measures impedance directly,
while the reflection coefficient
method measures reflection
coefficient and concerts it to
impedance. In both the RF I-V
2
The RF I-V method measures
impedance directly from a ratio
of voltage and current, without
converting the measured data.
Therefore, the RF I-V technique
maintains consistent accuracy
even if the impedance is
significantly larger or smaller
than 50 Ω. Thus for measuring
non-50 Ω components, the
HP 4291B using the RF I-V
technique is recommended.
High/Low impedance circuit
The HP 4291B employs highimpedance and low-impedance
circuits, as shown in Figure 4, to
expand the range of impedance
measurements. When measuring
a high-impedance device,
accurate measurement of the
DUT current is most critical. The
high-impedance circuit solves
this problem by connecting the
current detection circuit directly
in series with the DUT, ensuring
accurate DUT current
measurement and not measuring
current flowing in the voltage
sensing circuit.
On the other hand, when
measuring a low-impedance
device, the voltage across the
DUT is most critical. In this case,
the low-impedance circuit
connects the voltage detection
circuit directly to the DUT,
ensuring accurate voltage
measurement and not measuring
the voltage drop from the current
sensing impedance. By using the
right measuring circuit for the
impedance being measured, it is
possible to extend the range of
impedance magnitudes measured
for a given accuracy.
Figure 4. High-impedance and lowimpedance measuring circuits
High Accuracy Q and D
measurements
As shown in Figure 5,
the HP 4291B is capable of
evaluating a sample with Q = 100
within ±15 % accuracy at 1 GHz.
This capability is targeted for
evaluating the loss of low-loss
components at RF. The accuracy
of Q and D measurements depend
on the accuracy of phase
measurement. See Figure 6.
The HP 4291B improves the
accuracy of phase measurements
by requiring an additional phase
calibration step. (Conventional
one-port calibration uses open,
short, and 50 Ω load standards.)
This type of one-port calibration
does not provide satisfactory
accuracy for phase measurements
because of the phase uncertainty
of the 50 Ω standard. By using a
low-loss air capacitor as a phase
standard, the HP 4291B lowers
the phase uncertainty to 1 mrad or
less (corresponding to D = 0.001),
ensuring improved accuracy for
Q and D measurements.
Error-free 1.8m cable
extension
The cable connecting the main
body of the HP 4291B to the test
station has been extended to
1.8 meters without adding
additional errors. See Figure 7.
The long cable allows easy
access to remote DUT locations.
This temperature chamber,
scanner/handler, or custom test
setup, for example.
These two measuring circuits are
implemented as two different test
heads in the HP 4291B, so users
can select and switch the circuits
easily to optimize the measurement
range and accuracy to best
match the DUT impedance.
Figure 5. Q measurement accuracy
Figure 6. Q and D measurement and
phase measurement
3
Figure 7. Extended test cable
Normally, extending the cable
increases measurement error
due to increased noise,
temperature differentials, cable
resistance, etc. However, as
illustrated in Figure 8, the
HP 4291B measures the current
and voltage signals using the
same circuit and alternates the
measurement with fast timedivision multiplexing.
Since the measurements are
made at an alternating interval of
several milliseconds, the same
cable-induced errors occur in
both the current and voltage
measurement data. These errors
are then canceled out when the
impedance is obtain from the
ratio of voltage to current.
Furthermore, by measuring
current and voltage with the same
circuit, errors in the measuring
instrument caused by temperature
changes are offset in the same
manner, resulting in significant
temperature characteristic
improvement.
Figure 8. Time-division multiplexing
of cuurent/voltage measurement
Test fixtures have a large impact
on measurement accuracy,
especially at higher test
frequencies. An important factor
in getting accurate measurements
is eliminating errors introduced
by the DUT fixturing. Calibration
insures high accuracy at the plane
of calibration (measurement point
where the standards are applied),
but in actual practice, the test
fixture can add additional error
terms beyond the calibration
plane. This is why fixture error
compensation is so important.
Electrical properties of the test
fixture (which occur after the
calibration point), consist of a
phase rotation due to the
physical length of the electrodes
and other unwanted stray
parasitics between electrodes.
Both of these can cause significant
measurement errors in the RF
band. Conventional measuring
instruments often have no
convenient method to eliminate
them effectively. The HP 4291B
uses electrical length
compensation to remove the
errors caused by phase rotation
and OPEN/SHORT compensation
(at the DUT location in the fixture)
for removing fixture parasitic
impedance.
Conclusion
The HP 4291B RF impedance
and material analyzer provides
highly accurate impedance and
material measurements by
incorporating new technologies
and offering an integrated
package including a family of
SMD and material fixtures. The
analyzer overcomes many
limitations of conventional
impedance analysis and, for the
first time, provides an efficient
and accurate measurement
solution for passive component
analysis over the RF range.
For detailed technical information
of the HP 4291B RF impedance
and material analyzer, refer the
HP 4291B Technical Information
in page 5.
For more information, request
following literatures from your
local HP representative:
HP4291B 1.8 GHz impedance/Material
Analyzer Product Overview P/N 59661501E
HP 4291B Technical Specifications P/N
5966-1543E
Highly Accurate Evaluation of Chip
Capacitors using the HP 4291B
Application Note 1300-1 P/N 59661850E
Evaluating Chip Inductors using the HP
4291B Application Note 1300-2 P/N
5966-1848E
Permittivity Measurements of PC Board
and Substrate Materials using the HP
4291B and HP 16453A
Application Note 1300-3 P/N 59661847E
Permeability Measurements using the
HP 4291B and HP 16454A Application
Note 1300-4 P/N 5966-1844E
Electronic Characterization of IC
Package Application Note 1300-5 P/N
5966-1849E
Impedance Characterization of
Magneto-Resistive Disk Heads Using
the HP 4291B Impedance/Material
Analyzer Application Note 1300-6 P/N
5966-1096E
On-Chip Semiconductor Device
Impedance Mesurements Using the HP
4291B Application Note 1300-7
P/N 5966-1845E
Evaluating Temperature Characteristics
using a Temperature Chamber and the
HP 4291B Product Note
4291-2 P/N 5966-1927E
Impedance Measurements Using the HP
4291B and the Cascade Microtech
Prober Product Note 4291-3 P/N
5966-1928E
Dielectric Comnstant Evaluation of
Rough Surfaced Materials Product Note
4291-5 P/N 5966-1926E
4
Appendix.
New Technologies used in the High Frequency
Impedance Analyzer
HP 4291B RF Impedance/Material Analyzer
Technical information
Abstract
A new one-port impedance analyzer has been
developed for analysis of high frequency devices
and materials up to 1.8 GHz.
Traditionally, impedances near 50 Ω have been
measured accurately by the null method using a
directional bridge. However, this new analyzer
uses a voltmeter/ammeter method and offers
precise measurement capability over a wide
impedance range. Furthermore, a special
calibration method using a low-loss capacitor
realizes an accurate high-Q device measurement.
This paper describes the advantages of these
techniques. Impedance traceability of the
instrument will also be discussed.
Finally, many types of test fixtures are introduced,
because they are a key element in any test system.
1. Introduction
A general impedance measurement schematic
using two vector voltmeters is shown in Fig. 1. In
this case, the true impedance (Zx) of a device under
test (DUT) is determined by measuring the voltages
of any two different sets of points in a linear circuit.
Zx = K1 × (1)
where
K1, K2, K3 : complex constant
In the process of deriving the equation above,
linearity is assumed but reciprocity is not assumed.
Therefore, the existence of active devices in the
circuitry is not prohibited.
There are at most three unknown parameters
related to the circuit in the equation (1). Once we
know these parameters, we can calculate any
impedance of the DUT from the measured voltage
K2+Vr
1+K3×Vr
Vr : voltage ratio
E: signal source V1: vector voltmeter1
V2: vector voltmeter2 Zx: DUT
Figure 1. General schematic for impedance measurement
using two vector voltmeters
ratio (Vr). The procedure that estimates these
circuit parameters is called "calibration" and one
method is "Open-Short-Load (OSL) calibration."
Calculation of Zx from the measured voltage ratio
(Vr) according to equation (1) is called
"correction."
2. Transducer
We call a linear circuit such as the one in Fig. 1
(one that relates a signal source, two vector
voltmeters and a DUT) a "transducer."
Transducers are the key element in impedance
measurement. For example, two types of
transducers, the "directional bridge" and the
transducer in a "voltmeter/ammeter (V-I) method,"
are compared in terms of sensitivity to the gain
variance.
2-1. Directional bridge type
Directional bridges (see Fig. 2-1) are used in many
network analyzers.
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