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Hints
for
Successful
Impedance
Measurements
Application Note 346-4
How to evaluate electronic devices used in circuits
in order to achieve design performance.
1
Impedance Measurements for Engineers
Impedance is measured using a broadest frequency range (5Hz to
variety of techniques. The 110GHz). This technique delivers
advantages of each technique exceptional accuracy near 50 or
depend on test frequency, the 75, depending on the system.
impedance to be measured, as well
as preferred display parameters. LCR Meters and Impedance
Analyzers are differentiated
The Auto Balancing Bridge primarily by display properties.
technique is exceptionally An LCR Meter displays numeric
accurate over a broad impedance data, while an Impedance Analyzer
range (m to the order of 100M). can display data in either numeric
The frequency range this or graphic format. The techniques
technique can be applied to is from employed by these instrument
a few Hz to 110MHz. types are independent of analyzer
type, and can be RF-IV, IV or Auto
The IV and RF-IV techniques are Balancing Bridge (depending on
also very accurate over a broad frequency).
impedance range (m to M). The
frequency range this technique can Engineers perform impedance
be applied to is from 40Hz to measurements for a variety of
1.8GHz. reasons. In a typical application,
an electronic device used in a
The Transmission/Reflection designed circuit is characterized.
technique is applied over the Normally, a manufacturer states
only the nominal value of the
electronic device.
Engineering concerns as well as
production, procurement and
distribution decisions are based on
the quality of measurements. The
value of every electronic
component is determined by its
performance and/or stated values.
This performance determines the
quality of assembled products.
This guide provides useful
information when using the Auto
Balancing Bridge, IV or RF-IV
techniques.
'8 Hints for making Better
Network Analyzer Measurements'
is available as a guide to the
Figure 0-1 Accuracy Profile Transmission/Reflection
technique.
2
Impedance Parameters
Impedance is a parameter used to The Quality Factor (Q) and the
evaluate the characteristics of Dissipation Factor (D) are also
electronic devices. Impedance (Z) derived from resistance and
is defined as the total opposition a reactance. These parameters
device offers to the flow of an serve as measures of reactance
alternating current (AC) at a given purity. When Q is larger or D is
frequency. smaller, the quality is better. Q is
defined as the ratio of the energy
Impedance is represented as a stored in a component to the
complex, vector quantity. A polar energy dissipated by the
coordinate system is used to map component. D is sometimes called
the vector, where quadrants one "tan", since it is the tangent of the
and two correspond respectively to complimentary angle () to the
passive inductance and passive phase angle (). Both D and Q are
capacitance. Quadrants three and dimensionless quantities.
four correspond to negative
resistance. The impedance vector Figure 1-2 describes the
consists of a real part, resistance relationship between impedance
(R), and an imaginary part, and these derived parameters.
reactance (X).
Figure 1-1 shows the impedance
vector mapped in quadrant one of
the polar coordinate system.
Capacitance (C) and inductance
(L) are derived from resistance (R)
and reactance (X). The two forms
of reactance are inductive (XL) and
capacitive (XC).
Figure 1-1 Impedance Vector Figure 1-2 Capacitor and Inductor Parameters
3
Measurements Depend on Test Conditions
Stated values represent the The AC Voltage across the device
performance of a component under can be derived from the device
specific test conditions, as well as impedance, the source resistance,
the tolerance permitted during and the signal source output
manufacture. When circuit (Figure 2-3).
performance requires more
accurate characterization of a An automatic level control (ALC)
component, it is necessary to function maintains a constant
verify stated values, or to evaluate voltage across the DUT. It is
device performance at operating possible to write an ALC program
conditions (usually different than for instruments that have a level
manufacturers test conditions). monitor function, but not a built-in
ALC.
Frequency dependency is common
to all real-world components Control of measurement
because of the existence of integration time allows reduction
parasitics. of unwanted signals. The
averaging function is used to
Figure 2-1 describes ideal and reduce the effects of random noise.
parasitic frequency characteristics Increasing the integration time or
of a real-world capacitor. averaging allows improved
precision, but with slower
Signal level (AC) dependency is measurement speed.
exhibited in the following ways
(Figure 2-2): Detailed explanations of these test
Figure 2-1. Frequency Characteristics of a - Capacitance is dependent on parameters can be found in the
Capacitor
AC voltage level (dielectric instrument operating manuals.
constant (K) of the substrate).
- Inductance is dependent on AC Other physical and electrical
current level (electromagnetic factors that effect measurement
hysteresis of the core material). results include DC Bias,
temperature, humidity, magnetic
fields, light, atmosphere, vibration,
and time.
Figure 2-2. Signal Level Dependency Figure 2-3. Applied Signal & Constant Level Mechanism
4
Choose Appropriate Instrument Display Parameter
Many modern impedance All circuit components are neither
measuring instruments measure purely resistive nor purely
the real and the imaginary parts of reactive. A real-world component
an impedance vector and then contains many parasitic elements.
convert them into the desired With the combination of a
parameters. component's primary and parasitic
elements, a component performs
When a measurement is displayed like a complex circuit.
as impedance (Z) and phase (),
the primary element (R, C, or L) as Recent, advanced impedance
well as any parasitics are all analyzers have an Equivalent
represented in the |Z| and data. Circuit Analysis Function that
allows analysis of the
When parameters other than measurement result in the form of
impedance and phase angle are three or four element circuit
displayed, a two element models (Figure 3-2). Use of this
representation of the measured function enables a more complete
data is used. These two element characterization of a component's
models are based on a series or complex residual elements.
parallel circuit mode (Figure 3-1),
and are distinguished by the
subscript p for parallel or s for
series (Rp, Rs, Cp, Cs, Lp, or Ls).
Figure 3-1. Measurement Circuit Mode Figure 3-2 Equivalent Circuit Analysis Function
5
A Measurement Technique has Limitations
What's the number one question Instrument accuracy
from engineering and production? specifications for D or Q
measurements are usually
How accurate is the data? different than specifications for
other impedance terms.
Instrument accuracy is different
for different impedance values. In the case of a low loss device (low
Instrument accuracy is also D/High Q device), the R-value is
different for different measurement very small relative to the X-value.
technologies (reference Figure 0-1). Small changes in R result in large Q-
value changes (Figure 4-2).
To know the accuracy of a
measurement, compare the The measurement error is on the
measured impedance value of the order of the measured R-value.
DUT to the instrument accuracy for This can result in negative D or Q
the applicable test conditions. values.
Figure 4-1 shows that a 1 nF Be aware that measurement error
capacitor, measured at 1MHz includes error introduced by the
exhibits an impedance of 159. instrument as well as by the test
fixture.
Figure 4-1 Capacitor's Impedance and Test Frequency Figure 4-2. Concept of the Q error
6
Perform Necessary Calibration
Calibration is performed in order Some instruments offer the choice
to define a reference plane where of Fixed-mode or User-mode
the measurement accuracy is calibration. Fixed-mode
specified. Normally, calibration is calibration measures calibration
performed at the instrument's test standards at predetermined (fixed)
port. Corrections to raw data are frequencies. Calibration data for
based on calibration data. frequencies between the fixed,
calibrated points are interpolated.
A baseline calibration is
performed at service centers for Fixed-mode calibration sometimes
Auto Balancing Bridge instruments results in interpolation errors at
such that the specified accuracy those frequencies between the
can be realized for a period of time fixed, calibrated points. At higher
(usually twelve months) frequencies these errors can be
regardless of the instrument substantial.
settings. With these instruments,
operators do not require User-mode calibration measures
calibration standards. calibration standards at the same
frequency points the user has
Baseline calibration for non- Auto selected for a particular
Balancing Bridge instruments measurement. There can be no
requires that a set of calibration interpolation errors associated
standards be used after instrument with User-mode calibration.
initialization and setup. This hint
provides information that may be It is very important to recognize
helpful when using calibration that the operator-established
standards to establish calibration calibration is valid only for the test
for these instruments. conditions (instrument state) under
which calibration is performed.
Figure 5-1. Calibration Plane
7
Perform Necessary Compensation
Compensation is not the same as It is important to verify that error
calibration. The effectiveness of compensation works properly. In
compensation depends on the general, the impedance value for
instrument calibration accuracy, an Open condition should be
therefore compensation must be greater than 100 times the
performed after calibration has impedance of the DUT. In general,
been completed. the impedance value for a Short
condition should be less than 1/100
When a device is directly the impedance of the DUT.
connected to the calibration plane,
the instrument can measure within Open compensation reduces or
a specified measurement eliminates stray capacitance,
accuracy. Since a test fixture or while short compensation reduces
adapter is usually connected or eliminates unwanted resistance
between the calibration plane and and inductance of fixturing.
the device, the residual impedance
of the interface must be When an Open or a Short
compensated for in order to measurement is performed, keep
perform accurate measurements. the distance between the
UNKNOWN terminals the same as
Additional measurement error when the DUT is contacted. This
introduced by a test fixture or keeps parasitic impedance the
adapter can be substantial. The same as when measurements are
total measurement accuracy performed.
consists of the instrument
accuracy as well as error sources Perform Load compensation when
that exist between the device the measurement port is extended
under test (DUT) and the a non-standard distance, the
calibration plane. configuration uses additional
passive circuits/components (e.g.
balun, attenuator, or filter), or
when a scanner is used. The
impedance value of the Load must
be accurately known. A Load
should be selected that is similar
in impedance (at all test
conditions) and form-factor to the
DUT. Use a stable resistor or
capacitor as the LOAD device.
It is practical to measure a Load
using Open/Short compensation
and a non-extended fixture in
order to determine the Load
impedance. The values measured
can then be input as compensation
standard values.
Figure 6-1 OPEN/SHORT Compensation
8
Understanding Phase Shift and Port Extension Effects
Cable Length correction, port When working in the RF region,
extension, or electrical delay is calibration should be performed at
used to extend or rotate the the end of an extension cable. If
calibration plane to the end of a calibration standards cannot be
cable or the surface of a fixture. inserted, port extension can be
This correction reduces or used in this region for short and
eliminates phase shift error in the well-characterized distances.
measurement circuit.
An Auto Balancing Bridge and
When the measurement port is using non-standard cables or
extended away from the extensions, Open/Short/Load
calibration plane, the electrical compensation should be performed
characteristics of the cable effect at the terminus of an extension or
the total measurement fixture. HP Auto Balancing Bridge
performance. products use cable length
compensation for standardized test
To reduce these effects: cables (1, 2, or 4 meters). At the
- Make measurement cables as terminus of the standardized
short as possible. extension cable, the shields should
- Use well-shielded, coaxial normally be connected together.
cables to prevent influence from
external noise. Port Extension in any form has
- Use low-loss coaxial cables to limitations. Since any extension
keep from degrading accuracy, will contribute to losses in the
since the port extension method measurement circuit and/or phase
assumes lossless cable. error, it is imperative that the
limitations of the measurement
A phase shift induced error occurs technique be fully understood prior
due to the test fixture, which can to configuring an extension.
not be reduced using OPEN/
SHORT compensation.
Figure 7-1 Measurement Port Extension
9
Fixture and Connector Care
High quality electrical connections Technique:
insure the capability to make The use of a torque wrench and
precise measurements. At every common sense insures that
connection, the characteristics of damage does not occur when
the mating surfaces vary with the making repeated connections.
quality of connection. An Damage includes scratching and
impedance mismatch at mating deformation of the mating
surfaces will influence surfaces.
propagation of the test signal.
Maintenance:
Attention should be paid to the Many mating surfaces are
mating surfaces at test ports, designed to allow for the
adapters, calibration standards, replacement of parts that degrade
fixture connectors, as well as test with use. If a mating surface
fixture surfaces. cannot be repaired, regularly
scheduled replacement is advised.
The quality of connections depend
on the following Cleanliness:
- composition The use of non-corrosive/
- technique destructive solvents [de-ionized
- maintenance water, pure isopropyl alcohol
- cleanliness (IPA)] and lint-free wipes insures
- storage that the impedance at mating
surfaces is not influenced by
Composition: residual oils or other impurities.
It has been said that a chain is as Note that some plastics are
strong as the weakest link. The denatured with the use of IPA.
same is true for a measurement
system. If low-quality cables, Storage:
adapters or fixtures are used in a If a case is not provided with an
test circuit, the quality of the accessory, plastic caps should be
system is reduced to the lowest used to cover and protect all
quality interface. mating surfaces when not in use.
10
HP Impedance Product Lineup
The HP impedance product lineup marker function and programming Combinations analyzer series
offers the widest selection of function that ease evaluation of
equipment for your application. measurement results. The features The HP combination analyzer
Major impedance measuring provided within this series enable series provides three capabilities
instruments are introduced as characteristic evaluations at Lab/ (network measurement, spectrum
follows. For more information, R&D sections, as well as reliability measurement, and impedance
refer to "LCR meters, Impedance evaluations (temperature measurement) in one box. These
analyzers, and Test Fixtures characteristic evaluation, etc.) for instruments deliver broad
Selection Guide" (P/N 5952- QA purposes. functionality to engineers (from
1430E), "RF Economy Network circuit design to device
Analyzer " (P/N 5967-6316E) etc. evaluation). Graphical displays
are provided, which enable
LCR meter series analysis of device characteristics
for not only impedance, but also
The HP LCR meter series provides network and spectrum
the capability to easily and measurements.
accurately evaluate components
such as, capacitors, inductors,
transformers and
electromechanical devices. The
ability to apply specific
measurement conditions (test Network analyzer series
frequency, signal level, etc.) is
important in the R&D, production The HP network analyzer series
test and QA environments. allows impedance measurements
in higher frequencies like RF and
MW. The measuring technique, the
transmission/reflection technique,
can go to a much higher frequency
than is available with other
techniques. This series also
provides various sophisticated
functions, such as a graphical
display, marker function, and
programming function that ease
evaluation of measurement results.
The features provided within this
series enable evaluations in Lab/
Impedance analyzer series R&D as well as QA departments.
The HP impedance analyzer series
delivers the capability to observe
characteristic changes in device
performance as result from
changes in specific measurement
conditions. The characteristic
changes in device performance
can be displayed in a graphical
format on the instrument display.
This series provides various
sophisticated functions, such as a
11
For more information about Hewlett-
Packard test & measurement products,
applications, services, and for a current
sales office listing, visit our web site,
http://www.hp.com/go/tmdir. You can
also contact one of the following
centers and ask for a test and
measurement sales representative.
United States:
Hewlett-Packard Company
Test and Measurement Call Center
P.O. Box 4026
Englewood, CO 80155-4026
1 800 452 4844
Canada:
Hewlett-Packard Canada Ltd.
5150 Spectrum Way
Mississauga, Ontario
L4W 5G1
(905) 206 4725
Europe:
Hewlett-Packard
European Marketing Centre
P.O. Box 999
1180 AZ Amstelveen
The Netherlands
(31 20) 547 9900
Japan:
Hewlett-Packard Japan Ltd.
Measurement Assistance Center
9-1, Takakura-Cho, Hachioji-Shi,
Tokyo 192, Japan
Tel: (81) 426 56 7832
Fax:(81) 426 56 7840
Latin America:
Hewlett-Packard
Latin American Region Headquarters
5200 Blue Lagoon Drive
9th Floor
Miami, Florida 33126
U.S.A.
Tel: (305) 267-4245
(305) 267-4220
Fax:(305) 267-4288
Australia/New Zealand:
Hewlett-Packard Australia Ltd.
31-41 Joseph Street
Blackburn, Victoria 3130
Australia
Tel: 1 800 629 485 (Australia)
0800 738 378 (New Zealand)
Fax:(61 3) 9210 5489
Asia Pacific:
Hewlett-Packard Asia Pacific Ltd.
17-21/F Shell Tower, Times Square,
1 Matheson Street, Causeway Bay,
Hong Kong
Tel: (852) 2599 7777
Fax:(852) 2506 9285
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