SYS MENU MEASDISPFUNC : Z-

FREQ : 100.000 kHzLEVEL : 1.00 V

RANGE : AUTOBIAS : 0.000 VINTEG : LONG BIN

No.

BINCOUNT

LISTSWEEP

Vm : 50.00mVCORR: OPEN, SHORT

Im : 10.74mA

150.000 Ω:Z -90.000 deg:θ

θ

Agilent8 Hints for Successful Impedance MeasurementsApplication Note 346-4

Characterizing Electronic Componentsto Achieve Designed Circuit Performance

Contents

HINT 1. Impedance Parameters

HINT 2. Measurements Depend on Test Conditions

HINT 3. Choose Appropriate Instrument Display Parameter

HINT 4. A Measurement Technique has Limitations

HINT 5. Perform Calibration

HINT 6. Perform Compensation

HINT 7. Understanding Phase Shift and Port Extension Effects

HINT 8. Fixture and Connector Care

ImpedanceMeasurements forEngineersImpedance is measured using a vari-ety of techniques. A particular tech-nique is selected according to thetest frequency, the impedanceparameter to be measured and thepreferred display parameters.

The Auto-Balancing Bridgetechnique is exceptionally accurateover a broad impedance range (mΩto the order of 100 MΩ). The fre-quency range this technique can be applied to is from a few Hz to 110 MHz.

The IV and RF-IV techniques arealso very accurate over a broadimpedance range (mΩ to MΩ).These techniques can be appliedfrom 40 Hz to 3 GHz.

The Transmission/Reflectiontechnique is applied over the broadest frequency range (5 Hz to110 GHz). This technique deliversexceptional accuracy near 50 Ω or75 Ω.

LCR meters and impedance analyz-ers are differentiated primarily bydisplay properties. An LCR meterdisplays numeric data, while animpedance analyzer can display datain either numeric or graphic formats.

Figure 0-1. Accuracy Profile

The techniques employed by theseinstruments are independent of ana-lyzer type, and can be RF-IV, IV orAuto-Balancing Bridge (dependingon frequency).

Engineers perform impedance meas-urements for a variety of reasons. Ina typical application, an electroniccomponent used in a new circuitdesign is characterized. Normally,component manufacturers state onlynominal impedance values.

Design decisions, as well as deci-sions affecting the production of theassembled product, depend to somedegree on the impedance valuesattributed to the product’s compo-nents. The performance and qualityof the final product are thereforedetermined in part by the accuracyand thoroughness with which itscomponents are characterized.

This application note provides help-ful information for using the Auto-Balancing Bridge, IV and RF-IV tech-niques. Refer to Agilent ApplicationNote 1291-1, 8 Hints for MakingBetter Network AnalyzerMeasurements (literature number 5965-8166E) for information on theTransmission/Reflection technique.

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HINT 1.ImpedanceParametersImpedance is a parameter used toevaluate the characteristics of elec-tronic components. Impedance (Z)is defined as the total opposition acomponent offers to the flow of analternating current (AC) at a givenfrequency.

Impedance is represented as a com-plex, vector quantity. A polar coordi-nate system is used to map thevector, where quadrants one andtwo correspond respectively to pas-sive inductance and passive capaci-tance. Quadrants three and fourcorrespond to negative resistance.The impedance vector consists of areal part, resistance (R), and animaginary part, reactance (X).

Figure 1-1 shows the impedancevector mapped in quadrant one ofthe polar coordinate system.

Capacitance (C) and inductance (L)are derived from resistance (R) andreactance (X). The two forms ofreactance are inductive (XL) andcapacitive (XC).

The Quality Factor (Q) and theDissipation Factor (D) are alsoderived from resistance and reac-tance. These parameters serve asmeasures of reactance purity. WhenQ is larger or D is smaller, the quali-ty is better. Q is defined as the ratioof the energy stored in a componentto the energy dissipated by the com-ponent. D is the inverse of Q. D isalso equal to “tan δ”, where δ is thedielectric loss angle (δ is the com-plementary angle to θ, the phaseangle). Both D and Q are dimension-less quantities.

Figure 1-2 describes the relationshipbetween impedance and thesederived parameters.

Q =

D =

Capacitor

Inductor

RX L

X C

R

R

Z

ZjX L

jX C

-j

+j

R jX L

R - jX C

X L = 2πfL = ωL

X C =

=

1

ωC

2πfC1

δ

δ

θ

θ

Figure 1-1. Impedance Vector Figure 1-2. Capacitor and Inductor Parameters

3

HINT 2.MeasurementsDepend on TestConditionsA manufacturer’s stated impedancevalues represent the performance ofa component under specific testconditions, as well as the tolerancepermitted during manufacture.When circuit performance requiresmore accurate characterization of acomponent, it is necessary to verifythe stated values, or to evaluatecomponent performance under oper-ating conditions (usually differentthan the manufacturer’s test condi-tions).

Frequency dependency is commonto all components because of para-sitic inductance, capacitance andresistance.

Figure 2-1 describes ideal and para-sitic frequency characteristics of atypical capacitor.

Figure 2-1. Frequency Characteristics of a Capacitor

Signal level (AC) dependency isexhibited in the following ways (see Figure 2-2):

• Capacitance is dependent on AC voltage level (dielectric constant, K, of the substrate).

• Inductance is dependent on AC current level (electromagnetic hysteresis of the core material).

The AC voltage across the compo-nent can be derived from the com-ponent’s impedance, the sourceresistance, and the signal sourceoutput (Figure 2-3).

An automatic level control (ALC)function maintains a constant volt-age across the DUT (device undertest). It is possible to write an ALCprogram for instruments that have alevel monitor function, but not abuilt-in ALC.

Control of measurement integrationtime allows reduction of unwantedsignals. The averaging function isused to reduce the effects of randomnoise. Increasing the integrationtime or averaging allows improvedprecision, but with slower measure-ment speed. Detailed explanations ofthese test parameters can be foundin the instrument operating manuals.

Other physical and electrical factorsthat affect measurement resultsinclude DC bias, temperature,humidity, magnetic fields, light,atmosphere, vibration, and time.

(a) AC Voltage Level Dependency ofCeramic Capacitor Measurement

(b) AC Current Level Dependency ofCore Inductor Measurement

C

0

AC Test Voltage

Dielectric Constant High Dielectric

Constant Medium

Dielectric Constant Low

(No Dependency)

AC Test Current

L

0(No Dependency)

˜

R X

DUT

V dut

SignalSource

Source Resistance

Rs

V A

V dut = Vo *R2 + X2

VoRs

DUT

Feedback

(Rs+R)2 + X2

Vm

Vo Idut

A

Figure 2-2. Signal Level Dependency Figure 2-3. Applied Signal and Constant Level Mechanism

4

HINT 3.Choose AppropriateInstrument DisplayParameterMany modern impedance measuringinstruments measure the real andthe imaginary parts of an impedancevector and then convert them intothe desired parameters.

When a measurement is displayed asimpedance (Z) and phase (θ), theprimary element (R, C, or L) as wellas any parasitics are all representedin the |Z| and θ data.

When parameters other than imped-ance and phase angle are displayed,a two-element model of the compo-nent is used. These two-elementmodels are based on a series or par-allel circuit mode (Figure 3-1), andare distinguished by the subscripts“p” for parallel and “s” for series (Rp,Rs, Cp, Cs, Lp, or Ls).

No circuit components are purelyresistive or purely reactive. A typicalcomponent contains many parasiticelements. With the combination ofprimary and parasitic elements, acomponent acts like a complex cir-cuit.

Recent, advanced impedance analyz-ers have an Equivalent CircuitAnalysis Function that allows analy-sis of the measurement result in theform of three- or four-element cir-cuit models (Figure 3-2). Use of thisfunction enables a more completecharacterization of a component'scomplex residual elements.

Figure 3-1. Measurement Circuit Mode Figure 3-2. Equivalent Circuit Analysis Function

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HINT 4.A MeasurementTechnique HasLimitationsThe most frequently asked questionin engineering and manufacturing isprobably: “How accurate is thedata?”

Instrument accuracy depends on theimpedance values being measured,as well as the measurement technol-ogy employed (see Figure 0-1).

To determine the accuracy of ameasurement, compare the meas-ured impedance value of the DUT tothe instrument accuracy for theapplicable test conditions.

Figure 4-1 shows that a 1 nF capaci-tor measured at 1 MHz exhibits animpedance of 159 Ω.

Instrument accuracy specificationsfor D or Q measurements are usuallydifferent than accuracy specifica-tions for other impedance terms.

In the case of a low-loss (low-D/high-Q) component, the R-value isvery small relative to the X-value.Small changes in R result in largechanges in Q (Figure 4-2).

The measurement error is on theorder of the measured R-value. Thiscan result in negative D or Q values.

Be aware that measurement errorincludes error introduced by theinstrument as well as by the test fix-ture.

159

1 10 100 1K 10K 100K 1M10M 100M 1G

10M

1M

100K

10K

1K

100

10

1100m

1mF10mF100mF

100uF

10uF

1uF

100nF

10nF 1nF10pF

100fF 1fF

100pF 1pF10fF

Impe

danc

e (Ω

)

1 nF at 1 MHz

Impedance value of capacitor

Z =

(ω = 2πf)

Test frequency (Hz)

1ωC

Figure 4-1. Capacitor Impedance and Test Frequency Figure 4-2. Concept of the Q Error

6

HINT 5.Perform CalibrationCalibration is performed to define areference plane where the measure-ment accuracy is specified (Figure5-1). Normally, calibration is per-formed at the instrument's test port.Corrections to raw data are based oncalibration data.

A baseline calibration is performedat service centers for Auto-BalancingBridge instruments such that thespecified accuracy can be realizedfor a period of time (usually twelvemonths) regardless of the instru-ment settings. With these instru-ments, operators do not requirecalibration standards.

Baseline calibration for non-Auto-Balancing Bridge instrumentsrequires that a set of calibrationstandards be used after instrumentinitialization and setup. This hintprovides information that may behelpful when using calibration stan-dards to establish calibration forthese instruments.

Some instruments offer the choice offixed-mode or user-mode calibration.Fixed-mode calibration measurescalibration standards at predeter-mined (fixed) frequencies. Calibra-tion data for frequencies betweenthe fixed, calibrated points are inter-polated.

Fixed-mode calibration sometimesresults in interpolation errors at fre-quencies between the fixed, calibrat-ed points. At higher frequencies,these errors can be substantial.

User-mode calibration measures cali-bration standards at the same fre-quency points the user has selectedfor a particular measurement. Therecan be no interpolation errors asso-ciated with user-mode calibration.

It is important to recognize that theoperator-established calibration isvalid only for the test conditions(instrument state) under which cali-bration is performed.

Calibration Defines a Reference Plane atWhich Measurement Accuracy is Specified.

Calibration Plane

Z Analyzer

Z?

Calibration Standards

Short Open LoadLow-LossCapacitor*

OS0Ω –90o50Ω

*Agilent 4291B, 4286A and 4287A

Figure 5-1. Calibration Plane

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HINT 6.PerformCompensationCompensation is not the same as calibration. The effectiveness ofcompensation depends on theinstrument calibration accuracy,therefore compensation must beperformed after calibration has beencompleted.

When a device is directly connectedto the calibration plane, the instru-ment can measure within a specifiedmeasurement accuracy. Since a testfixture or adapter is usually connect-ed between the calibration plane andthe device, the residual impedanceof the interface must be compensat-ed for to perform accurate measure-ments.

Additional measurement error intro-duced by a test fixture or adaptercan be substantial. The total meas-urement accuracy consists of theinstrument accuracy plus the errorfrom sources that exist between theDUT and the calibration plane.

It is important to verify that errorcompensation is working properly. Ingeneral, the impedance value for anopen condition should be greaterthan 100 times the impedance of theDUT. In general, the impedancevalue for a short condition should beless than 1/100 the impedance of theDUT.

Open compensation reduces or elim-inates stray capacitance, while shortcompensation reduces or eliminatesthe unwanted resistance and induc-tance of fixturing.

When performing an open or a shortmeasurement, keep the distancebetween the UNKNOWN terminalsthe same as when the DUT is contacted. This keeps parasiticimpedance the same as when meas-urements are performed.

Perform load compensation whenthe measurement port is extended anon-standard distance, the configu-ration uses additional passive cir-cuits/components (for example, abalun, attenuator, or filter), or whena scanner is used. The impedancevalue of the load must be accuratelyknown. A load should be selectedthat is similar in impedance (at alltest conditions) and form-factor tothe DUT. Use a stable resistor orcapacitor as the LOAD device.

It is practical to measure a loadusing open/short compensation anda non-extended fixture to determinethe load impedance. The valuesmeasured can then be input as com-pensation standard values.

Figure 6-1. OPEN/SHORT Compensation

8

HINT 7.Understanding PhaseShift and PortExtension EffectsCable length correction, port exten-sion, or electrical delay is used toextend or rotate the calibrationplane to the end of a cable or thesurface of a fixture. This correctionreduces or eliminates phase shifterror in the measurement circuit.

When the measurement port isextended away from the calibrationplane (Figure 7-1), the electricalcharacteristics of the cable affect thetotal measurement performance. Toreduce the resulting effects:

• Make measurement cables as short as possible.

• Use well-shielded, coaxial cables to prevent influence from external noise.

• Use low-loss coaxial cables to keep from degrading accuracy, since the port extension method assumes lossless cable.

A phase-shift-induced error occursdue to the test fixture, which cannot be reduced using OPEN/ SHORTcompensation.

When working in the RF region, cali-bration should be performed at theend of an extension cable. If calibra-tion standards cannot be inserted,port extension can be used in thisregion for short and well-character-ized distances.

When using the Auto-BalancingBridge technique with non-standardcables or extensions, open/short/loadcompensation should be performedat the terminus of an extension orfixture. Auto-Balancing Bridge prod-ucts use cable length compensationfor standardized test cables (1, 2, or4 meters). At the terminus of thestandardized extension cable, theshields should normally be connect-ed together.

Port extension in any form has limi-tations. Since any extension willcontribute to losses in the measure-ment circuit and/or phase error, it isimperative that the limitations of themeasurement technique be fullyunderstood prior to configuring anextension.

ImpedanceMeasuringInstrument

Extension Cable TestFixture

Device Under Test

Figure 7-1. Measurement Port Extension

9

HINT 8.Fixture andConnector CareHigh-quality electrical connectionsinsure the capability to make precisemeasurements. At every connection,the characteristics of the mating sur-faces vary with the quality of con-nection. An impedance mismatch atmating surfaces will influence propa-gation of the test signal.

Attention should be paid to the mat-ing surfaces of test ports, adapters,calibration standards, fixture con-nectors, and test fixtures. The quali-ty of connections depends on thefollowing:

• composition• technique• maintenance• cleanliness• storage

CompositionIt has been said that a chain is asstrong as the weakest link. The sameis true for a measurement system. Iflow-quality cables, adapters or fix-tures are used in a test system, theoverall quality of the system isreduced to that of the lowest-qualityinterface.

TechniqueThe use of a torque wrench andcommon sense insures that damagedoes not occur when making repeat-ed connections. Damage includesscratching and deformation of themating surfaces.

MaintenanceMany mating surfaces are designedto allow for the replacement of partsthat degrade with use. If a matingsurface cannot be repaired, regularlyscheduled replacement is advised.

CleanlinessThe use of non-corrosive/non-destructive solvents (such as de-ion-ized water and pure isopropylalcohol) and lint-free wipes insuresthat the impedance at mating sur-faces is not influenced by residualoils or other impurities. Note thatsome plastics are denatured with theuse of isopropyl alchohol.

StorageIf a case is not provided with anaccessory, plastic caps should beused to cover and protect all matingsurfaces when not in use.

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Agilent Technologies’Impedance ProductLineupAgilent offers the widest selection ofimpedance measuring equipment foryour applications. An overview ofthese instruments is given below.For more information, refer to theproduct literature listed at the endof this note.

LCR Meters LCR meters can easily and accurate-ly evaluate components such ascapacitors, inductors, transformersand electromechanical devices. Theability of these instruments to applyspecific measurement conditions(such as test frequency and signallevel) is important in the R&D, pro-duction test and QA environments.

Impedance Analyzers Agilent impedance analyzers canmeasure characteristic changes incomponent performance resultingfrom changes in specific measure-ment conditions. The characteristicchanges in performance can be dis-played in a graphical format. Theseanalyzers provide sophisticatedfunctions, such as markers and

programming, which ease evaluationof measurement results. They alsohave features that enable character-istic evaluations for R&D, as well asreliability evaluations (includingtemperature characteristics) for QApurposes.

Network Analyzers Network analyzers allow impedancemeasurements at RF and microwavefrequencies using the Transmission/Reflection technique. Their graphicaldisplays have markers and program-ming functions that simplify theanalysis of measurement results.Agilent network analyzers are suit-able for both R & D and QA use.

Combination Analyzers Combination analyzers from Agilentprovide three capabilities—vectornetwork, spectrum and impedancemeasurements—in one box. Theseinstruments deliver broad function-ality to engineers over a wide rangeof applications from circuit design tocomponent evaluation.

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For more assistance with your test & measurement needs go to

www.agilent.com/find/assist

Or contact the test and measurementexperts at Agilent Technologies(During normal business hours)

United States:(tel) 1 800 452 4844

Canada:(tel) 1 877 894 4414(fax) (905) 206 4120

Europe:(tel) (31 20) 547 2000

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New Zealand:(tel) 0 800 738 378 (fax) 64 4 495 8950

Asia Pacific:(tel) (852) 3197 7777(fax) (852) 2506 9284

Product specifications and descriptionsin this document subject to change without notice.

Copyright © 2000Agilent TechnologiesPrinted in USA 06/005968-1947E

Product Literature1. LCR Meters, Impedance

Analyzers, and Test Fixtures Selection Guide, literature number 5952-1430E.

2. RF Economy Network Analyzer, literature number 5967-6316E.

Key Web Resourseswww.agilent.com/find/component_test

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