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Optimizing Electronic Component and MaterialImpedance Measurements

Application Note 369-1Taking Full Advantage ofThe HP 4284A Precision LCRMeter's Versatility

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CONTENTSPage

Introduction 1

HP 4284A Precision LCR Meter 1

The Versatility of the HP 4284A 2

Evaluating the Level and Bias Dependencyof High Dielectric Constant Ceramic Capacitors 4

Automation of Multi-Frequency Evaluationsof Film Capacitors 6

Improving the Cored Coil Evaluation Methodby Means of Constant-Current Measurement 8

Evaluation of the Saturation Characteristicsand Self-Resonant Frequencies of Magnetic Heads 9

Conclusion 11

1

Introduction

Impedance measurements are a basic means ofevaluating materials and electronic components thatsupport the technical innovations of electronicinstruments and equipment. Impedance characteristicsof the components used in circuits or of materialsvary with frequency, signal level or the signalcombined DC-bias voltage. For an engineer engagedin circuit design or component development, preciseevaluation of impedance under the actual operatingconditions is the first step towards obtaining highquality circuit design, and for the development andmanufacture of high-performance components.

This application note describes the evaluation ofelectronic components and materials that will givefull play to the wide range of frequency, signallevels, and DC bias settings.

HP 4284A Precision LCR Meter

Sophisticated Measuring Functions that Meetthe Demands for High Quality

The HP 4284A precision LCR meter provides highlyaccurate and efficient impedance measurement ofelectronic components and materials which enablesit to give excellent cost/data performance. With awide 20 Hz to 1 MHz frequency range and a reliablebasic accuracy of 0.05%, it answers the measurementneeds of testing high quality components. Attachoption 001 and you get a maximum test signal levelof 20 Vrms/200 mArms and a ±40 V DC bias whichallows it to meet the measurement conditions thatensure fast and highly reliable evaluation of electroniccomponents and materials characteristics. In addition,a constant-voltage/constant-current functionmaintains the test signal at a constant level toguarantee measurements are well within JIS andMIL standards.

Low-Cost and Highly Efficient Measurements inAny Measurement Environment

The outstanding basic functions of the HP 4284A andthe wealth of available Options and accessories thatextend these functions depending on the measurementobjective, answer all of the measurement needs ofresearch laboratories, screening inspection, qualitycontrol, incoming inspections, etc. You can easilycreate an automatic test system with options such asthe HP-IB interface which includes buffer memory asstandard, a handler interface and a scannerinterface. The ergonomic design of the front panel,the large easy-to-read backlit Liquid Crystal Displaymakes it possible to take in measurement conditionsand results at a glance. All measurement conditionscan easily be set up using the display cursor, thearrow keys, and the softkeys. Furthermore,measurement setups can be recorded on a memorycard to raise efficiency and to prevent mistakes.Thus the efficiency of all stages of measurement,from measurement setup through data collection andanalysis have been improved while costs have beenlowered.

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Key Specifications

HP 4284A Precision LCR Meter

Test 20 Hz to 1 MHzFrequency 8610 points

Number of 6 digitsDigits

Measurement |Z|,R,X: 0.01mΩ to 99.999MΩparameters |Y|,G,B: 0.01nS to 99.9999S

C: 0.01fF to 9.99999FL: 0.01nH to 99.9999kHD: 0.000001 to 9.99999Q: 0.01 to 99999.9

Basic |Z|,C and L: 0.05%accuracy D: 0.0005

Test Signal Voltage:5mVrms to 2VrmsLevel Range Current:50µArms to 20mArms

Constant Test Voltage:10mVrms to 1VrmsSignal Level Current:100µArms to 10mArmsRange

Measurement 40ms/190ms/830ms at 1kHzTime

Test Cable 0, 1, 2, and 4 metersLength (2 and 4 meters are option)

Comparator Primary Parameter: 10 binsFunction Secondary Parameter: HIGH/IN/LOW

Internal 1.5V and 2VDC bias

Test signal Voltage:5mVrms to 20VrmsLevel Range Current:50µArms to 200mArms

Constant Test Voltage:10mVrms to 10VrmsSignal Level Current:100µArms to 100mArmsRange

Internal ±40V with 0.1% accuracyDC bias

HP 4284A Option 001

The Versatility of the HP 4284A

Research and Development of New Materialsand Low Loss Electronic Components

Conventional LCR meters do not have the accuracynor the resolution required for measuring the newlow-loss electronic components and materialsresulting from the latest materials technology. TheHP 4284A has a basic accuracy of C: 0.05%, D: 0.0005,and a 0.000001 D resolution which allows it tomeasure high quality electronic components andmaterials, The 6-digit resolution of the display meansthat even minor changes in temperature and humiditycharacteristics are not missed. The powerful OPEN/SHORT/LOAD error correction capability reducesmeasurement errors to a minimum.

Quality Assurance of Materials and ElectronicComponents and Incoming Inspection

For quality assurance and incoming inspection,measurements on the two frequency points, 1 kHzand 1 MHz are essential to meet JIS and MIL standards.Since no previous LCR meter could cover bothstandard frequency points, two meters had to beused. The HP 4284A covers these frequency pointswith its wide measurement frequency range of 20 Hzto 1 MHz. Consequently, it covers all the principalstandard frequencies required for quality assuranceand incoming inspection, thus reducing the purchasingcosts of measurement equipment and reducingmaintenance costs. Furthermore, when measuringimpedance frequency characteristics, no discontinuityof data is caused by instrument error. A powerful LOADcompensation function has been added in addition tothe conventional OPEN/SHORT compensationfunction to perform total error compensation.

Thus even if the measurement values of the vendordiffers from those of the customer due to instrumenterror, if can be canceled out by using the LOADcompensation function and a working standard.

Evaluating the High Frequency Characteristicsof Switching Power Supplies

The power supply is an extremely important part ofany electronic instrument. The ever increasingsophistication of electronic components and theirminiaturization has lead to a demand for powersources that are equally compact and low cost. The

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switching power supply, the immediate answer tothis demand, has become quite common. The recentadvances in materials and circuit technology hasled to a trend to raise the switching frequency.However, raising the operating frequency also causesa rise in the loss characteristics of the componentsused, and the parasitics in the components usedcannot be ignored. This makes it important to evaluatethe impedance of the components used at the actualoperating frequency. The HP 4284A, by virtue of itscapacitance and dissipation factor accuracy of 0.05%and 0.0005, respectively, up to 1 MHz, makes a vitalcontribution to high reliability design.

Evaluating Test signal / DC-Bias Dependency ofFerroelectric Materials

The working life of a switching power supply is oftenshortened due to the aluminum electrolytic capacitorselectrolytic drying up. The increase in switchingfrequency has led to employ of small value capacitors,and to the switching over to other types of highreliability capacitors. High dielectric constantceramic capacitors are regarded as particularlypromising. The capacitance of these capacitors variesgreatly with the AC/DC voltage applied to them, andwith temperature changes. This effect increases asthe dielectric constant of the capacitor's dielectricmaterial increases and with increasing values ofcapacitance. For this reason due care has to be paidin the design stage. When these capacitors are usedas filter capacitors in the input and output of aswitching power supply, the effective capacitance maybe lower than the design value causing an increasein output noise. When the load becomes inductive incharacter, the transient response deteriorates. Thuscapacitors must be evaluated under actual operatingconditions. However, conventional LCR meters hadthe following limitations when used for this type ofmeasurement.

An External Circuit (Power Amplifier) is Required

The measurement accuracy cannot be foretold.

The current of the test signal cannot be increasedand high values of capacitance cannot be measuredat high frequencies.

The DC Bias Sources were also Limited.

Only spot bias built-in

Only an external bias terminal was supplied andan external computer, and an HP-IB controlledpower supply were required.

An HP 4284A equipped with Option 001 can makemeasurements at up to 1 MHz and at signal levels ofup to 20 Vrms / 200 mArms, and can apply a DC biasof up to ±40 V. Since no external circuits are needed,and calibrations can be performed with the optionsinstalled, measurement efficiency and accuracy arenot sacrificed. The full 6-digit display resolution makesit possible to monitor even minimal temperature andhumidity changes. All HP 4284A functions can becontrolled by HP-IB, the DC bias can be set with ahigh accuracy of 0.1% which makes the HP 4284Aideal for multi-frequency C-V measurements.

The Physical Properties of Liquid CrystalMaterials

Due to its compact size and low power consumption,Liquid Crystal Displays are used in pocket TVs,wordprocessors, computers, etc., and new applicationsare appearing all the time. LCDs are now used in awide variety of applications and manufacturers mustblend different types of liquid crystal materials toobtain the required characteristics, and these mixturesmust be critically tested to determine whether or notthey meet the required characteristics. The voltagelevel of the test signal of conventional LCR meterswas not adequate as a measurement voltage severaltimes greater than the threshold voltage (1.2 V - 3 Vfor TN LCDs) could not be applied. This made itnecessary to use the complicated curve fittingmethods to derive an experimental value, or to usean external circuit to amplify the test signal.

An HP 4284A equipped with Option 001 can makemeasurements over a frequency range of 20 Hz to 1MHz at a test signal voltage of 5 mV to 20 Vrms. Thismakes it possible to simplify measurement methodsby using approximation calculations based on thelinear relationship between the test signal voltageand the high voltage capacitance characteristics. Anexternal circuit is not required, and since calibrationcan be performed with the options installed,measurements can be made without sacrificingefficiency and accuracy.

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Constant-Current Evaluations of MagneticMaterials, Coils and Magnetic Heads.

In order to raise the Q of magnetic core coils andmagnetic heads, magnetic materials with a highrelative permeability are used. The magnetic,temperature, and humidity properties of a core aregreatly influenced by the use of core materials whichhave high relative permeability. Consequently, theimpedance value varies with the strength of the testsignal and with the strength of the DC bias current.This is the reason why a constant magnetic field isused to evaluate magnetic core coils, and magneticheads. Since the strength of the magnetic field isproportional to the current and to the number of turnsin the coil, it is possible to make a constant currentmeasurement as long as the number of turns in thecoil is held constant. However, conventional LCRshave the following limitations.

Only a spot voltage can be selected as theconstant-voltage signal source.

The measurement voltage is not HP-IBcontrollable.

The measurement current cannot be monitored.

For the above reasons, constant-current measurementsare either impossible, or a computer must be used forcontrol, this requires longer setup time and is not apractical solution. The HP 4284A has a built-in AutomaticLevel Control function (ALC) to facilitate constant-current measurements. The built-in microprocessormonitors the measurement current and uses thisfeedback information to control the test signalvoltage. This means that efficiency is not sacrificedby having to transfer data to an external computer.

Higher Productivity in the Production ofElectronic Instruments and Components

Improving productivity is an on going concern formanufacturers. On the production line reducing themeasurement time required is extremely importantsince it raises productivity and reduces costs. Inresearch departments the increasing number ofmeasurements to be made, and the ever larger numberof materials to be measured make higher measurementspeed and efficiency an absolute necessity. Thedilemma with conventional LCR meters is that accuratemeasurement require longer measurement time andif the measurement time is reduced, accuracysuffers. The HP 4284A offers the best solution to this

dilemma in that it can make measurements up to1 MHz in 30 ms measurement time in the SHORTmode and with a basic accuracy of 0.1%. This is avast improvement in efficiency and accuracy. Theunique list sweep function allows the programmingof a maximum of ten measurement points and havingthe result displayed in tabular form without the useof an external computer.

The HP 4284A can perform automatic screeninginspections on the production line fast and accuratelyby installing Option 201 or202 handler interface.Maximum measurement speed can be maintained atall times even when samples that cause internal shortcircuits are connected.

The following sections will give detailed measurementexamples using the HP 4284A.

Evaluating the Level and Bias Dependencyof High Dielectric Constant CeramicCapacitors

Aluminum electrolytic capacitors were used inconventional power switching supplies as filtercapacitors. However, the recent trend is to developpower switching supplies of ever higher frequenciesto make them more compact, lighter and cheaper.This trend has led to a demand for filter capacitorsthat are equally compact, and developers are beginningto look for other types of capacitors. In some powerswitching supplies high capacitance multi-layeredceramic capacitors are being used for their low ESRcharacteristics at high frequencies and their longlife at high temperatures.

As shown in Figure 1 and Figure 2, when a DC biasvoltage is applied to the high capacitance multi-layered ceramic capacitors which use high dielectricconstant dielectrics, the capacitance varies withpolarity. Since the ripple of the switching power supplyincreases with variations in filter capacitor'scapacitance, the material must be tested undermeasurement conditions that are identical to theactual operating conditions. By installing Option 001in the HP4284A, test signal levels of 5 mV - 20 Vrms,50 µA - 200 mArms and a DC bias of ±40 V can beused for measurement. Since no external circuitsneed to be employed and all required options can becalibrated when installed in the HP 4284A, allmeasurements can be performed without sacrificingaccuracy and efficiency.

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Thus the HP 4284A will quickly perform all evaluationsrequired in component design to improve thereliability and functions of switching power supplies,and help to make their development more efficient.When used with a scanner interface, the HP 4284Acan also be used for environmental chamber testing.

Since the HP 4284A can store compensation data fora maximum of 128 channels, it is easy to build areliable temperature and humidity test system.

Figure 1. DC Bias Dependence of Capacitance

Figure 2. Loss Coefficient of DC Bias Dependence

Figure 3. Signal Voltage Dependence of Capacitance

Figure 4. Signal Voltage Dependence of Loss Coeffi-cient

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Automation of Multi-FrequencyEvaluations of Film Capacitors

Film capacitors, which are used in motors and powersupplies to reduce noise, have to be highly reliable.The evaluation of the dissipation factor (D) isparticularly valuable in preventing heat and fire inthe capacitor due to ripple current. The followingwill describe making high speed and highly accuratemeasurements with the HP 4284A on the productionline and in quality assurance departments.

Screening Tests of Film Capacitors

The capacitance of film capacitors are normallytested at 1 kHz, the JIS and MIL standard frequency.As shown in Figure 5 the D value increases thefrequency increases and it is therefore usual toperform tests at 10 kHz and 100 kHz. The HP 4284Adoes not only make high resolution C-Dmeasurements at 1 kHz, but can make programmedmeasurements using the list sweep function at 1 kHz,10 kHz, and 100kHz and perform thesemeasurements in order automatically.

Figure 5. Frequency Characteristics of Loss coefficientof Film Capacitors (by the HP 4194A)

The HP 4284A can cover all the required measurementfrequencies. Six full digits of resolution for allmeasurement parameters, and a maximum dissipationfactor resolution of 0.000001 makes the HP 4284Aideal for testing low-loss capacitors. The capacitanceand dissipation factor accuracy are 0.05% and 0.0005at 1 MHz, respectively, which will increase yieldfigures, and improve quality on the production line.The HP 4284A increases efficiency. It has an

excellent built-in comparator which allows 10 binsorting of C, and IN/OUT tests of D or ESR. Thehandler interface Option facilitates connection toautomatic equipment. An extension cable of 4 m canbe connected and the accuracy of measurementsperformed at the end of the cable are guaranteed.

Recently automatic equipment with built-in scannershave been constructed with the aim of raisingthroughput. When the scanner interface is installedin the HP 4284A the combination can perform multi-channel compensation for a maximum of 128 channels.Since OPEN/SHORT/LOAD compensation can beperformed for each channel, errors can be reducedto a minimum to guarantee accurate measurements.The unique list sweep function of the HP 4284A allowsyou to program up to fen sweep points and to displayin tabular form without using an external computer.Thus it is possible to perform measurements at 1 kHz,10 kHz and 100 kHz in order, or to test the C limitvalue at 1 kHz and the D limit values at 10 kHz and100 kHz. When the handler interface is installed, thetest result at each point can be output.

Note:The list sweep is performed at high speed so it isnecessary to set an appropriate measurement delaytime to allow for the sample response time.

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The HP4284A's memory card function can store entiremeasurement setups, including comparator limitinformation, and list sweep table. This reduces thetime required for production line setup and eliminatesinstrument setup errors. A turn-key system can bebuild so that a special setup is automatically loadedfrom the memory card when the power is turned ON.

1 Option 006 2m/4m Cable Length Operationfunction must be used.

The capacitance and dissipation factor accuracy forthe HP 4284A are 0.05% and 0.0005 at 1 MHz,respectively, which makes it ideal for quality controlof low loss capacitors in quality assurancedepartments. A powerful LOAD compensationfunction has been added, in addition to the OPEN/SHORT compensation function. It uses a workingstandard as a reference and makes sure that themeasured value is compensated to yield an accuratemeasurement result under any measurementconditions.

List Sweep Display Menu Correction Menu

Comparator BIN Count Menu

The comparator built into the HP 4284A canautomatically perform bin counts up to 999999 is aconvenient feature when making sampling inspections.It can be programmed to emit a beep to announcewhen the limit test result is OUT.

The menus, the measurement conditions which givethe measurement results, and the measurementresults can be printed out by an HP-IB connectedprinter to facilitate the recording of necessary data.The HP 4284A's display cursor, arrow keys, andsoftkeys make it easy to check the measurementcondition settings, and measurement results. Duringsorting tests the operator can freely select the'Normal measurement', 'BIN NO.' and 'BIN count'display page to suit the application. The memorycard can store up to ten instrument setups which theoperator can choose by selecting a number on thecatalog menu. This eliminates set up errors andsimplifies operation.

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All these features make the HP 4284A precision LCRmeter suitable for testing film capacitors.

Improving the Cored Coil EvaluationMethod by Means of Constant-CurrentMeasurement

The permeability of the magnetic material used inmagnetic coils is not constant, but varies with thestrength of the magnetic field, temperature, and otherfactors. When the current fed through a coil woundaround a magnetic material is gradually increased,the magnetic flux density B changes as shown inFigure 6, Except at the origin, it is proportional to thestrength of the magnetic field at the start, after whichit steeply increases and then falls off. The saturationpoint is reached when all of the magnetic axis of themagnetic domains within the core material arealigned.

Catalog Menu Figure 6. Magnetic Characteristics

Consequently, also the inductance of the coil varieswith the strength of the magnetic field. Thus todetermine the value of a magnetic cored coil, it hasto be measured at a constant magnetic field level.That is, if the number of turns in the coil are heldconstant, a constant current can be used for themeasurement.

H = nI [AT/m]n: number of turns I: current

Even though constant voltage signal sources wereused in conventional LCR meters to set the signalsource level to the same voltage and DC biasconditions, the measurement values varied frommeter to meter. This was due to the fact that theoutput impedance of the signal source was differentfor different meters, and that the current flowingthrough the sample was also different. Table 1shows an example.

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Current monitor value ( ) denotes output impedanceof signal source

Table 1. Differences in Measurement Values forDifferent Meters

For this reason constant-current measurements withconventional LCR meters were achieved by using anexternal computer to monitor the signal sourcecurrent, and to control the signal source voltage.Since data transfers had to be made via an HP-IBbus, efficiency suffered and the need for an externalcomputer raised system costs. Using the HP4284A'sbuilt-in ALC function2 simplifies constant-currentmeasurements. The built-in microprocessor controlsthe signal current and high efficiency is guaranteed.

In addition, the HP 4284A has a comparator functionwhich is convenient for performing GO/NO-GOproduction line testing. The handler interface makesit possible to connect the LCR meter to automaticequipment which is useful for performing automaticscreening inspections of magnetic cored coils.

2 AUTOMATIC LEVEL CONTROL Function

Evaluation of the SaturationCharacteristics and Self-ResonantFrequencies of Magnetic Heads

Magnetic heads are used in audio, video, and inmagnetic disk devices of digital equipment. Highquality heads are required for analog applications,and high density heads are demanded in the digitalfield. A wide frequency range, core materials withhigh permeability, and high saturation magnetic fluxdensity are required to produce high quality, highdensity magnetic heads. Raising the self-resonantfrequency of the coil is one way to widen itsfrequency range. This in turn makes it necessary toevaluate the self-resonant frequency and othermagnetic head characteristics during thedevelopment stage.

The following is a description of how the HP 4284Acan be used in R&D to evaluate magnetic heads.

The evaluation of heads for magnetic disks involvethe following difficulties and demands.

Since the characteristics vary with the strength ofthe magnetic field, the evaluation must be performedat constant magnetic field levels, That is, the headimpedance should be measured using a constantcurrent test signal.

Impedance frequency characteristics are requiredwhen performing L-Q measurements at severalfrequencies to find the resonance point.

A wider range of level characteristics at larger/smaller test signal currents, and higher frequencycharacteristics are now required.

Accurate Measurements are not Deterioratedby Test Fixture Influences

When conventional LCR meters were used for thistype of measurement, a computer had to be connectedto monitor the measurement current, and themeasurement voltage level had to be adjusted foreach measurement frequency. The measurementfrequency and the test signal level had limitationsthat prevented the measurement under the desiredconditions which made a thorough evaluationimpossible.

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Evaluation of Multi-Frequency Characteristicsusing Constant Current

Even when the HP 4284A's measurement frequencyis changed, it is still possible to make constant-currentmeasurements across a wide current range of 100 µAto 100 mA3. The built-in ALC function has simplifiedconstant-current measurements since it eliminatesthe need to adjust the test signal level every time themeasurement conditions and the test devices arechanged, as was the case with conventional LCRmeters. Furthermore, the measurement frequencyrange of 20 Hz to 1 MHz, and with 8610 settablemeasurement frequency points, the evaluation offrequency characteristics using the constant currentmeasurement technique is easy. This means that theHP 4284A is capable of performing fastmeasurements of resonant frequency which isessential for evaluating magnetic headcharacteristics. With the use of an externalcomputer, data can be output to a computer via anHP-IB interface for graphic representation to make iteasier to determine the head characteristics and topromote development efficiency.

Figure 7 shows the measurement results of thefrequency characteristics of a floppy disk head bymeans of the constant current measurementtechnique.

Since test signal current level can be varied up to200 mA3, and the List Sweep function makes itpossible to sweep a maximum of ten signal levels,the level characteristics of magnetic heads areeasily obtained. A measurement example fordetermining magnetic head level characteristics isshown in Figure 8.

Figure 7. Constant Current Measurements of the |Z|-fCharacteristics of magnetic Heads

Highly Accurate and Highly ReliableMeasurements

The HP 4284A is provided with OPEN/SHORT/ LOADcompensation functions, three powerful compensationfunctions which reduce the influence of test fixturesto a minimum. In addition, excellent repeatabilityguarantees accurate measurements even when thetest fixture is changed to accommodate differenttypes of magnetic heads. Thus the HP 4284A providesfast and easy constant-current measurements. Astest signal levels can be set freely and list sweepsare possible, the HP 4284A is a single instrumentsolution for performing all of the measurementsrequired to evaluate magnetic heads. This willimprove the efficiency in developing and evaluatingmagnetic heads in R&D departments and raisereliability.

3 When Option #001 is installed.

Figure 8. Measurement Example of Magnetic HeadLevel Characteristics

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Conclusion

High accuracy impedance measurements are not theonly requirement for the correct evaluation ofelectronic components. Precise measurementcondition setups are also important.

Thus,

Measurement Frequency

Test Signal Level: Voltage/Current

DC Bias: Voltage/Current

Will make it possible to select components, toimprove reliability, and to raise the quality ofequipment design, only when these threemeasurement conditions are adjusted to meet theobjective of the evaluation, and only when they arecorrectly set up. The increased pace of precisemeasurements on the production line will raise thereliability of inspection. The HP 4284A PrecisionLCR Meter will raise the quality in development, inproduction, and in Quality Assurance for bothcomponent manufacturers and electronic equipmentmanufacturers.

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