HP-AN369-12_Measurement of Impedance of Magnetic Heads

8/14/2019 HP-AN369-12_Measurement of Impedance of Magnetic Heads

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Preface

The market for magnetic heads isexpected to experience furtherexpansion due to a wide variety of industrial applications as well asconsumer electronic applications.In consumer electronics, the VTR

(video tape recorder) marketforms a large market for magneticheads. The digital VCR (videocassette recorder) and VTRmarkets are expected to expand,

benefiting the magnetic headmarket. In the industrial field,along with the market expansionof computer-related devices, themarket for both hard and floppydisks has continued to grow. In

addition, there is a high demandfor tape drives used for computer

back up.

In these areas, magnetic headswill be required to operate athigher frequencies to providehigher quality. The movement

towards higher picture quality,demonstrated in S-VHS's and evenhigher picture quality in highergrade VTR's, demands magneticheads to record and read signalsat higher frequencies. Hard andfloppy disk drives process amassive amount of data, thusrequiring high speed and highdensity recording and reading of the data.

In order to accurately check thequality of magnetic heads tosatisfy these requirements, it isnecessary to conductmeasurement under theconditions close to actualoperation in terms of operatingfrequency and applied current. Forthis purpose, it is important toconduct measurement at constantcurrent and to accurately measurethe impedance value even at highfrequencies. An LCR meter used

for measurement of magneticheads is required to measure atconstant current and at a widerange of frequencies. As well as

being highly accurate, it must beeasy and cost effective to use.Development of a measuringdevice satisfying all theserequirements at frequencies over1 MHz had been long awaited.

(Table 1 shows "Applications of Magnetic Heads, Typical

Operating Frequency andMeasurement Range for each Application")

This application note will presentsolutions which make use of theHP 4285A, when such magneticheads operating at relatively highfrequencies are inspected on the

production line, and assessed fortheir device characteristics.

Measurement of Impedance of Magnetic Headsin Conducting an Inspection on the Production Line

and Assessing Device Characteristics

Application Note 369-12The HP 4285APrecision LCR Meter


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How to Use an LCR Meter forMeasuring Magnetic Heads

When magnetic heads areassessed using an LCR meter, thefollowing items are normallymeasured. The objectives formeasurement and assessment arealso listed in the following.

The Items to be Measured andConditions for ConductingMeasurement of MagneticHeads on the Production Line

A. The Items Measured

1) L (Inductance)Compared with the prescribed

value obtained from data onmagnetic heads of good quality,check for any abnormality.

Measurement objectivesa) Checking for defects or flaws of the magnetic head materials

b) Checking for the number of coilturnsc) Checking for abnormalconfiguration including the widthof head gap

2) Q (Purity of reactance)

Measurement objective Assessment of losses such aseddy-current loss

3) | Z | - (Impedance)Compared with the prescribed

value obtained from data onmagnetic heads of good quality,check for any abnormality in thematerials and/or configuration of

magnetic heads.

Measurement objectiveChecking for the resonantfrequency

B. Measurement Conditionsand Method

Conducting measurement atconstant current (The constantcurrent should be applied inconducting measurement becausethe magnetic head characteristics

change depending on the currentlevel.)Covering the range of frequencyrequired for measuring magneticheads and providing high accuracyGo/No-Go measurement(Measurement to see if a magnetichead operates or not.)Interface available for measuringscanners which are capable of multidevice measurement

The Items Measured andConditions for Assessing theMagnetic Head Characteristics

In addition to the items andconditions for measurement onthe production line mentionedabove, assessment of devicecharacteristics requires thefollowing:

Sweep measurement at multiple

frequenciesSelection of multiplemeasurement parametersFlexible setting of measurementconditions such as frequencies to

be measured at and the input levelof voltage, current and electric

power

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Application ofMagnetic Heads

Typical OperatingFrequency

Measurement Range foreach Application

Measurement atConstant Current

Audio Head 12.5 KHz 1 KHz 100 uAp-p

VHS Head ~5 MHz 5 MHz 100 uAp-p

High8 VCR head ~7.7 MHz 5 MHz 1 mAp-p

High Definition VTR Head 20M~76 MHz Various Various

Professional Digital VCR Head 30~40 MHz Various Various

6mm Digital VCR Head ~22 MHz 10 MHz 40 mAp-p

Thin Film Head ~40 MHz 1 MHz 30~40 mAp-p

MR Head ~100 MHz 1 MHz 10~14 mAp-p(MR)30~40 mAp-p(Ind)

Floppy Disk Head ~100 KHz 1 KHz 4~7 mAp-p

DDS-DAT Head ~5 MHz 5 MHz 24 mAp-p

Table 1. Application of Magnetic Heads, Typical Operating Frequency and Measurement Range


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Problems of ConventionalDevices used for MeasuringMagnetic Heads, and Solutionsbrought by the HP 4285A and

its Features

The HP 4285A's special featureswill provide a powerful solution toeach problem conventionalmagnetic head measuring devicesface.

1) Measurement at ConstantCurrentFor accurate assessment of magnetic heads, impedance must

be measured at constant current because the inductance value of amagnetic head changes dependingon the current level. Mostconventional measuring devicesdo not allow measurement atconstant current when thefrequency is over 1 MHz. Evenwhen this is possible, you mightfind them unsatisfactory, becauseyou cannot select the value of constant current.

The HP 4285A is equipped with Automatic Level Control (ALC).This ALC function allows you toset the current level

(100 A-20 mA) as you wish. Onceyou set the level, the HP 4285Aautomatically controls the current,thus making it easy to performmeasurement of magnetic headsat constant current.

2) A Wide Measuring Range of Frequency and High AccuracyWith conventional LCR meters, itis technically difficult to measurewith high accuracy at highfrequencies up to tens of MHz.

The HP 4285A performsmeasurement at a wide range of frequency ranging from 75 kHz to30 MHz, and ensures highaccuracy within 0.1%. It isequipped with a powerful errorcorrection function called"Open/Short/Load" to eliminate theerror residual of the testingdevice.

3) Easy-to-use Functions on theProduction Line, includingGo/No-Go MeasurementIn order to save time and labor for

switching devices beforemanufacturing a new lot on the

production line, it is essential thata device allow as simple set-upsas possible, including a set-up of measurement conditions. Thedevice is also required torecognize if a magnetic head isoperating as desired or not.

The HP 4285A is capable of storing ten sets of set-upinformation such as measurement

conditions in its internal memory,and ten more sets on a memorycard. This feature permits easyset-up for measurement on the

production line. In addition, theHP 4285A is equipped with a

variety of functions useful for production such as a simple,flexible set-up of Go/No-Gomeasurement and BIN's. Aninterface is available toaccommodate scanners andhandlers.

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Graph 1. Inductance Characteristics of the Magnetic HeadFrequency (HZ) (Measured using the HP 4194A)

Graph 2. Impedance Characteristics of the Magnetic HeadFrequency (HZ) (Measured using the HP 4194A)


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4) Sweep Measurement atMultiple Frequencies

A measuring device used forassessment of magnetic head

characteristics needs to have afunction which specifically selectsmultiple frequencies for sweepmeasurement.

The HP 4285A allows sweepmeasurement by entering up toten frequencies of your choice. Italso presets the limit to themeasured value for everyfrequency selected, thus allowingeasy Go/No-Go measurement.

5) Setting up FlexibleMeasurement Conditions Assessment of magnetic headcharacteristics requires setting upflexible measurement conditions.The HP 4285A has twentycombinations of presetmeasurement parameters. Thus,you only need to choose one. Youcan enjoy flexibility of setting upmeasurement conditions such asthe frequency to be measured, theinput voltage/current level and the

direct current bias.

An Example

The measurement functions of theHP 4285A will be presented using

measurement of hard diskmagnetic heads as an example.

1) The Hard Disk Magnetic HeadsMeasured and theirCharacteristicsa) Device measured: hard diskmagnetic heads

b) Device characteristics

The impedance characteristics of the hard disk magnetic headsmeasured (Ls - Q, | Z | - ) areshown in graphs 1 and 2.(Measured using the HP 4194A.)

In measuring magnetic heads onthe production line, theircharacteristics are not measuredat all levels of frequency. Instead,the actual method used isdesigned to achieve highmeasurement efficiency. Asmentioned above, the methoddetermines Go or No-Go bymeasuring the value of parametersat specifically selectedfrequencies and setting up limit

values.

2) Set-up for MeasurementThe set-up for measurement isshown in diagram 3.

HP 4285A

HP 16047CTest Fixture

MagneticHead

Diagram 3. Set-up for Measurement

3) ResultsThe hard disk magnetic headsused in assessment of inductanceand impedance characteristicsdemonstrated in graphs 1 and 2were measured using theHP 4285A, equipped with specialfeatures such as a sweepmeasurement function at flexiblefrequencies, a Go/No-Gomeasurement function and a BINfunction. The results aredescribed in the following.

Chart 4 shows the results of sweep measurement in which theinductance characteristics Ls - Qare measured at ten selectedfrequencies. Chart 5 shows how toset up frequencies to be measuredat, and the high and low limit

values. On the display as shown inChart 5, the selected frequenciesto be measured at are set up in theFREQ [Hz] column, the high andlow limit values needed forGo/No-Go measurement areentered for each frequency level inthe HIGH and LOW columns, andthe selected measurement

parameter in the LMT column.

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asshown in the chart 4 displays themeasurement results at eachfrequency level and the results of

Go/No-Go measurement in theCMP column. Chart 6 displays theresults of binning in which thedevices are measured for theirimpedance value | Z | at a specificfrequency, and then classified intothe nine preset bins (BIN's1 through 9) or OUT OF BINS. Thedesired impedance value is set inthe NOM column, and the lowerand upper limit values from thedesired value are set for the BIN's1 through 9 in the LOW and HIGHcolumns, respectively.

The COUNT column shows thenumber of devices classified intothat bin. The lower and upperlimit values can be entered by theabsolute value as well as by

percentage, which allows flexiblesetups. The limit value of usedas the second parameter can also

be set in the row labeled "2nd."

Conclusion

A magnetic head measuringdevice must have a function which

maintains constant current. TheHP 4285A allows you to set up anyspecific value of constant currentwithin the range of 100 A-20 mA,and to automatically control it.Furthermore, the HP 4285A isequipped with a function tomeasure magnetic heads with highaccuracy even at high frequency(75 kHz -30 MHz) which cannot becovered by the conventional LCRmeters. In addition to this feature,it is also equipped with aGo/No-Go function, a memorycard, and several functions whichmake it easy to use the device onthe production line. Therefore, theHP 4285A is the most suitabledevice for performingmeasurements on magnetic headsoperating at relatively highfrequencies. This LCR meter alsoserves various other purposes,including measurement andassessment of magnetic storagecomponents such as rotarytransformers used in VTR's.

The HP 4285A offers powerfulsolutions to problems thatconventional LCR meters cannotovercome in measuring magneticheads. The HP 4285A is furnishedwith functions ranging from

production line inspection toassessment of devicecharacteristics.

If a device must be measured athigher frequencies due to itshigher operating speed, theHP 4219A RF impedance/material

analyzer or the HP 4286A RF LCRmeter may best suit your needs.

Chart 4. Sweep Measurement Results of Ls - Q, and Go/No-Go MeasurementResults

Chart 5. Setting up Ls - Q SweepMeasurement Frequencies andGo/No-Go Limit Values

Chart 6. Results of BINNING based onthe Impedance Value | Z |

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For more information on Hewlett-PackardTest and Measurement products,applications, or services please call yourlocal Hewlett-Packard sales office. Acurrent listing is available via the Web

through AccessHP at http://www.hp.com. If you do not have access to the internet, please contact one of the HP centers listed below and they will direct you to yournearest HP representative.

United States:Hewlett-Packard CompanyTest and Measurement Organization5301 Stevens Creek Blvd.Bldg. 51L-SCSanta Clara, CA 95052-80591 800 452 4844

Canada:Hewlett-Packard Canada Ltd.5150 Spectrum Way

Mississauga, OntarioL4W 5G1(905) 206 4725

Europe:Hewlett-PackardEuropean Marketing CentreP.O. Box 9991180 AZ AmstelveenThe Netherlands

Japan:Hewlett-Packard Japan Ltd.Measurement Assistance Center9-1, Takakura-Cho, Hachioji-Shi,Tokyo 192, JapanTel: (81 426) 56 7832

Fax: (81 426) 56 7840

Latin America:Hewlett-PackardLatin American Region Headquarters5200 Blue Lagoon Drive9th FloorMiami, Florida 33126U.S.A.(305) 267 4245/4220

Australia/New Zealand:Hewlett-Packard Australia Ltd.31-41 Joseph StreetBlackburn, Victoria 3130

Australia1 800 629 485

Asia Pacific:Hewlett-Packard Asia Pacific Ltd17-21/F Shell Tower, Times Square,1 Matheson Street, Causeway Bay,Hong KongFax: (852) 2506 9285

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Data subject to changeCopyright 1997Hewlett-Packard CompanyPrinted in U.S.A. 1/975965-6663E

HP-AN369-12_Measurement of Impedance of Magnetic Heads

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