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Microprocessor control broadens the capabilities of thisspeedy LCR meter and makes it readily adaptable to BCDor HP-IB automatic systems.

by Masahiro Yokokawa and Keiki Kanafuji

quency, and connect the component to the test termi-nals. The instrument automatically switches to thecorrect measurement range, selects the preferred cir-cuit mode, and presents results on the 31/2 digit dis-plays in about 250 ms without any time-consumingbalancing adjustments. The user, however, can havefull manual cont:rol of the instrument at any timesimply by pressing the appropriate front-panelpushbuttons.

The front-panel[ control arrangement is similar tomanually controlJled instruments (see Fig. 1) speed-ing familiarizatio][l with the instrument. As a furthersafeguard agains:t the possibility of unintentionalmissettings, the RANGE and CIRCUIT MODE functionsautomatically revlert to AUTO whenever the L, C, or Rfunction keys are pressed after manual modes havebeen in use. Since most measurements made with thiskind of instrument are made on capacitors, when it isfirst turned on the new LCR meter automatically setsitself for the CD mode using a I-kHz test frequency.

F OR AN ELECTRONIC CIRCUIT to meet perfor-mance goals, the values of the components used

in assembling the circuit must fall within certainranges, some wide, some narrow. Despite increasingsophistication in the design and manufacture of pas-sive circuit components, differences between sup-posedly identical components do exist and the rangesof values encountered often exceed acceptable limits.

Thus, most electronic laboratories, quality-controllabs, and receiving departments are equipped to mea-sure the actual values of the components with whichthey are concerned, a process that is not only timeconsuming, but one that often introduces errors of itsown. To speed these measurements, and reduce thehuman errors that arise in this activity, late-modelLCR meters, such as the Model 4261A described in arecent issue of the HP Journal,1 have been designedwith a high degree of automation.

A new LCR meter, Model 4262A, uses a micro-processor to further automate procedures while in-creasing the instrument's capabilities without incur-ring significantly greater costs. To measure the valueof a passive component with this instrument (Fig. 1),~it is only necessary to select the measurement func-tion and loss parameter-i.e., resistance (R), capaoi-tance (C), or inductance (L), and dissipation factor (D)or quality factor (Q)-select the appropriate test fre-

Enhanced CapabilityBesides autoranging, self-triggering, and automatic

selection of the appropriate circuit mode (series orparallel), the mil::roprocessor brings several othencapabilities to the instrument at little cost. Deviationmeasurement is one. When the I1LCR key is pressed,

Fig. 1. Model 4262A LCR Meter,shown here with the comparator

option installed, measures the in-ductance (L), capacitance (C), re-sistance (R), dissipation factor (D),and quality factor (Q) of compo-nents with 3V2 digit resolution.Three test frequencies (120 Hz,1 kHz, and 10 kHz) enable mea-

surements of a wide range ofcomponent values.

18

Fig. 2. Capacitance and inductance ranges over which theequivalent series resistance (ESR) can be measured with theModel 4262A LCR Meter. The 10-kHz test frequency extendsthese ranges by 6! factor of 10 over those usually found inmeters of this type.

addition to the customary 120-Hz and l-kHz test fre-quencies. The 10-lcHz test frequency extends thelow-end C and L measurement ranges respectively to10 pF and 10 JLH fulll scale, giving the new instrumentthe ability to measure components over exceptionallywide ranges: inductance from 0.01 JLH (the limit ofresolution) to 1999 H, capacitance from 0.01 pF to19.99 mF, resistance from 1 mD. to 19.99 MD., dissipa-tion factor from 0.001 to 19.9, and quality factor from0.05 to 1000. With these wide ranges, the instrumentmay be used to measure RF coils, dielectric materials,electrolytes, the internal resistance of batteries, andthe high dissipation factor of delay lines, as well as

the measurement value currently on display is stored,the display resets to zero, and the present measure-ment range is held. The result of the next measure-ment is then displayed as the difference between thenew measurement value and the stored value. Besideschecking device deviations during incoming inspec-tion, this mode is also useful for monitoring thechanges in device performance caused by variationsin temperature or bias voltage.

For go/no-go measurements, a comparator optionprovides two pairs of thumbwheel switches on thefront panel, one pair for LCR and one for DQ. Once highand low limits are established with these switches, agreen light turns on when measured values are withinthe selected limits and a red light turns on when theyare outside the limits. Electrical indications are alsoprovided at a rear-panel connector.

Self test is another capability obtained at low costwith the microprocessor. At turn on, all LED indi-cators and all segments of the display digits light upmomentarily to verify that all are functioning. Then,when the SELF TEST button is pressed while the inputterminals are open (if the C measurement function isselected), or shorted (with the L or R measurementfunction), the instrument tests its digital section andthe process amplifier and phase detector/integrator inthe analog section, through five ranges. If all goeswell, the word PASS appears in the LCR window andthe user can be assured that the instrument is func-

tioning correctly (this, however, is not a check on theinstrument's accuracy). If there is a problem, the wordFAIL is displayed, the range is held, and a codenumber indicating the location of the problem ap-pears in the DQ window.

Other capabilities that the microprocessor gives thenew LCR meter include low-cost HP interface bus(HP-IB)* and BCD-output options. In earlier LCR me-ters, an HP-IB interface very often cost more than theinstrument itself but because of the microprocessor,the cost of the HP-IB interface for Model 4262A is lessthan one-fifth that of earlier interfaces. The new LCRmeter may thus be interfaced readily to a printer for

logging measurement results, or to a desktop com-puter and/or other instruments for programmedmeasurements and statistical analyses of measure-ment results.

When the new instrument is equipped with theHP-IB option, all functions except DC BIAS are pro-

grammable through the HP-IB. The current status ofthe instrument (FUNCTION, CIRCUIT MODE, AUTORANGE,etc.) is made visible by illumination of the LED indi-cators in the corresponding front-panel keys.

Wide-Ranging MeasurementsThe new instrument has a la-kHz test frequency in

*Hewlett-Packard's implementation of IEEE Standard 488-t975.

19

the values of discrete components.In particular, the new LCR meter can measure the

ESR (equivalent series resistance) of a capacitor to verylow values, a significant measurement if the capacitoris to be used for bypass applications. With the 10-kHztest frequency, the reactance of a wide range ofcapacitors becomes low enough for the ESR to be asignificant, and measurable, part of the total impe-dance, as plotted in Fig. 2. At 10 kHz, the resolution ofan ESR measurement is 1 mn. for capacitors largerthan 10 JLF. A five-terminal input (two for voltage,two for current, and one for a guard) is provided tominimize errors. Internally-generated bias levels of1.5V, 2.2V and 6V are also provided, primarily formeasurements on electrolytic capacitors.

Small values of capacitance, such as the junctioncapacitance of semiconductor devices, can be mea-sured with the low-level (50 mY) test signal level thatis available. Normally, the test signal level is 1 V rms.

Typical measurement accuracy is 0.2%. On thelow-end ranges, the test-fixture parasitic reactancescan affect the accuracy of the measurement, so front-panel C and L offset controls are provided to null outparasitics up to 10 pF and 1 JLH. The compensationtechnique is explained in Fig. 3.

Internal OperationsModel 4262A Jl.CR Meter finds the values of 1, c, R,

D, and Q by determining the vector ratio of the voltageacross the device under test (DOT) to the currentflowing through the device, in the same manner as theModel 4261A LC:R Meter.1

A block diagram is shown in Fig. 4. The voltageacross the device, is represented by e1 in the diagramand the current by voltage e2, which is proportional tothe current flo~Ting through range resistor RR inse;ries with the unknown. Op amp A3 assures accu-rate current flow by maintaining the LOW input ter-minal at virtual ground.

The four-phase generator supplies a signal shiftedin precise increments of 90° with respect to its inputfor use as a reference by the phase detector. The integ-rator and comparator are part of a dual-slope A-to-Dconverter of the type widely used in digital volt-meters.

As an example of how all this fits together, assumethat a capacitor is being measured in the parallel-circuit mode. Voltage e1 is applied to the phase detec-tor and voltage e2, shifted 90°, serves as the phasedetector reference. The output of the phase detectoris applied to the integrator, which starts from the zero

Fig. 3. Compensating Cirt its neutralize the effect of the test fixture's stray capacitance (CoYand residual inductance (L .Amplifier A1 inverts the test signal; the CZEROADJUSTcontrol allows

adjustment of the currentth gh C1 so it equals and thus cancels the current through Co' Trans-former T1 supplies a curren~ to amplifier A6 equal to that flowing through the unknown, and the

combination of C2, A7, and:R4 shifts the signal goo. R4 is adjusted so C2R3R4 = Lo, cancelling

thf:i effect of Lo at the input to amplifier A5.

20

Fig. 4. Simplified block diagram of the Model 4262A LCR Meter. Under control of the micro-

processor, the circuits find the ratios of the real and imaginary parts of the voltage across the OUTwith respect to the current (or vice versa), from which the device parameters can be derived.

computation.Similarly, by suitable choice of e1 or e2 as the phase

reference, and proper choice of the phase shift, timeperiods proportional to the other measurement quan-tities can be obtained.

Digital DesignThe microprocessor reads the keyboard, operates

the displays, the comparator and the HP-IB option,performs the self test, and controls the many opera-tions of the measurement cycle, such as autorangingand the A-to-D conversion. The various measurementroutines, the comparator algorithm, the self-test prog-ram, and other instrument programs are stored in a4K-byte ROM. The microprocessor is the samecontrol-oriented type as that used in the HP Models3455A and 3437 A Digital Voltmeters.2 It uses parallelarchitecture to achieve high speed but does not have agreat deal of computation capability. What little

level and charges for a fixed period of time. Voltagee2 is then applied to the phase detector input and it isalso used as the phase reference but shifted 1800 togive a negative output, which is used to dischargethe integrator.

As shown in the flow chart of Fig. 5, the micro-processor determines the time of discharge by ac-cumulating clock pulses during the discharge in-terval, the end of discharge being indicated by thecomparator when it senses the return to the zero level.The discharge time is proportional to the quadraturepart of the vector ratio e2/el' which is proportional tothe capacitance.

The counts accumulated during the discharge timeare stored in a register within the microprocessor.The stored count can be used directly as the displayedquantity in a capacitance measurement because of thechoice of the range resistor RR and the clock fre-quency (31 kHz), which eliminates the need for any

21

22

!~~ofFig. 6. Comparator option requires little hardware becaus

the capabilities provided by the microprocessor.

itlyin

arithmetic there is to be done is accomplished molby incrementing or decrementing counts storedregisters.

The manner in which this microprocessor SYS1enables operating features to be added at little co~illustrated by the comparator option, diagramme<Fig. 6. Each digit of each thumbwheel connects to Iof the ten lines of the comparator data bus by meana diode. The diode cathodes for each switch are tielcommon to the switch contact. When the front-paCOMPARATOR ENABLE key is pressed, the microprolsor initiates a stored program that sequentilgrounds the common contacts of the thumbwheand for each one senses which line of the data bugrounded. The comparator settings can thus be stcfor use during subsequent measurement comlJsons. This technique minimizes the amounhardware needed to implement the compar;function.

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AcknowledgmentsSpecial thanks are due to Hiroshi Sakayori,

evaluated the 10-kHz C standard, Shigeo KamiYImuch help, and Dave Okuyama, who developedmicrocomputer cross-assembler to be run on an2100 Computer. Yoshimasa Shibata contribute,the mechanical design, and industrial designdone by Kazunori Shibata. Thanks are due R argroup manager Shiro Kito and marketing manShigeki Mori who provided significant suggestand encouragement throughout the project.

I

h~fo~I

theIHP1

towas

dDigerlons

Fig. 5. Flow chart of measurement routine.

23

Fig. 7. Model 4262A LCR Meter,shown here without options, has afront panel arrangement thatfacili-tates manual control when manual

control is preferred to automatic

operation. With the HP-IB optioninstalled.. all PUShbutt~ functionsare programmable through the HPinterface bus (all options are field

installable).

that Calibrates Itself Automatically;" J.E. McDermid, J.B.Vyduna, and J.M. Gorin, "A High-Speed System Voltmeterfor Time-Related Measurements," Hewlett-Packard Jour-nal, February 1977.

References1. S. Hashimoto and T. Tamamura, "An Automatic Wide-Range Digital LCR Meter," Hewlett-Packard Journal, Sep-tember 1976.2. A. Gookin," 4 Fast-Reading, High-Resolution Voltm~ter

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