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Microprocessor control broadens the capabilities of this
speedy LCR meter and makes it readily adaptable to BCD
or HP-IB automatic systems.
by Masahiro Yokokawa and Keiki Kanafuji
F OR AN ELECTRONIC CIRCUIT to meet perfor- quency, and connect the component to the test termi-
mance goals, the values of the components used nals. The instrument automatically switches to the
in assembling the circuit must fall within certain correct measurement range, selects the preferred cir-
ranges, some wide, some narrow. Despite increasing cuit mode, and presents results on the 31/2digit dis-
sophistication in the design and manufacture of pas- plays in about 250 ms without any time-consuming
sive circuit components, differences between sup- balancing adjustments. The user, however, can have
posedly identical components do exist and the ranges full manual cont:rol of the instrument at any time
of values encountered often exceed acceptable limits. simply by pressing the appropriate front-panel
Thus, most electronic laboratories, quality-control pushbuttons.
labs, and receiving departments are equipped to mea- The front-panel[ control arrangement is similar to
sure the actual values of the components with which manually controlJled instruments (see Fig. 1) speed-
they are concerned, a process that is not only time ing familiarizatio][l with the instrument. As a further
consuming, but one that often introduces errors of its safeguard agains:t the possibility of unintentional
own. To speed these measurements, and reduce the missettings, the RANGE and CIRCUIT MODEfunctions
human errors that arise in this activity, late-model automatically revlert to AUTOwhenever the L, C, or R
LCR meters, such as the Model 4261A described in a function keys are pressed after manual modes have
recent issue of the HP Journal,1 have been designed been in use. Since most measurements made with this
with a high degree of automation. kind of instrument are made on capacitors, when it is
A new LCR meter, Model 4262A, uses a micro- first turned on the new LCR meter automatically sets
processor to further automate procedures while in- itself for the CDmode using a I-kHz test frequency.
creasing the instrument's capabilities without incur-
ring significantly greater costs. To measure the value Enhanced Capability
of a passive component with this instrument (Fig. 1), Besides autoranging, self-triggering, and automatic
~it is only necessary to select the measurement func- selection of the appropriate circuit mode (series or
tion and loss parameter-i.e., resistance (R),capaoi- parallel), the mil::roprocessor brings several othen
tance (C),or inductance (L),and dissipation factor (D) capabilities to the instrument at little cost. Deviation
or quality factor (Q)-select the appropriate test fre- measurement 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 of
component values.
18
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 the
new measurement value and the stored value. Besides
checking device deviations during incoming inspec-
tion, this mode is also useful for monitoring the
changes in device performance caused by variations
in temperature or bias voltage.
For go/no-go measurements, a comparator option
provides two pairs of thumbwheel switches on the
front panel, one pair for LCRand one for DQ. Once high
and low limits are established with these switches, a
green light turns on when measured values are within
the selected limits and a red light turns on when they
are outside the limits. Electrical indications are also
provided at a rear-panel connector.
Self test is another capability obtained at low cost
with the microprocessor. At turn on, all LED indi-
cators and all segments of the display digits light up
momentarily to verify that all are functioning. Then,
when the SELFTESTbutton is pressed while the input
terminals are open (if the C measurement function is
selected), or shorted (with the L or R measurement
function), the instrument tests its digital section and
the process amplifier and phase detector/integrator in
the analog section, through five ranges. If all goes
well, the word PASS appears in the LCR window and
the user can be assured that the instrument is func-
tioning correctly (this, however, is not a check on the
instrument's accuracy). If there is a problem, the word
FAIL is displayed, the range is held, and a code
number indicating the location of the problem ap-
pears in the DQ window.
Other capabilities that the microprocessor gives the
new 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 the
instrument itself but because of the microprocessor, Fig. 2. Capacitance and inductance ranges over which the
the cost of the HP-IB interface for Model 4262A is less equivalent series resistance (ESR)can be measured with the
than one-fifth that of earlier interfaces. The new LCR Model 4262A LCR Meter. The 10-kHz test frequency extends
these ranges by 6! factor of 10 over those usually found in
meter may thus be interfaced readily to a printer for
meters of this type.
logging measurement results, or to a desktop com-
puter and/or other instruments for programmed
addition to the customary 120-Hz and l-kHz test fre-
measurements and statistical analyses of measure- quencies. The 10-lcHz test frequency extends the
ment results.
low-end C and L measurement ranges respectively to
When the new instrument is equipped with the
10 pF and 10 JLHfulll scale, giving the new instrument
HP-IB option, all functions except DC BIAS are pro-
the ability to measure components over exceptionally
grammable through the HP-IB. The current status of wide ranges: inductance from 0.01 JLH (the limit of
the instrument (FUNCTION,CIRCUITMODE, AUTORANGE,
resolution) to 1999 H, capacitance from 0.01 pF to
etc.) is made visible by illumination of the LED indi-
19.99 mF, resistance from 1 mD. to 19.99 MD., dissipa-
cators in the corresponding front-panel keys.
tion factor from 0.001 to 19.9, and quality factor from
0.05 to 1000. With these wide ranges, the instrument
Wide-Ranging Measurements may be used to measure RF coils, dielectric materials,
The new instrument has a la-kHz test frequency in
electrolytes, the internal resistance of batteries, and
*Hewlett-Packard's
implementationof IEEEStandard488-t975. the high dissipation factor of delay lines, as well as
19
the values of discrete components. Internal Operations
In particular, the new LCRmeter can measure the Model 4262A Jl.CR Meter finds the values of 1, c, R,
ESR (equivalent series resistance) of a capacitor to very D,and Q by determining the vector ratio of the voltage
low values, a significant measurement if the capacitor across the device under test (DOT) to the current
is to be used for bypass applications. With the 10-kHz
flowing through the device, in the same manner asthe
test frequency, the reactance of a wide range of Model 4261A LC:RMeter.1
capacitors becomes low enough for the ESR to be a A block diagram is shown in Fig. 4. The voltage
significant, and measurable, part of the total impe- across the device, is represented by e1 in the diagram
dance, as plotted in Fig. 2. At 10 kHz, the resolution of and the current by voltage e2,which is proportional to
an ESR measurement is 1 mn. for capacitors larger the current flo~Ting through range resistor RR in
than 10 JLF.A five-terminal input (two for voltage, se;ries with the unknown. Op amp A3 assures accu-
two for current, and one for a guard) is provided to rate current flow by maintaining the LOWinput ter-
minimize errors. Internally-generated bias levels of minal at virtual ground.
1.5V, 2.2V and 6V are also provided, primarily for The four-phase generator supplies a signal shifted
measurements on electrolytic capacitors. in precise increments of 90