Document 364-1

Calibration, Compensation and Correlation
Introduction
When comparing published inductance values, engineers
require a common basis for comparison. Ideally, a 1 uH
inductor from one manufacturer would be equivalent to a
1 uH inductor from every other manufacturer. For that to
be true, all manufacturers would have to use the same
standards to design and measure their inductors.
The National Institute of Standards and Technology (NIST)
provides very limited standards for inductors. The standards require coaxial connections, styles are limited toair core or non-saturable core inductors, the inductancevalue range is 10 nH to 100 mH, and the frequency range
is limited from 10 kHz to 250 MHz. These standards are
not suitable for comparing modern chip inductors andpower inductors with their wire lead or surface mount (SM)constructions.
As surface mount inductors were developed, new instruments and fixtures were developed to measure them. As
instrumentation and fixture technology improved, the accuracy of measurements improved. The relatively rapidchange in measurement technology resulted in the manufacture of many different fixture designs. The instrumentation and fixtures used are generally based on the best
available technology at the time for the inductor type, size,inductance value, and test frequency.
Since inductance standards have not kept pace with inductor technology, each manufacturer is responsible forthe accuracy and precision (repeatability) of its inductor
measurements. The following discussion explains how
Coilcraft uses calibration, fixture compensation, and correlation (where appropriate), to ensure the accuracy andprecision of its measurements.

A typical calibration process involves connecting several
measurement standards to the instrument. The calibration
standards should have values that cover the entire measurement range of inductance. The instrument is adjusted
so that it reads the labeled value of the standard. The
instrument is considered calibrated when it displays thecorrect value of each standard.

Calibration Plane
The calibration plane is where measurement accuracy isdefined using the calibration standards. Ideally, it is locatedat the instrument terminals where the inductor is connected
for measurement.
In order to provide a durable and repeatable method ofconnecting calibration standards to the instrument, standard connectors — such as type N, 7 mm, or 3.5 mmthreaded connectors — are typically used. Wire lead andsurface mount inductors do not have threaded terminals,
therefore a test fixture is required to connect the part to theinstrument.

Fixture Compensation
The Fixture Compensation Plane, shown in Figure 1, is
where the inductor is connected to the test fixture. The test
fixture extends the measurement plane a physical distance from the calibration plane. The test fixture also adds
parasitic resistance, inductance, and capacitance to themeasurement. Fixture compensation “extends” the calibration plane to the test fixture plane and reduces theeffects of fixture parasitics. Fixture compensation is similar
to calibration in that standards of known value are used to
set the instrument to read the correct value of the standard
at the fixture measurement plane.

The Importance of Calibration
Accuracy statements assume that a standard of the “true”
value exists to which a measurement can be compared.
In the absence of an available standard, a defacto (generally accepted) standard may be used. Defacto calibration standards are typically supplied by the instrument
manufacturer. The accuracy of the standards is based on
physical principles and measurements traceable to national standards.
Calibration sets the instrument to an established measurement condition (or state) using physical standards.
Ideally, after each calibration session, the instrument is set
to the same standard conditions, resulting in an established and repeatable measurement condition.

Fixture compensation plane

Instrument

Calibration plane

Figure 1

Calibration Standards
Coilcraft performs an instrument calibration before every
measurement session. The selection of calibration standards depends on the instrument to be calibrated and therequired accuracy. The accuracy of calibration standards
has increased with improvements in impedance analyzer
technology. The extremes of impedance are: a short circuit

Specifications subject to change without notice.

Document 364-1

Revised 09/16/03

Document 364-2(lowest impedance), and an open circuit (highest impedance). Most impedance analyzers and network analyzers
that are used to measure inductance have both open andshort calibration standards. Some 50 Ohm (characteristicimpedance) analyzers also have a 50 Ohm load calibration standard.
An ideal inductor causes a 90° phase angle between thevoltage across and the current through the inductor. Accurate quality factor (Q) measurements depend on an accurate measurement of the phase angle. A very small error in
the phase angle measurement can lead to a large error in
the displayed Q value. Typical open and 50 Ohm load calibration standards do not have perfect 0° phase characteristics. Calibrating with an accurate phase calibration
standard, such as a low loss air capacitor, can reduce thephase angle measurement error and improve Q measurements. Air capacitor standards are available for use withmany higher-end impedance analyzers.

Fixture Compensation Standards
Typical fixture compensation includes open and short compensation. When measuring series inductance and Q, theopen standard is the air gap between the test terminals
with no part connected. The short standard is a very low
impedance (high conductivity) metal bar or wire with dimensions approximately the same as the inductor to bemeasured. Coilcraft uses high quality gold-plated shortingbars for the short standard to ensure high conductivity andgood contact with the test terminals.
Some instruments allow a load compensation to be performed. A standard 50 Ohm or 75 Ohm surface mount orwire lead resistor (load) may be used, but real resistors
always have some parasitic inductance and capacitancethat add error to the measurement. Combining two resistors of double the fixture compensation load value in parallel cuts the parasitic inductance in half, but may morethan double the parasitic capacitance. In most cases, since
the parasitic effects of a fixture compensation load standard are not known, load compensation is not performed.

Correlation of Chip Inductor Values
It is often assumed that after instrument calibration andfixture compensation, the value displayed by an instrument is “the” correct or true or absolute value. This requires that all combinations of instrument, fixture, calibration
standards, and fixture compensation standards yield thesame value when measuring the same inductor. Improve-ments in instrumentation and fixture technology lead tomore accurate measurements, but still can result in different measured values of the same component.
Test equipment used in the past to design and test ourinductors required special fixtures to accommodate smaller
components, particularly surface mount devices. The inaccuracies of the original test equipment and fixture became part of the component’s measurement. As a result,a part that measured 100 nH on one instrument and fixture, may measure 95 nH on a different instrument andfixture. To overcome variations due to different instruments
and fixtures Coilcraft uses a correlation process. Correlation ensures that all manufactured inductor values areconsistent with their original design value.
This process uses standards (correlation pieces) that areequivalent in size and electrical character to the actual
production parts. These correlation pieces were created,measured, and labeled (“tagged”) with the measured value
at the time of design. Measurements of inductors with thesame part number are compared to measurements of thecorrelation standards. The product measurements arecompensated for the difference between the new measurement of the correlation piece and that of its taggedvalue. The compensated measurements are then compared to specification, providing consistent verification required by the customer.
We continue to manufacture our surface mount chip inductors to their original correlation standards, ensuringthat the electrical parameters are consistent over time. Bymanufacturing to a correlation standard, we ensure thatan inductor we produce today or in the future will have thesame value as the original design.

Correlation Is Not Always Required
On low inductance value components with tight tolerances,such as chip inductors, correlation is used for tolerance
control. The large inductance values of power inductors
require the use of ferrite or powdered iron core material.
The permeability tolerance of ferrites and powered ironcore materials is typically 20%, although tighter tolerances
can be requested at a premium. The difference in inductance measurements between different instruments is typically very small compared to the 20% tolerance range ofpower inductors. Therefore, correlation is not necessary
for power inductors.

Specifications subject to change without notice.

Document 364-2

Revised 10/20/03