Testing Inductors atApplication Frequencies

Why testing at actual circuit frequency is critical to ensuring desired inductor performance

Introduction
The accurate measurement of an inductor has always been
more difficult than the measurement of other passive components. The primary difficulty with coil measurements liesin the fact that coil inductance and its efficiency are quite
frequency dependent. Similarly, coil parasitics (distributedcapacitance and core/copper resistive losses) vary dramatically with frequency. The measurement of a coil at theapplication frequency, so-called “use frequency testing,” ismore representative of the basic value of the component
in circuit than testing at traditional standard frequencies.
Often, the value of a measurement frequency is specified
for measurement convenience alone. If the measurement
frequency is not the circuit (or “use”) frequency, the resultof testing generally will not yield the same inductance value
or display the same efficiency as seen by the intended
circuit. Because equipment and methods now allow more
flexibility in test frequency selection, inductors should betested at the actual frequency of use, particularly if tighttolerances are required.

Inductor parameters
The primary electrical parameters of an RF inductor areinductance, Q (efficiency), the direct current resistance(DCR) and self-resonant frequency (SRF). All primaryelectrical parameters are design controlled, although not
independently. The measured inductance and Q are often highly dependent on the test instrumentation and thefrequency at which the test is performed.

Test equipment
The appropriate selection of a method of test largely determines the accuracy of a measurement. Instrumentationand test methods vary for individual electrical parameters,
and every instrument has further limitations in terms ofrange, frequency, and error. Parasitics and their effects
associated with test fixturing is a significant consideration
in a measurement. In all cases, the instrument, fixturing,frequency, and current (if applicable) must be specified inorder to have a repeatable and reliable test.
An impedance analyzer is generally used for the inductance and Q measurements of an inductive component.
The SRF can be measured by noting the zero crossingof the phase on a network analyzer. The DCR is usually
measured on a low ohmmeter.
The selection of instrumentation also influences measuredvalues. The influence of an instrument is the result of thevarious measurement methods and frequencies employedby each piece of equipment.
Part of the discrepancy can be attributed to the differentinstruments, but the majority is due to the different frequencies. In general, the proper instrument to specify is one
that is accurate and repeatable at the frequency required.
The latest-available impedance analyzers for measuring
L and Q of RF inductors have accurate phase calibration
standards and measure as high as 3 GHz to facilitate
testing at application frequencies.

Q is the ratio of imaginary to real impedance, and is related to the efficiency of an inductor as the ratio of stored
energy to dissipated energy. Q describes the energy storage capability of an inductor as directly proportional to
the frequency at which it is being measured and inversely
proportional to the resistive (copper and core) losses thecomponent displays at the same frequency.
DCR is the resistance of the wire used in the inductor as
seen by a direct, non-varying current. DCR gives us an ideaof the insertion loss the inductor will add to the circuit by
its simple insertion into the circuit. DCR also indicates how
much energy the inductor will waste in simply heating up.
SRF is the frequency above which the component nolonger functions as an inductor. The SRF is determined by
the parallel combination of the (low frequency) inductanceand the effective capacitance accumulated between theturns of the coil.

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Traditional method of specifying andtesting inductors
Before the advent of impedance analyzers, Q meters wereused to measure L and Q. These meters were cumbersome
to use and had low accuracy. They also required setup “standards” which were similar to correlation pieces. Correlation
pieces are similar to standards in that they are identified bothby the vendor and user as having a particular value. Correlation pieces are deemed to be the standard for a particularinductor and are used to set up each instrument every timea test is performed. Correlation is still the most accurate
method, resulting in very little error, excellent repeatability
and is applicable at any frequency. However, the method ofcorrelation has significant logistical disadvantages: the establishment and accountability of specific correlation pieces
between the manufacturer and the customer.
The other electrical parameters, SRF and DCR are generally
specified along with L and Q but there is rarely any reference
to the test method used. The lack of a specific method of
testing SRF and DCR reflects the fact that the inductanceis the dominant parameter and one that requires the mostdiligent control.

Application frequency testing

Figure 1. Inductance change with frequency for threedifferent inductors

The basic complication with the traditional testing method
is that the coils are tested at one frequency and used atanother frequency. The graphs in Figures 1 and 2 show thefundamental problem with the traditional testing method.

Inductance vs frequency
Figure 1 shows a log sweep of inductance vs. frequency
for three different inductors. When these parts are tested
at a standard frequency, they appear to be identical interms of inductance. At the actual circuit frequency, thesecoils are quite different. These three coils can represent
three different designs or three different coils of the same
design. If the use frequency is the point where all three
converge, then these coils are effectively the same. If thecircuit frequency is significantly different from the test frequency, then the inductance at the use frequency cannot
generally be implied from the test frequency.
Even if these coils had not deviated from each other, there
is still a change in inductance. The consequence of an inductance shift with frequency is generally observed whena design requires a specific inductance and the particularcoil does not perform as expected. The shift is generally
attributed to fixturing differences and circuit parasitics andthe specified value is compensated accordingly.
Figure 2 shows another consequence of testing at a frequency other than the application frequency. It displays aninductance vs. frequency sweep for a particular coil. The
normal inductance is indicated at both frequencies alongwith their tolerances. If this coil had been specified as a
5% inductor at the test frequency, it would have resulted
in a 10% tolerance at the application frequency. Theselimits could have compressed over frequency or spreadeven more, depending on the coil. The fact that the limits
are not constant with respect to each other results in aloss of tolerance control over frequency.

Figure 2. Tolerance change with frequency for aninductor
In all cases where the inductance changes with frequency,
testing at the application frequency results in better
specification control. Testing at the application frequency,
particularly with correlation, can be an extremely effective and superior method of determining the application
suitability of a coil.

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Specifying Inductor Tests at Application
Frequencies
The procedure to specify and the electrical test for an
inductor at use frequencies is as follows:Inductance1.

Specify the nominal inductance.

2.

Specify the test instrument, fixture, calibration andfixture compensation methods, and frequency.

3.

Specify the inductance tolerances.
a. Determine the allowable tolerance as a percentage using 6 sigma or other appropriate methods.
b. Evaluate the instrument error for the nominal impedance at the test frequency. Subtract this errorpercentage from the allowable tolerance found instep 3a.
c. Establish the instrument and fixture repeatability.
Subtract this error percentage from the result of
step 3b. This result is the tolerance to specify. The
manufacturer should test to the specified tolerance without correlation as all errors have beenaccounted for.

Q
1.

Specify the absolute minimum Q (allowable minimum).
Determine the allowable minimum using 6 sigma orother appropriate methods. The customer should test
to this tolerance.

2.

Specify the test instrument, fixture, calibration andfixture compensation methods, and frequency.

3.

Specify the Q minimum for manufacture.
a. Evaluate the instrument error for the nominal
impedance at the test frequency. Adjust the allowable minimum found in step 1 (i.e., increase theminimum Q by an amount equal to the instrument
error).
b. Establish the instrument and fixture repeatability.
Adjust the new allowable minimum found in step
3a (i.e., increase the minimum Q by an amountequal to the test repeatability). This result is thetolerance to specify. The manufacturer should
test to the specified tolerance without correlation
because all errors have been incorporated into
the final adjustment of the Q specification.

a. Evaluate the instrument error for the nominalresistance. Adjust the allowable maximum foundin step 1 (i.e., decrease the maximum DCR by anamount equal to the instrument error).
b. Establish the instrument and fixture repeatability. Adjust the new allowable maximum found instep 3a (i.e., decrease the maximum DCR by anamount equal to the test repeatability). This result is the tolerance to specify. The manufacturer
should test to the specified tolerance without correlation because all errors have been incorporated
into the final adjustment of the DCR specification.

SRF
SRF is typically not a specification. A typical reference
value or typical minimum value may be published. Qand SRF will track the measured inductance value.
Therefore, only inductance typically needs to be tested
for qualification.

Specifications With Correlation
The use of correlation in conjunction with use frequency
testing can eliminate errors due to different fixture andenvironmental factors and allow the specifications to betightened. It may not be needed for higher inductancevalues. Correlation can be used with any of the inductor
parameters but is most often applied to inductance and Q.

Conclusion
Coil inductance and Q are frequency dependent and testing methods have further influence on these parameters.
Specifying and testing at the actual circuit frequency is
an appropriate method of controlling inductor parameters.
Use frequency testing ensures that components areconsistent with their intended application.
The primary inductor parameters are interrelated design
functions of the coil. The inductor specification shouldtake into account the component variations and measurement system errors.

DCR
1.

Specify the absolute maximum DCR (allowable maximum). Determine the allowable maximum using 6
sigma or other appropriate methods.

2.

Specify the test instrument and fixture.

3.

Specify the DCR maximum for manufacture.

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