Methods of 100 dB RF Performance
Verification
of Facility Filters
Thomas D. Geist
Kermit 0. Phipps
Power ElectronicsApplication Center
Knoxville, Tennessee
Power Electronics Application Center
Knoxville, Tennessee
EMI/RFI facility filters specified to 100 dB are
usedin governmentand commercialinstallationsto prevent
unwantedsignalson power lines. The main standardusedto
measurefilter insertion loss (synonymousfor matched
impedanceattenuation),is MIL-STD-220A. But becauseMILSTD-220Awas developedonly for matchedimpedance
communicationsystems,it has beenlabeledunrealistic. Since
then,otherstandardsand methodshavebeendevelopedthat
attemptto model “real world” attenuationby requiring varying
sourceand load impedance.To date,however,noneof these
measuringmethodshave replacedMIL-STD-220A. This paper
supportsthe continueduseof the much-beleaguered
MIL-STD220A as the preferredchoicefor high frequencyperformance
characterization,Additionally, this paperillustrateswhy none
of the existing methodsare sufficient to fully verify 100 dB
attenuation.Furthermore,in recognizingMlL-STD-220A’s
shortcomings,this paper suggeststhe developmentof a new
facility filter standardthat addressespower quality issues,
critical RF performancefactors,and filter/shieldedroom
equipmentincompatibility issuesthat previously have not
beenconsidered.
h&IMODS
Abstract:
BACKGROUND
Since 1995,the EPRI Power ElectronicsApplications Center
(PEAC) hasbeenengagedin evaluatingfilter standardsand
methodsof filter characterization.Theseefforts are directedat
bringing aboutstandardizedtest protocolsfor the effective
measurementand specificationof facility filters. This paper
reportsthe results of the project’s investigationinto facility
filter RF performancefor filters rated at 100 dB. After
conductinga literaturesearch,PEAC selectedfour standards
and/ormethodsthat apply to 100 dB RF performance
characterization,
They are:
1. MIL-STD-220A
2. CISPR 17
3. ARP-4244
4. Current Injection ProbeMethod
Although the Current Injection ProbeMethod appearsas part
of ARP-4244, it is not itself a standard.The use of AC power
differentiatesthe Current Injection ProbeMethod enoughthat
it warrantsseparateattention.
0-7SO3-j015-4/9Sl$lO.~~0 1998IEEE
MIL-STD-220A
Implementedin 1959 by the United StatesArmy Signal corps,
MIL-STD-220A, Method of Insertion Loss Measurement[ 11,
was designedto rate Radio FrequencyInterference(RFI)
capacitors,which are usedwith some mobile radio transceivers
when they experienceradio interferencefrom the automobile’s
ignition system.MIL-STD-220A test setup is shown in
Figure 1. However, MIL-STD-220A, basedon 1952’sMILSTD-220and on previous filter capacitorstandardsdating from
the beginningof military radio communications,is commonly
usedtoday to rate industrial and commercialfacility filters.
Figure 2 shows a low-frequency plot of a facility filter
performancecurve as measuredusing MIL-STD-220A. The
test methodspecifiedin MIL-STD-220A was developedfrom
techniquesfor measuringinsertion loss in matchedimpedance
communicationsystems,typically 50 or 75 ohms. However,
most applicationsof filters are in areaswhere impedanceis not
matched.Thus, measurementsmade using MIL-STD-220A
test methodswill yield different insertion-lossvalueswhen
made in an unmatchedsystem,particularly at low
frequencies-100 kHz and below. Even though this problem
has beenrecognizedfor more than thirty years,filter
manufacturersand userscontinueto rely on MIL-STD-220A
test methodsto specify performance.
159
I
I
I
I
I-
-1
Tracking
Generator
Spectrum
Analyzer
Unit Under Test
///////////////////////////
Ground Plane
Figure
1. MIL-STD-220A
Test
Setup.
achievean appreciablefrequencyrange-10 kHz to 10 MHz-a
compromisemust be reachedbetweena high turns ratio,
resultingin reducedbandwidth, and a large voltagedivision,
resulting in high insertion loss. The impedancetransformers,
therefore,have a 50 dB insertion loss [4]. G iven realistic
measurementequipment,CISPR 17 providesgood resultsfor
about 60 to SOdB of dynamic measurementrange.However,
the transformerinsertion loss effectively restrictsthis method
from useon 100 dB filters (seeFigure 4).
100
50-bhm
insertion
loss
-
100,000
10,000
1,000
Frequency (Hz)
Figure
using
CISPR
2. Typical performance
MIL-STD-220A.
curve as measured
17
Answeringthe needto addressunmatchedimpedance,CISPR
17 specifiedproceduresfor measuringfilters underconditions
of mismatchedload and sourceimpedance.It provideda means
of performingmeasurementsin both symmetrical(common)
and asymmetrical(differential) modes[3] [4]. Createdin the
mid 197Os,The InternationalElectrotechnicalCommission
developedCISPR 17, Methods of Measurement of
Suppression Characteristics of Passive Radio Intelference
Filters and Suppression Components [2], to provide a
universalstandardthat would allow variouslaboratoriesand
manufacturersto comparefilter measurementresults.It is
basedon the conceptof combining extreme-valuetheory of
mismatchwith a statisticaldata baseof load and source
impedance[3] [4]. Becausethe method,shownin Figure 3,
measuresfilter performanceusing extremevaluesof sourceand
load impedance,it is usually called the worst-case,or extremevalue, test method.After initial measurements
are made,the
TrackingGeneratorand SpectrumAnalyzerarereversed.
BecauseImpedancetransformersare not readily available
commercially,and becausethe methodis limited in dynamic
range,CISPR17hasneverbeen widely used.In order to
Figure 4. CISPR 17 Measurement
showing
dynamic range due to impedance transformer
insertion
loss.
ARP-4244
As anothermore recentattempt to rectify the shortcomingsof
MIL-STD-220A, a committee of the Society of Automotive
Engineers(SAE), AE-4P, developedthe Aerospace
Recommended
PracticeARP-4244standard.In this standard,
sourceimpedanceis changedby addingor removinga LineImpedanceStabilizationNetwork (LISN), and load impedance
is changedthrough an adjustableresistivebank [5]. In the
setup,shown in Figure 5, R.Fcurrent is injected into the
systemby an RF current injection probe (CIP) and the
measurementof attenuationis made with an W-receiving
currenttransformer(CT). This CIP methodis an enhancement
of the buffer networksusedin MIL-STD-220A for load testing
andprovidesa wide-frequencymeasurement
range.
Measurements are taken
,c----.-----.-.-----,---. in this configuration __-
Ground Plane
Figure
3. CEPR
limited
17 Test Setup.
RF Port
Terminated
with 50 Ohm
16”
10 pfh
Isolated
DC Power
SUPPlY
<,
50 pH LEN
+
/\
30 mf
short as practical and two
inches above copper
groundplane, except at
terminations, where different
height is mandated.
Figure
5. ARP-4244
Filter
t
Generator
RF Port
Terminated
with 50 Ohm
Setup
for
Differential
The objective of testing a filter under load is to determine if
the seriesinductor saturates. However, as the ARP-4244
standardpoints out, it is not necessaryto use the CIP method
to test for saturationabove the inductor resonantfrequency,
because at that frequency,the inductors are effectively
capacitors,as shown in Figure 6. Therefore, above the
resonantfrequency, which is typically less than 5 MHz, the
impedanceseenby a noise source is actually the line
impedanceand not the seriesinductor. As per the standard,line
impedanceabove 5 MHz is generally between 40-300 ohms.
Thus, the use of the 50-ohm impedancetest method, MILSTD-22OA,is a reasonableprocedureabove 5 MHz.
Pad
RF Performance
With 3 dB
Pad
Measurement.
However, ARP-4244 is vague in its requirement for testing
differential-modeperformance. The test procedurerequires
adjusting the resistive load to produce the dc peak equivalent of
rated current. Unfortunately, this current requirement is open to
interpretation.Nonlinear loads can produce either less peak
current or more peak current than a resistive load dependingon
the design of the filter series inductor. If, for example, the
inductor is designedfor nonlinear current, the inrush due to
nonlinear loading will cause voltage flat topping that will
limit the peak current. If, on the other hand, the inductor is not
designedto support the nonlinear current, the inductor will
saturateand the peak current will be higher than the resistive
load. In both cases,however, the current peak will be higher
than the peak current for a sinusoidal-shapedcurrent waveform.
Figures 7 and 8 show the voltage and current wave shapesof a
thirty-amp 100 dB facility filter under twenty-five percent
switch-modepower supply loading (nonlinear). Given the
uncertainty as to what the rated current, or more correctly
proper “measurement”current should be, more work is needed
to define a method that measuresRF performanceunder “real
world” loading conditions.
5 MHz
Frequency
Figure 6. Typical
frequency.
curve
showing
impedance
versus
1Gl
200
Current
‘---
Injection
Probe
‘Technique
Another method of eliminating the low-frequency matched
impedanceproblem of MIL-STD-220A is to use current probes
for both the injection and reception of RF signals. A typical
setup is shown in Figure 9 (A good description of the method
can be found in [6], [7] and [8]). As in the SAE ARP-4244
method, the CIP method allows for sourceimpedancevariation
by adding an LISN or an air-core inductor in serieswith the
feed-throughcapacitor (not shown).
150
--ml2
-100
9\
-150
I
-200
Figure 7. Flat-topped
with 25% nonlinear
voltage
loading.
of a facility
20 ----
filter
---
-
15-----A10 -
/
I
\\
Crest Factor = 2.05
\
--_
-20
Figure 8. Current
wave-shape
with 25% nonlinear
loading.
of a facility
filter
While not a new idea-its beginnings can be traced back to at
least the mid 1960s-a survey of filter manufacturersindicates
that its use has been very limited, perhapsdue to the difficulty
in obtaining a high dynamic range. A 100 dB measurement,
for example,requiresa CIP capableof transferringappreciable
amountsof power from the signal source to the test circuit.
This then leads to difficulties in shielding the current injection
probe so that it does not radiate and couple to the filter output.
Additionally challenging is the elimination of erroneous
measurementsdue to sneak ground paths, unintentional
radiatedcoupling, and other undesiredpaths.
On the other hand, the advantageof the CIP method is that it
allows variation of filter sourceand load impedancesto any
desiredvalue so that actual installation conditions can be
simulated. This simulation means that testing under AC
loading using both linear and nonlinear loads is possible.
Figure 10 shows that a difference in measuredfilter
performancemay exist between resistive (30 Amp R) and
nonlinear loading (30 Amp PC, as noted in Figure 10) for two
different filters. The difference in measuredperformance,
however, is a result of different filter design quality.
162
Figure
9. Typical
CIP
measurement
setup
shortcomings.They each fall short in either the frequency
rangeover which measurementsare madeor becauseof an
70
80
100,000
inability to test under realistic load conditions. However, Table
1 illustratesthat when used togetherthere are two methodsthat
can accommodateall the needsof RF performance
measurements
for facility filters rated at 100 dB: (1) the CIP
methodallows for realistic load testing at lower frequencies
(down to 10 kHz and below), and (2) MLSTD-220A allows
for measurementof RF performanceat high frequencies(above
5 MHz).
1 ,ooo.ooo
Frequency(tk)
i
Figure 10. CIP measurement results for linear
nonlinear
loading, for two similar filters with
different
manufacturers.
Which method should
Characterization?
be used for
and
RF
The backgroundresearchdescribedin this paperalong with
extensivetesting conductedby PEAC shows that no existing
facility-filter RF performancetest standardor method is
completelyadequate.Each standardor methodhasone or more
WHAT’S NEXT
The researchrepresentedin this paperis part of the ongoing
effort to provide a standardizedmeasurementset and realistic
specificationfor facility filters. The creation of a new standard
that will provide meansfor addressingfacility-filter power
quality issues,critical RF performancefactors, and
filter/shieldedroom equipmentincompatibility issuesis a
logical next step to this research.What is neededis a standard
with broadindustry recognition.The existing standardsfall
short.
MIL-STD-220A specifies only 50-ohm impedanceand,
therefore,does not accuratelymeasureperformanceat
frequenciesbelow 5 MHz. Rather than changing MIL-STD220A, a new non-military standardis neededfor facility filter
RF performance verification, but as yet, none exists. CISPR
17 is a European standard,and is also not well-known in the
United States.Similarly, becauseSAE ARP-4244 is an
automotive standard,other industries may not be aware of it.
Table
1. Comparison
of facility
Frequency Range
5 MHz - 22 GHz
AC
Loading
MIL-STD-220A
CEPR17
ARP-4244
CIP Method
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
No
Yes
RF1 Corporation,
Sy Rubin and Robbert Hassast
Fil-Coil, Lester Hammer, Paul Alessandrini and
Romesh Rajput
6 Elmag Division
l
Mike Leone
MTK. Gil Geiger
l
LCR Electronics,
of Lindgren,
Terry Forsyte and
Nissin Isankof and Jerry Womer
REFERENCES
[I]
RF performance
Frequency Range
lOkHz-5MHz
We would like to thank the following filter manufacturesfor
their time, cooperation, efforts and full support in this project.
l
filter
Standard or
Method
ACKNOWLEDGMENTS
l
Given the broad range of filter uses-medical, information
technology, government, and others-the need exists for
international and national standardssupportedby both IEEE
and ANSI. Once this standardis developedand adopted,filter
userswill no longer need to rely on inadequatestandardswhen
specifying and purchasing facility filters. Instead, filter users
will have a new standardthat addressesthe needsof today and
tomorrow.
MlL-STD-220A,
Method of Insertion Loss Measurement,
15 December 1959.
[2] CISPR 17, Methods of measurement of suppression
characteristics of passive radio interferencefilters and
suppression components, Geneva: IEC, 1981, Section
4.2.2.2.
methods
Varying Source
and Load
Impedance
No
Yes
Yes
Yes
[3] H. Weidmann and W. J. McMartin, Two Worst-Case
Insertion Loss Test Methods for Passive Power-Line
Interference Filters,
[4] A. Tucker, 100/O. 1 Ohm Filter Terminations and Other
Extreme-Valued Affects, 1989 International Symposium on
EMC, 8-10 Sept., 1989, Nagoya, Japan, p. 828-833, Vol. 2
[5] AerospaceRecommendedPracticeARP-4244,
Recommended Insertion Loss Test Methods for EMI Power
Line Filters, Preparedby SAE Committee AE-4P, 9 July
1996.
[6] J.R. Fischer et at., Evaluating jilters in situ under heavy
load currents and normal working impedances, presentedat the
IEEE Electromagnetic Compatibility Symposium,
Washington, DC., July 1967.
[7] Robert E. Hassett,EM Filter Characteristics and
Measurement Techniques, InterferenceTechnology Engineers’
Master (ITEM) 1997.
[8] S. Eisbruck and F. Giordano, A Survey of Power-Line
Filter Measurement Techniques, IEEE Transactionson
Electromagnetic Compatibility, Vol. EMC-10, No. 2 June
1968.
16-c