IEC 61 726 Edition 3 0 201 5 09 INTERNATIONAL STANDARD Cable assemblies, cables, connectors and passive microwave components – Screening attenuation measurement by the reverberation chamber method IE[.]
I E C 61 ® Edition 3.0 201 5-09 I N TE RN ATI ON AL S TAN D ARD C abl e as s em bl i es , cabl es , n ectors an d pas s i ve m i crowave com pon e n ts – IEC 61 726:201 5-09(en) S cree n i n g atte n u ati on m e as u re m e n t b y th e reverbe rati on ch am ber m eth od T H I S P U B L I C AT I O N I S C O P YRI G H T P RO T E C T E D C o p yri g h t © I E C , G e n e v a , S wi tz e rl a n d All rights reserved Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either IEC or IEC's member National Committee in the country of the requester If you have any questions about I EC copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or your local I EC member National Committee for further information IEC Central Office 3, rue de Varembé CH-1 21 Geneva 20 Switzerland Tel.: +41 22 91 02 1 Fax: +41 22 91 03 00 info@iec.ch www.iec.ch Ab ou t th e I E C The I nternational Electrotechnical Commission (I EC) is the leading global organization that prepares and publishes I nternational Standards for all electrical, electronic and related technologies Ab o u t I E C p u b l i ca ti o n s The technical content of IEC publications is kept under constant review by the IEC Please make sure that you have the latest edition, a corrigenda or an amendment might have been published I E C Catal og u e - webstore i ec ch /catal og u e The stand-alone application for consulting the entire bibliographical information on IEC International Standards, Technical Specifications, Technical Reports and other documents Available for PC, Mac OS, Android Tablets and iPad I E C pu bl i cati on s s earch - www i ec ch /search pu b The advanced search enables to find IEC publications by a variety of criteria (reference number, text, technical committee,…) It also gives information on projects, replaced and withdrawn publications E l ectroped i a - www el ectroped i a org The world's leading online dictionary of electronic and electrical terms containing more than 30 000 terms and definitions in English and French, with equivalent terms in additional languages Also known as the International Electrotechnical Vocabulary (IEV) online I E C G l os sary - s td i ec ch /g l oss ary More than 60 000 electrotechnical terminology entries in English and French extracted from the Terms and Definitions clause of IEC publications issued since 2002 Some entries have been collected from earlier publications of IEC TC 37, 77, 86 and CISPR I E C J u st Pu bl i s h ed - webstore i ec ch /j u stpu bl i sh ed Stay up to date on all new IEC publications Just Published details all new publications released Available online and also once a month by email I E C C u stom er S ervi ce C en tre - webstore i ec ch /csc If you wish to give us your feedback on this publication or need further assistance, please contact the Customer Service Centre: csc@iec.ch I E C 61 ® Edition 3.0 201 5-09 I N TE RN ATI ON AL S TAN D ARD C abl e as s em bl i es , cabl es , n e ctors an d pas s i ve m i crowave com pon en ts – S creen i n g atte n u ati on m eas u re m e n t b y th e reverbe rati on ch am ber m eth od INTERNATIONAL ELECTROTECHNICAL COMMISSION ICS 33.1 20.1 0; 33.1 20.30 ISBN 978-2-8322-2893-7 Warn i n g ! M ake su re th a t you obtai n ed th i s pu bl i cati on from an au th ori zed d i s tri bu tor ® Registered trademark of the International Electrotechnical Commission –2– I EC 61 726:201 © I EC 201 CONTENTS FOREWORD Scope Norm ative references Terms and definitions Basic description of the reverberation chamber method 5 Measurement of the screening attenuation of the device under test (DUT) 6 Description of the test set-up 6.1 Reverberation cham ber 6.2 Mode stirrer 6.3 Antennas 6.4 Test equipm ent 6.5 Device under test (DUT) 6.6 Linking devices Measurement procedure 7.1 General 7.2 Measurement of the DUT 2.1 General 2.2 Standard m easurem ent 2.3 Fast m easurem ent 7.3 Measurement of the insertion loss of the cavity 7.4 Control of the test set-up 4.1 Dynamic range 4.2 I nsertion loss of the cham ber 1 4.3 Measurem ent of a calibrator 1 4.4 Measurem ent of lossy DUT 1 7.5 Revolution speed of the mode stirrer 1 7.6 Test frequencies 1 7.7 Voltage standing wave ratio (VSWR) Evaluation of the test results Annex A (inform ative) Relationship between transfer im pedance and screening attenuation Annex B (informative) Example of a calibrator Bibliography Figure – Exam ple of a test set-up Figure B.1 – Basic construction details I EC 61 726:201 © I EC 201 –3– INTERNATIONAL ELECTROTECHNICAL COMMI SSION C AB L E AS S E M B L I E S , C AB L E S , C O N N E C T O RS AN D P AS S I VE M I C RO WAVE C O M P O N E N T S – S C RE E N I N G AT T E N U AT I O N M E AS U RE M E N T B Y T H E RE VE RB E RAT I O N C H AM B E R M E T H O D FOREWORD ) The I nternational Electrotechnical Com m ission (I EC) is a worldwide organization for standardization com prising all national el ectrotechnical com m ittees (I EC National Com m ittees) The object of I EC is to prom ote international co-operation on all questions concerning standardization in the electrical and electronic fields To this end and in addition to other activities, I EC publishes I nternational Standards, Technical Specifications, Technical Reports, Publicly Availabl e Specifications (PAS) and Guides (hereafter referred to as “I EC Publication(s)”) Their preparation is entrusted to technical com m ittees; any I EC National Com m ittee interested in the subject dealt with m ay participate in this preparatory work I nternational, governm ental and nongovernm ental organizations liaising with the I EC also participate in this preparation I EC collaborates closely with the I nternational Organization for Standardization (I SO) in accordance with conditions determ ined by agreem ent between the two organizations 2) The form al decisions or agreem ents of I EC on technical m atters express, as nearl y as possible, an international consensus of opinion on the relevant subjects since each technical com m ittee has representation from all interested I EC National Com m ittees 3) I EC Publ ications have the form of recom m endations for international use and are accepted by I EC National Com m ittees in that sense While all reasonable efforts are m ade to ensure that the technical content of I EC Publications is accurate, I EC cannot be held responsible for the way in which they are used or for any m isinterpretation by any end user 4) I n order to prom ote international uniform ity, I EC National Com m ittees undertake to apply I EC Publications transparently to the m axim um extent possibl e in their national and regional publications Any divergence between any I EC Publication and the corresponding national or regional publication shall be clearly indicated in the latter 5) I EC itself does not provide any attestation of conform ity I ndependent certification bodies provide conform ity assessm ent services and, in som e areas, access to I EC m arks of conform ity I EC is not responsible for any services carried out by independent certification bodies 6) All users should ensure that they have the latest edition of this publication 7) No liability shall attach to I EC or its directors, em ployees, servants or agents including individual experts and m em bers of its technical com m ittees and I EC National Com m ittees for any personal injury, property dam age or other dam age of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this I EC Publication or any other I EC Publications 8) Attention is drawn to the Norm ative references cited in this publication Use of the referenced publications is indispensabl e for the correct application of this publication 9) Attention is drawn to the possibility that som e of the elem ents of this I EC Publication m ay be the subject of patent rights I EC shall not be held responsible for identifying any or all such patent rights I nternational Standard I EC 61 726 has been prepared by I EC technical comm ittee 46: Cables, wires, waveguides, R.F connectors, R.F and m icrowave passive components and accessories This third edition cancels and replaces the second edition, published in 999 This edition constitutes a technical revision I t takes into account the latest developm ents in the design of reverberation chambers as described in I EC 61 000-4-21 , which is also referencing this standard as a possible test method Furtherm ore, an alternative measurement procedure is added which is able to reduce the measurem ent time needed –4– I EC 61 726:201 © I EC 201 The text of this standard is based on the following docum ents: FDI S Report on voting 46/551 /FDI S 46/569/RVD Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table This publication has been drafted in accordance with the I SO/I EC Directives, Part The comm ittee has decided that the contents of this publication will rem ain unchanged until the stability date indicated on the I EC web site under "http://webstore.iec.ch" in the data related to the specific publication At this date, the publication will be • • • • reconfirmed, withdrawn, replaced by a revised edition, or amended A bilingual version of this publication may be issued at a later date I EC 61 726:201 © I EC 201 –5– C AB L E AS S E M B L I E S , C AB L E S , C O N N E C T O RS AN D P AS S I VE M I C RO WAVE C O M P O N E N T S – S C RE E N I N G AT T E N U AT I O N M E AS U RE M E N T B Y T H E RE VE RB E RAT I O N C H AM B E R M E T H O D S cope The requirements of m odern electronic equipm ent have indicated a dem and for a method for testing screening attenuation of microwave com ponents over their whole frequency range Convenient test m ethods exist for low frequencies and com ponents of regular shape These test m ethods are described in the relevant I EC product specifications (e.g I EC 621 53-4-3) For higher frequencies and for com ponents of irregular shape, a new test method has becom e necessary and such a test m ethod is described in this I nternational Standard This I nternational Standard describes the measurement of screening attenuation by the reverberation cham ber test method, sometim es named mode stirred chamber, suitable for virtually any type of microwave com ponent and having no theoretical upper frequency limit I t is only limited toward low frequencies due to the size of the test equipment, which is frequencydependent and is only one of several methods of m easuring screening attenuation For the purpose of this standard, exam ples of microwave components are waveguides, phase shifters, diplexers/m ultiplexers, power dividers/combiners etc N o rm a t i ve re fe re n c e s The following documents, in whole or in part, are norm atively referenced in this docum ent and are indispensable for its application For dated references, only the edition cited applies For undated references, the latest edition of the referenced docum ent (including any amendm ents) applies I EC 61 96-1 , Coaxial communication cables – Part 1: Generic specification – General, definitions and requirements IEC TS 621 53-4-1 , Metallic communication cable test methods – Part 4-1: Electromagnetic compatibility (EMC) – Introduction to electromagnetic screening measurements IEC 61 000-4-21 , Electromagnetic compatibility (EMC) – Part 4-21: Testing and measurement techniques – Reverberation chamber test methods Te rm s a n d d e fi n i t i o n s For the purposes of this docum ent, the term s and definitions given in I EC 61 96-1 and I EC 61 000-4-21 apply B a s i c d e s c ri p t i o n o f t h e re ve rb e t i o n c h a m b e r m e t h o d The reverberation chamber method for measurement of the screening attenuation of microwave com ponents consists of exposing the device under test (DUT) to an alm ost homogeneous and isotropic electromagnetic field and then measuring the signal level induced into the device –6– I EC 61 726:201 © I EC 201 These conditions are achieved by the use of a shielded enclosure, which acts as an oversized cavity (in terms of wavelength), with a high quality factor I ts boundary conditions are continuously agitated by a rotating reflective surface (m ode stirrer), m ounted within the cham ber, which enables the field to approach homogeneous and isotropic conditions during one revolution Electromagnetic power is fed to the chamber by means of an input or transmitting antenna The strength of the field inside the chamber is measured through a reference antenna The ratio of the injected power (input antenna) to the received power (reference antenna) is the insertion loss of the cavity The insertion loss is strongly frequency dependent and is also dependent on the quality factor of the cavity More detailed explanation on the m easurem ent facility can be found in I EC 61 000-4-21 It has been shown that, due to the isotropic field, any antenna placed inside the cavity behaves as if its gain was unity [2] , therefore no directional effect is to be expected I f the device under test is electrically short, its screening attenuation will be directly related to usual transfer param eters ( Zt and Zf) I f the device under test is not electrically short, the screening attenuation m ay still be related to Zt and Zf in some sim ple cases (evenly distributed leakage, periodically distributed leakage) using sum ming functions derived from antenna network theory M easurement of the screeni ng attenuati on of the devi ce under test (DU T) The m easurem ent of screening attenuation is based on the comparison of the electromagnetic field power outside the DUT to the electrom agnetic field power induced into the DUT The screening attenuation is then defined as: as = − 10 log 10 PDUT PREF (1 ) − D ins (2) or as =− log PDUT PINJ where PDUT PREF PI NJ D ins 6.1 is is is is the the the the power coupled to the device under test (W); power coupled to the reference antenna (W); power injected into the chamber (W); insertion loss of the chamber in decibels (dB) Descri pti on of the test set-up Reverberati on ch amber The used reverberation chamber shall be com pliant to I EC 61 000-4-21 I n general, a reverberation cham ber is a shielded enclosure having any shape A perfect cubic shape should be avoided for optimum performance at lower frequencies I t shall be m ade of conductive materials (copper, aluminium or steel) and shall not contain lossy materials The size of the cavity depends on the lowest test frequency For a sufficient test facility, a num ber of at least 00 m odes need to be present at this frequency The upper frequency lim it depends _ Figures in square brackets refer to the Bibliography I EC 61 726:201 © I EC 201 –7– on the quality of the shielding enclosure and cables Furtherm ore, the sensitivity of the used measurem ent instruments lim its the maximum useable frequency 6.2 M ode stirrer The mode stirrer shall be large with respect to wavelength and be bent at angles to the walls of the cham ber The mode stirrer shall be at least two wavelengths from tip to tip at the lowest test frequency 6.3 Antennas The reverberation cham ber is equipped with input and reference antennas Both antennas shall present lim ited resonances in the frequency range and shall not introduce losses; their return loss shall be better than dB For convenience, the sam e antenna should be kept for the whole frequency range However, strongly polarised and directional antennas m ay disturb m easurements due to lack of isotropic field state This is checked during the calibration of the reverberation cham ber according to I EC 61 000-4-21 6.4 Test equipment The essential test equipment and components required for an automated screening attenuation measurem ent are shown in Figure Preamplifiers, amplifiers and other control equipment may also be included in order to improve perform ance The generator and the spectrum analyser shall have a com mon, highly stable frequency reference Reverberation chamber PINJ Input antenna 50 Ω Frequency synthesizer Spectrum analyser PDUT DUT Modestirrer Stepper motor Reference antenna PREF Insertion loss measurement (calibration) otherwise 50 Ω load Motor control Computer IEC Figure – Example of a test set-up 6.5 Device under test (DUT) To avoid resonances, the DUT is inserted into a loop (made of semi-rigid coaxial cable) having a length of more than four wavelengths at minimum frequency The other ports of the DUT should be term inated with m atched loads having a screening attenuation at least dB better than the DUT The assembly is then placed inside the chamber in any orientation and location, the coupling zone being inside the area of hom ogeneous field according to I EC 61 000-4-21 –8– I EC 61 726:201 © I EC 201 This is usually the case if a minim um distance from the cham ber panels of one wavelength at the lowest frequency is kept I f the DUT is a cable, it shall be ensured that the connectors used are those recom mended for the particular type of cable, in order to m inimize interface losses I f the cable is to be used in a bent form , than it shall be tested within the lim itations imposed by a relevant standard or the manufacturer Both ends of the loop are connected to the outputs from the chamber One end is term inated with a m atched load and the other end is connected to the spectrum analyzer I t is also acceptable to terminate the DUT inside the cham ber, in which case the second leg of the loop shall be replaced by a single wire, one end being electrically linked to the DUT, the other end to a panel of the cham ber For the purpose of this m ethod of m easurem ent, waveguides and waveguide accessories (WUT) are not coaxial devices Therefore, they require to be connected to the appropriate waveguide to coaxial transition(s) in order to be tested in the reverberation cham ber The measurement of the dynamic range, insertion loss and coaxial calibrator shall be carried out with the waveguide to coaxial transition assembled in the test circuit in the same manner as for the testing of the WUT The design of the waveguide to coaxial transitions shall be such that their input and output return loss shall be better than dB Their design shall ensure that when they are assem bled into the test circuit, with a highly screened waveguide in place of the WUT, the total screening effectiveness (dynam ic range) shall be at least dB better than the specification for the WUT 6 Lin kin g devices Linking devices are normally 50 Ω coaxial lines having a screening attenuation at least dB better than the DUT Depending on practical considerations, semi-rigid or sem i-flexible cables may be used All linking lines shall be characterized for attenuation at all test frequencies prior to starting the test (attenuators, cable assemblies, etc.) Equation (2) shall be corrected, taking into account the insertion losses of linking devices: as =− P 0log DUT PINJ − Δ ins − XL (3) where XL is the insertion loss of all linking devices inside or outside the cham ber and is expressed in decibels (dB) These corrections m ay be included as part of the test program me for an automated test system They shall be checked periodically and, at least, during calibration of the test system 7.1 M easurement procedure G en eral Different approaches are acceptable depending on the perform ance of the equipment: – – – – discrete tuning (step positioning of the mode stirrer); continuous tuning (constant rotation of the stirrer); peak power acquisition on one revolution of the mode stirrer; averaged power calculation on one rotation of the m ode stirrer I EC 61 726:201 © I EC 201 –9– When deciding on a measurement procedure, it shall be recognized that: – discrete tuning is slow and requires a large num ber of sample m easurements to be taken per revolution of the mode stirrer (200 is a usual value up to 20 GHz) This does, however, result in the acquisition of more accurate m easurem ents; – averaged power calculation during one revolution of the m ode stirrer dram atically decreases the dynam ic range of the method I n this case, acquisitions shall be recorded in watts (W) and not in dBm The m easurem ent procedure described here is very econom ical in time, but requires a modern and stable spectrum analyser 7 M eas u rem en t of th e D U T G en eral Depending on the available measurem ent instruments and the need on dynamic range, there are two possibilities on m easurem ent Stan d ard m eas u rem en t The standard measurement offers a high dynamic range, especially if power controlled amplifiers are used at the output of the generator The synthesized generator is connected to the input antenna and set to deliver a constant power at a fixed frequency The mode stirrer is set to rotate at a constant speed (for example, revolution every s) The spectrum analyser is connected to the output of the device under test I ts resolution filter is centred on the em itting frequency of the synthesizer and is fixed (SPAN 0: demodulator mode) The spot scans the screen during a period which is equal to the tim e of one revolution of the mode stirrer The resulting trace which appears on the screen shows the evolution of the power as a function of the angular position of the mode stirrer After one com plete revolution of the mode stirrer, the m aximum value of the power is recorded Screening attenuation is then calculated, taking into account the attenuation of links and insertion loss of the cavity (equation (3)) The sam e procedure is repeated for all the required test frequencies F as t m eas u rem en t For a faster measurem ent, a spectrum analyser with synchronized tracking generator is used [5] The resolution bandwidth is set according to the requirem ents on dynam ic range Furtherm ore, the m axim um hold function has to be used To calibrate the cham ber set up and to determ ine the insertion loss, a first measurement is perform ed in which the spectrum analyser input is connected to the reference antenna and the tracking generator output is connected to the input antenna The mode stirrer is turning continuously with e g revolution every s The analyser needs to be set into a continuous sweep m ode with m ax hold function To ensure that a sufficient amount of independent samples have been recorded, the sweep tim e of the spectrum analyser and the revolution time of the stirred must not be equivalent or integer multiple of each other I f no changes in the m ax – 10 – I EC 61 726: 201 © I EC 201 hold values can be recognized, the insertion loss of the chamber has been determ ined Therefore, at least 20 sweeps are necessary For the determination of the screening attenuation, the tracking generator output has to be connected to the DUT instead of the input antenna and the m easurem ent sequence from the first step is repeated Then the screening attenuation sim ply can be calculated by subtracting the result from the first step from the one of the second step This can either be done in a post processing step as well as by using the thru-calibration of m odern spectrum analysers with build in tracking generators (therefore the first step is used to calibrate the system ) To check the maxim um dynamic range, the DUT needs to be replaced by a shielded match load and the second m easurem ent has to be perform ed with this configuration I f the result is not fulfilling the needs on dynam ic range, the resolution bandwidth hast to be reduced I t has to be kept in m ind that this might enlarge the m easurem ent time a lot because a reduced measurem ent bandwidth is coming along with an enlarged sweep time 7.3 Measurement of the insertion loss of the cavity The synthesized generator is connected to the input antenna and set to deliver a constant power at a fixed frequency The mode stirrer is also set to rotate at a constant speed All param eters, i.e rotation speed and spectrum analyzer set-up (except for input attenuator and reference level) shall be the sam e as those used during DUT measurement The spectrum analyser is connected to the output of the reference antenna After a complete revolution of the mode stirrer, the m aximum value of the power is recorded The insertion loss, D ins , of the chamber is then calculated: D ins = − P 0log REF PINJ + XL (4) where PI NJ is the input power (W); PREF is the output power (W); XL is the insertion loss of linking devices (dB) NOTE The insertion loss D i n s of the cham ber is a function of frequency and is a characteristic of each reverberation cham ber I t depends on construction param eters such as conductivity of panels, geom etry, lossy m aterials inside the cavity, coupling through apertures, and m easurem ent param eters such as rotation velocity of the m ode stirrer and bandwidth of the spectrum filter I t m ay be m easured either before each DUT m easurem ent or m ade part of the test program m e for an autom ated test system 7.4 7.4.1 Control of the test set-up Dynamic range Prior to taking a set of measurements, the dynam ic range of the test set-up shall be checked using the sam e linking devices (cables, connectors) and terminations as for the DUT, except that the DUT shall be replaced by a highly screened device The dynamic range shall be at least dB better than the specification of the DUT I EC 61 726:201 © I EC 201 – 11 – I n s e rt i o n l o s s o f t h e c h a m b e r I f the insertion loss of the chamber is part of the program me, in an automated test system, its suitability shall be checked before each set of measurements, a DUT being installed into the cham ber The spectrum analyzer shall be connected to the output of the reference antenna, which will be measured as if it was a DUT I f a preamplifier is used, it should not be overloaded; calibrated attenuators shall be added to the output of the reference antenna, as required, in order to prevent overload The screening attenuation should oscillate ( ± dB) around a dB value, or around the value for the attenuators, if used A systematic discrepancy indicates that the modelling of insertion loss is incorrect, either due to losses in the DUT (see 4.4) or to antenna problems M e a s u re m e n t o f a c a l i b t o r A calibrator is a device having stable screening attenuation An exam ple of a calibrator is gi ven in Annex B Such a calibrator should be measured during the full calibration procedure for the test set-up, and com pared to previous m easurem ents This enables the detection of any deviations or malfunctioning of the test set-up The calibrator is the subject of on-going study to derive its theoretical screening attenuation from Zt and Zf, in order to provide the necessary data for calibration of the test equipment 4 M e a s u re m e n t o f l o s s y D U T Some inaccuracy may occur when m easuring a lossy DUT, due to insufficient moding of the reverberation chamber This m ay be checked by verifying that during one revolution of the mode stirrer, the ratio between the m aximum and the minimum power at the output of the reference antenna exceeds 20 dB 7.5 R e v o l u t i o n s p e e d o f t h e m o d e s t i rre r The speed of the m ode stirrer has two effects on the test results: – broadening of the frequency spectrum delivered by the synthesized generator; – levelling of power peaks and gaps Due to both these effects, the sam e revolution speed and the same bandwidth of the analysis filter of the spectrum analyser should be used for both DUT and insertion loss m easurem ents I f this is not done, a system atic error up to dB could appear at higher frequencies I n practice, one turn every s is a good compromise between accuracy, m easurem ent dynam ic range and tim e saving T e s t fre q u e n c i e s The m ode stirred m ethod exhibits significant changes in measured screening attenuation for close frequencies ( ± dB) This is due to real wave impedance at the maxim um coupling position of the m ode stirrer (the averaged wave impedance is 377 Ω , but the real wave impedance may vary widely [2]) To m aintain accuracy, an adequate num ber of test frequencies should be taken One hundred points per decade is an adequate value For narrow-band m easurem ent, the screening attenuation value is the average of at least closely spaced frequencies – 12 – 7.7 I EC 61 726: 201 © I EC 201 Voltage standing wave ratio (VSWR) The individual components of the m easurem ent system should be of good quality, with an input and output return loss of dB or better This applies especially to all com ponents, cables and instrum entation in the signal paths between both the reference antenna and the DUT This requirement may be difficult to achieve for som e DUTs, in which case a graph of return loss against frequency shall be included in the documentation The return loss shall be ≥ dB Masking attenuators may also be used Measurem ent shall be limited to the frequency range that can only be propagated by transverse electromagnetic modes (TEM) Evaluation of the test results By using the described test m ethod, the screening attenuation of the DUT is measured For the reasons given in 6, the ideal curve is not a sm ooth one Whatever the frequency steps are, the m easurements will oscillate around a mean curve The order of magnitude of the oscillations is ± dB The true screening attenuation under 377 Ω conditions is said to be the average curve Care shall be taken not to confuse normal oscillations with resonances of the DUT These resonances m ay be distinguished by subdividing the frequency step, in which case normal oscillations will still present the same aspect whereas a resonance peak will be clearly apparent When loading the mode stirrer cham ber with the DUT, the results shall be corrected by the corresponding power level difference m easured with the receiving antenna I EC 61 726:201 © I EC 201 – 13 – Annex A (informative) Relationship between transfer impedance and screening attenuation For a single hole leakage, the proposed relationship between the transfer im pedance parameters and the screening attenuation is: Z t + Z f = Z1 × Z × −as / (A.1 ) where Zt is the surface transfer impedance ( Ω ); Zf is the capacitive coupling im pedance ( Ω ); Z1 is the characteristic impedance of the internal system (usually 50 Ω ); Z2 is equal to 377 Ω ; a s is the screening attenuation For distributed leakages (ideal cables for exam ple), this relation becomes: Z2t + Z2f = × Z1 × Z × −[as + S (D / λ )] / D2 (A.2) where S(D/λ ) is a sum ming function; is the length of the coupling zone; λ is the free space wavelength (m) ( λ ≡ c/f) D ( ) πD cos φ − ε π sin λ dφ S( D / λ ) = log1 π πD 0 cos φ − ε λ ∫ ( ) (A.3) where φ is the angle coordinate in a cylindrical coordination system to be integrated from 0° to 80° ε is the relative permittivity of the cable Measurem ent experience shows that these form ulae are accurate up to GHz For higher frequencies, they m ust be used with caution and the correct value for comparison should then be the screening attenuation NOTE This m ethod does not allow Zt and Zf to be calculated separatel y However, this is not usually a problem since Zf is often equal to NOTE For electrically l ong cables (m ore than 0, λ at lowest test frequency), the screening attenuation can be assum ed to be nearly constant versus frequency and length of the DUT when its surface transfer im pedance increases by 20 dB/decade This behaviour can be explained with the sum m ing function, see I EC TS 621 53-4-1 – 14 – I EC 61 726: 201 © I EC 201 Annex B (informative) Example of a calibrator The centre part of the calibrator is a 50 Ω airline End connectors are of a highly screened type such as SMA Two holes diametrically opposed are drilled in the outer screen of the airline, see Figure B ∅D t holes ∅ d IEC D = 4, × –3 m d = t = 2, × –3 m Figure B.1 – Basic construction details As d = t, Zf is negligible Zt can be com puted using the following formulae (see [3] and [4]): Zt = d [1 −7 /( D )] × f × e −3 , 68 (B ) where f is the frequency (Hz) Alternatively or where v µ0 f −3, 68 t Z t = d 2ì f × e d πD −3 , 68 t −7 Z t = × 2× d v × f × e d 3D is the num ber of holes (in this application v = 2); is π × –7 (Vs/Am ); is the frequency in hertz (Hz) The predicted screening attenuation in a reverberation cham ber is (see Annex A) (B 2) (B.3) I EC 61 726:201 © I EC 201 – 15 – a s = −20log1 Zt Z1 × Z2 (B.4) if Z1 = 50 Ω and Z2 = 377 Ω then as = [– 20 log1 Zt / (Ω )] + 46 (dB ) (B.5) With the values listed above, a s at GHz is typically + 94 dB with the slope of –20 dB/decade The Zt formula applies m ainly for a triaxial or injection line test set-up having longitudinal currents on the calibrator Thus a perfect correlation between experimental m easures and theoretical values cannot be expected Nevertheless these form ulae can be used to estim ate the order of m agnitude of the predicted screening attenuation Further studies are continuing in order to derive m ore accurate form ulae for use during the reverberation cham ber test set-up – 16 – I EC 61 726: 201 © I EC 201 Bibliography [1 ] I EC 621 53-4-3, Metallic communication cable test methods – Part 4-1: Electromagnetic compatibility (EMC) – Surface transfer impedance – Triaxial method [2] CRAWORD, M L and KOEPKE, G H.; "Design, Evaluation and Use of a Reverberation Chamber for Performing Electromagnetic Susceptibility/Vulnerability Measurements" Technical Note 902, National Bureau of Standards [3] KADEN, H.; "Loch und Schlitzkopplungen Zwischen Koaxialen Leitungssystemen" Z.Angew.Physik, 3, 951 , 44 [4] KADEN, H ; Wirbelströme und Schirm ung in der Nachrichtentechnik, Springer Verlag, Berlin/Göttingen/Heidelberg 959, Zweite Auflage [5] HI LLGÄRTNER, M and PEI ER, D ; “Effiziente Bestim mung der Kabelschirm dämpfung bei hohen Frequenzen in Modenverwirbelungskamm ern“, in EMV 2006, VDE Verlag, Berlin, 2006 _ I N TE RN ATI O N AL E LE CTRO TE CH N I CAL CO M M I S S I O N 3, ru e d e Va re m bé P O B ox CH -1 1 G e n e va S wi tze rl a n d Te l : + 41 F a x: + 22 91 02 1 22 91 03 00 i n fo @i e c ch www i e c ch