TEST METHODS FOR SCREENING AND BALANCE OF COMMUNICATION CABLES

Transcription

1 533 4 ES MEHODS FOR SCREENING AND BALANCE OF COMMUNICAION CABLES. Häher *, B. Mud ** *ALCAEL, Germay; ** bedea Berkeho ud Drebes GmbH, Germay (Priciple cotact: thomas.haeher@wu.acab.alcatel.de / bmud@bedea.com) Abstract: With the ew triaxial test method all parameters, describig the screeig eectiveess o balaced ad ubalaced cables, ca be measured i oe test set-up.. Itroductio he requecy rage o symmetrical trasmissio lies - i.e. balaced pairs or quads has icreased up to the GHz rage. Especially i the ield o data trasmissio, e.g. i applicatios o horizotal loor wirig or data commuicatio, a high screeig eectiveess respectively a good electromagetic compatibility (EMC) is required. I order to measure the screeig eectiveess o those cables, ew measurig procedures have bee developed. he ew procedures are ow uder discussio i the Europea ad iteratioal stadardisatio [,,3,4,5,6,7,8,9,7,8,9]. For balaced cables the EMC is described by the couplig atteuatio a c. he couplig atteuatio a c covers both eects, the screeig atteuatio o the scree ad the ubalace o the trasmissio lie. he screeig eectiveess o cable screes is described by the surace traser impedace Z i the lower requecy rage ad the screeig atteuatio a s i the upper requecy rage. he symmetry is described by the ubalace atteuatio o the balaced trasmissio lie. With the ew triaxial measurig procedure all o those parts o the screeig eectiveess surace traser impedace, screeig atteuatio, ubalace atteuatio ad couplig atteuatio ca be measured with oe test set-up. he ubalace atteuatio is measured as the dierece betwee couplig atteuatio ad screeig atteuatio. I this way it is possible to measure the ubalace atteuatio up to the GHz requecy rage with balus, havig o cetre tap.. Physical basics. Geeral couplig equatios It is expediet to use the cocept o operatioal atteuatio with the square root o power waves, like i the deiitio o scatterig parameters [3,]. he geeral couplig traser uctio is the deied as: U Z P () P U Z he electromagetic iluece betwee the cable ad the surroudig is i priciple the crosstalk betwee two lies ad is caused by capacitive ad magetic couplig. At the P Fig. : Equivalet circuit or geeral couplig traser uctio ear ed the magetic ad capacitive couplig add where at the ar ed they subtract [,,,3,4]. he couplig over the whole cable legth is obtaied by itegratig the iiitesimal couplig distributio alog the cable with the correct phase. he phase eect, whe summig up the iiitesimal coupligs alog the lie is expressed by the summig uctio S [3,]. Whe the cable atteuatio is eglected S could be expressed by the ollowig equatio. S ( l ) ( ± β) ( β ± β ) l si β l exp( j ( β + β) l ) () For high requecies the asymptotic value becomes: S ( β ± β ) l Ad or low requecies the summig uctio becomes: S (4) Z Z P S log scale S U U S (l ) c Z ; ε r ; v ; γ Z ; ε r ; v ; γ (l ) c Fig. : Summig uctio S U U P Z Z log (l ) (3)

2 534 4 he poit o itersectio betwee the asymptotic values or low ad high requecies is the so called cut-o requecy c. his requecy gives the coditio or electrical log cables: c c, l (5) π ε ± ε r r. Scree related parameter I [3,,3] the couplig through the scree is described i detail. he ollowig gives a short summary o the literature... Surace traser impedace I may cases, above all i the lower requecy rage, the screeig eectiveess o cable screes is described by the traser impedace Z. It is, or a electrically short peace o cable, deied as the quotiet o the logitudial voltage measured o the secodary side o the scree to the curret i the scree, caused by a primary iducig circuit, related to uit legth [5]. Although the traser impedace Z covers oly the galvaic ad magetic coupligs it is commo practice to use it as a quatity as well, which icludes the eect o the couplig capacitace C through the cable scree []. I this case it is amed equivalet traser impedace Z E, icludig the eects o galvaic, magetic ad capacitive couplig. Z I I l << λ/4 U I l.. Couplig admittace, capacitive couplig impedace For the determiatio o the proper couplig capacitace there is, as stadardised quatity, the capacitace couplig admittace Y. he couplig admittace, or a electrically short peace o cable, is deied as the quotiet o the curret i the scree caused by the capacitive couplig i the secodary circuit to the voltage i the primary circuit related to uit legth [5]. U Fig. 3: Deiitio o traser impedace l << λ/4 (6) Z F Z Z Y (8)..3 Couplig traser uctio he traser impedace Z ad the capacitive couplig impedace Z F o homogeeous screes are costat alog the cable legth. he itegratio alog the cable could the be easily solved. For matched lies the couplig traser uctio is the expressed by [3,]: l ( ZF ± Z ) S (9) s, Z Z khz khz MHz MHz c c GHz GHz Fig. 5: Calculated couplig traser uctio (l m; ε r,3; ε r ; Z F) Fig. 5 shows the eect o the summig uctio S. I the cable becomes electrical log (eq. 5) the the couplig traser uctio starts to oscillate with a costat evelope...4 Screeig atteuatio he screeig atteuatio is deied as the logarithmic ratio o the power ed ito the matched cable ad the maximum peak power i the matched outer circuit, i a requecy rage where the cable is electrical log [8,5]. From Fig. 4 we observe, that the maximum peak power or electrical log cables is costat over the requecy. [ max{ Ev( ) Ev( )}] P a s log log ; () P,max.3 Ubalace atteuatio Screeed balaced pairs may be operated i the dieretial mode (balaced) or the commo mode (ubalaced). I the dieretial mode oe coductor carries the curret +I ad the other coductor carries the curret I; the scree is without curret. I the commo mode both coductors o the pair carry hal o the curret +I/; ad the scree is the retur U C U S Z S U S I Z S a) Fig. 4: Deiitio o couplig admittace Y I jω (7) C U l he through capacitace C ad thus the capacitive couplig admittace Y are depedet o the permittivity ad geometry o the outer circuit. I order to have a quatity which is ivariat o the permittivity ad geometry o the outer circuit ad is also comparable to the traser impedace Z the capacitive couplig impedace Z F is itroduced [3,8,] U U Z U U U Z U b) Fig. 6: a) Dieretial mode o a screeed pair (SP) b) Commo mode o a screeed pair (SP)

3 535 4 path with the curret I, comparable to a coaxial cable. Uder ideal coditios respectively with ideal cables both modes are idepedet o oe aother. Actually both modes iluece each other. Diereces i the diameter o the core isulatio, uequal twistig ad dieret distaces o the cores to the scree are some reasos or the ubalace o the pair. he usymmetry is caused by the capacitive ubalace to earth e (cross-usymmetry) ad the dierece o the iductace ad resistace betwee the two wires r (logitudial-usymmetry) [,4,6,]. e C C () ( R + jω L) ( R + jω ) r () L he couplig betwee the two lies is the expressed by: l u, ) γ 4 Zdi Zcom ( γ di + com ) [ jω e( x) Zdi Zcom + r( x ] e l γ u, ) 4 Z Z di com ( γ di com ) ( l x) [ jω e( x) Zdi Zcom r( x ] e x dx (3) (4) Where Z di is the characteristic impedace o the dieretial mode (balaced) ad Z com o the commo mode (ubalaced). hese are i priciple the same couplig traser uctios compared to the couplig through the scree. he itegral could oly be solved i the distributio o the usymmetry alog the cable legth is kow. For a usymmetry beig costat alog the cable legth, the traser uctio results i the same way as or cable screes (see..3) ( j ez Z ± r) u, ω di com S (5) Zdi Z 4 com I the the cable is electrical log we the have the same pheomeo as or the couplig through the scree. Depedig o the velocity dierece betwee the dieretial ad commo mode circuit the evelope o the traser uctio approaches a costat value which is requecy ad legth idepedet. However i the velocity dierece is zero, the the traser uctio at the ar ed icrease by per decade over the whole requecy rage (S ). I praxis we have small systematic coupligs together with statistical coupligs. hus u, icrease by approx. per decade ad u, by less the per decade [4]. he ubalace atteuatio is deied as the logarithmic ratio o the power which couples rom oe mode ito the other mode, related to the eedig power. a u, log (6) he measuremet o the ubalace atteuatio up to the MHz rage is uder discussio i the iteratioal stadardisatio [7,8,9]. Fig. 7 illustrates the priciple setup or the measuremet o the ubalace atteuatio. l dx i MHz Fig. 8: Ubalace atteuatio o m wiax 5 Ohm a u, a u, Ela u, Fig. 8 shows the measurig results o m o the wiax 5Ω cable with PE isulatio ad PE itersheath ad thus a velocity dierece o early zero..4 Couplig atteuatio As discussed above, balaced cables which are drive i the dieretial mode will, due to irregularities i the cable symmetry, radiate a part o the iput power. For uscreeed balaced cables (UP) this radiatio is depicted by the ubalace atteuatio a U. For screeed balaced cables (SP), the disturbig power rom the pair is additioal atteuated by the outer scree. he ubalace causes a curret i the scree which is the coupled by the traser impedace ad capacitive couplig impedace ito the outer circuit. Cosequetly the total eectiveess agaist electromagetic disturbaces o shielded balaced cable (SP) is the sum o the ubalace atteuatio a U o the pair ad the screeig atteuatio a S o the scree. Sice both quatities usually are give i a logarithmic ratio, they may simply be added ito the couplig atteuatio a C. ac au + a (7) s 3. riaxial test set-up With the triaxial set-up the surace traser impedace Z, the screeig atteuatio a S, the couplig atteuatio a C ad the ubalace atteuatio a U ca be measured. 3. Priciple o the triaxial set-up ELa u, Ubalace atteuatio at ear ed Ubalace atteuatio at ar ed, icludig cable atteuatio Equal level ubalace atteuatio at ar ed, excludig cable atteuatio O the basis o the kow reversibility o primary ad secodary measurig circuits, the ew measurig set-up, preseted i Fig. 9, actually meets the IEC 696-, despite the iterchage o geerator ad receiver. he beeits o eedig the ier system, which is termiated by its characteristic impedace, are the matchig o the geerator ad relectio ree wave propagatio over the cable legth. he triaxial test set-up cosists o a tube o brass or alumiium with a ier diameter o about 4 mm. he legth is,5 m to m, whe measurig the traser au, a u, U di, U di, L U com, Z com Zcom U com, Fig. 7: Measurig set-up or ubalace atteuatio geerator 4 termiatio load cable uder test 5 measurig receiver 3 measurig tube L couplig legth Fig. 9: Priciple o the triaxial set-up

4 536 4 impedace, ad m to 4 m or more whe measurig the screeig atteuatio or the couplig atteuatio. he total legth o m or more may be achieved by screwig sigle parts o tubes (RF-tight) together. At the ear ed the scree uder test is short circuited with the measurig tube. he electrical waves which are coupled rom the ier system ito the outer system over the whole cable legth are travellig i both directios. At the short circuited ed they are totally relected, so that at the measurig receiver the superpositio o ear- ad ar ed couplig ca be measured as the disturbace voltage ratio (8) U U he maximum requecy which ca be measured i the triaxial set-up result rom the deiitio o the wave propagatio o trasversal electromagetic waves (EMwaves) ad is give by: c g (9) π ε D + d ( ) r where d is the outer diameter o the braid o the cable, D is the ier diameter o the measurig tube ad ε r is the resultig dielectric permittivity o the outer system. With a ier diameter o 4 mm o the tube ad a outer diameter o about 3,5 mm o the braid, the maximum requecy o the system is about 4, GHz. 3. Advatages o the triaxial set-up Due to the closed measurig ixture atteuatio values up to 5 respectively traser impedace s i the µ-ohm rage ca be measured without additioal ampliiers. he advatages are i detail: isesitive agaist electromagetic disturbaces rom outside o radiatio o electromagetic power high dyamic rage > 5 high reproducibility simple measurig set-up ast preparatio o the cable sample measuremet o screeig atteuatio a S ad traser impedace Z i oe go with oe test ixture. 3.3 Measuremet o the traser impedace he measuremet o the traser impedace o a electrical short peace o cable is doe accordig to IEC 696- despite o the iterchage o geerator ad receiver (see Fig. 9). he maximum requecy up to which the traser impedace is measured i the triaxial set-up is give by: co l < () ε r 3.4 Measuremet o the screeig atteuatio he screeig atteuatio o electrically log cables is measured accordig to IEC 46A/3/CDV. he cable is electrically log whe: c l () π ε ε r r Due to the variable legth o the tube, the requecy limit may be varied i a wide rage. A detailed descriptio is give i [,,]. 3.5 Measuremet o the couplig atteuatio O the basis o the absorbig clamp method [5] the measuremet o the couplig atteuatio with absorbig clamps has bee ivestigated ad is uder study i the stadardisatio [6, 8]. Aother method with ijectio clamps is described i [5, ]. As described above the couplig atteuatio is the combied result o screeig atteuatio ad ubalace atteuatio. Because the ubalace atteuatio is legth depedet to a ucertai amout the measuremet is doe o a cable legth o m. he absorbig clamp method has two major drawbacks, the strog iluece o the outer circuit o the measurig results ad reduced sesitivity caused by the high (approx. 5 ) operatioal atteuatio o the clamp [3]. hus a ew procedure to measure the couplig atteuatio has bee ivestigated based o the procedure to measure the screeig atteuatio i the triaxial set-up [7,8]. he cable uder test (CU) is cetred i the tube ad ed i the dieretial mode via a balu. he scree o the CU is coected to the tube at the geerator side. he opposite ed o the cable is matched with a symmetrical/asymmetrical resistor etwork which matches the dieretial ad the commo mode circuit. he matchig etwork ad the coectio to the scree is shielded by a screeig case which orms the ier coductor o the outer system together with the scree uder test. scree uder test geerator balu measurig tube symmetrical/ asymmetrical load receiver Fig. : Set up to measure the couplig atteuatio i the triaxial set-up. At the iput ed o the receiver the couplig atteuatio a c ca be measured as logarithmic ratio o the dieretial power ed ito the cable ad the power i the outer circuit. he operatioal atteuatio o the measurig leads ad the balu shall be subtracted durig the calibratio procedure. o measure a certai ubalace atteuatio o the pair, a legth o about m is required. Whe this cable legth o about m is arraged betwee the balu ad the measurig tube o the eedig side o the CU, the sesitivity o the set up is reduced by the operatioal atteuatio o this cable legth. o improve the sesitivity, i a urther desig o the set up, the required cable legth o about m is arraged at the receiver side o the tube. he tube at the receiver side is ow ope ad the voltage o the secod system is picked up by a eedig through coectio. I this way, ull sesitivity is give, oly reduced by the operatioal atteuatio o the balu ad the measurig leads [9]. he errites at the receiver side o the tube act as a high resistace matchig load, parallel to the iput resistor o the receiver as well as a geerator scree uder test balu measurig tube absorber receiver remaiig cablelegth approx. m symmetrical/ asymmetrical termiatio Fig. : Set up to measure the couplig atteuatio i the modiied triaxial set-up.

5 537 4 absorber to avoid uwated ilueces. Fig. show the measured screeig atteuatio o the cable type RG 58 i a m tube i logarithmic ad liear requecy scale. Oe measuremet has bee doe i the commo triaxial set-up ad the other i the modiied triaxial set-up (ope tube) with a cable legth o m b a ,, b i MHz i MHz Fig. : screeig atteuatio o RG 58 measured with the commo ad modiied triaxial set-up a) logarithmic requecy scale b) liear requecy scale I priciple we have the same results whe measured with the commo or the modiied triaxial set-up. he ripples at low requecies i the modiied set-up are caused by the errites which have poor properties at low requecies. he dierece at high requecies i the period o the ar ed crosstalk is caused by the dieret couplig legth. I the commo triaxial set-up the couplig legth has bee cm ad i the modiied triaxial set-up 4 cm. But the Evelope, which represets the screeig atteuatio is equal. Fig. 3 show the measurig results o the couplig atteuatio i logarithmic ad liear requecy scale o a shielded twisted pair cable with idividual pair scree ad overall scree (Cat 6). he measuremet was doe i the modiied triaxial set-up with a couplig legth o 4 cm ad a cable legth o m. At low requecies the atteuatio is icreasig due to the 5 decreasig traser impedace o the cable scree (logitudial oil with a gap). At high requecies the atteuatio is decreasig due to the decreasig ubalace atteuatio. he cut o requecy (eq.5) o that cable with a couplig legth o 4 cm is approx. 8 MHz. Behid that requecy we are talkig about the couplig atteuatio which is legth idepedet. 3.6 Evaluatio o the ubalace atteuatio Fig. 4 shows the measuremet o the ubalace atteuatio a U with balus accordig to Fig. 7, the screeig atteuatio a S as well as the couplig atteuatio a C o a screeed twisted pair (wiax 5) measured i the triaxial set-up. he curve "a C - max(a S)" is calculated as the dierece o the couplig atteuatio a C ad the max. value o the screeig atteuatio a S. he evelope o the resultig curve is early idetical to the measured ubalace atteuatio a U. (he slope dow o the curves at requecies > 5 MHz is caused by o liearities o the balus). By measurig the screeig atteuatio ad the couplig atteuatio, the ubalace atteuatio o the pair up to high requecies ca be evaluated, with balus havig o cetre tap. a/ i MHz Fig. 3: couplig atteuatio o a commo Cat 6 cable, couplig legth 4 cm a) logarithmic requecy scale b) liear requecy scale a U a C-max(a S) a S a C /MHz a i MHz Fig. 4: Curves o ubalace, screeig atteuatio, couplig atteuatio ad calculated ubalace o a screeed twisted pair (wiax 5). a U: a S: a C: a C-max(a S): measured ubalace atteuatio (ar ed) measured screeig atteuatio (commo mode) measured couplig atteuatio (dieretial mode) calculated ubalace atteuatio

6 Coclusio Up to ow, the measuremet o the traser impedace Z, the screeig atteuatio a S ad the couplig atteuatio a C required two measurig set-ups, e.g. the triaxial set-up, or the lie ijectio set-up, ad the absorbig clamp set up. With the ew triaxial measurig set-up both, the traser impedace Z i the lower requecy rage as well as the screeig atteuatio a S i the higher requecy rage up to more tha 4 GHz ca be measured. Furthermore the couplig atteuatio ad the ubalace atteuatio o screeed symmetrical cables ca be measured. Due to the closed measurig ixture atteuatio values o more tha 5 respectively traser impedaces i the µ-ohm rage may ow be measured easily, without additioal ampliiers or a shielded cabi. 5 Further ivestigatio Further ivestigatios should be take cocerig the behaviour respectively the iterpretatio o measurig results o uscreeed pairs (UP). he measured couplig atteuatio icreases with icreasig the diameter o the measurig tube, due to decreasig capacitace ubalace to earth. Such measurig results, (opposite to the results o screeed pairs), will give o iormatio o the EMCbehaviour o the uscreeed pair uder istallatio, e.g. o a metallic cable tray. Literature [] Breitebach, O., Häher.: Kabelschirmug im Übergag vo MHz- zu GHz-Frequeze. tz Bd. 46(993) H.8, S [] O. Breitebach,. Häher, B. Mud: Screeig o cables i the MHz to GHz requecy rage exteded applicatio o a simple measurig method. Colloquium o screeig eectiveess measuremets, Savoy Place Lodo, 6 May 998, Reerece No:998/45 [3] L. Halme, R. Kytöe: Backgroud ad itroductio to EM screeig (shieldig) behaviours ad measuremets o coaxial ad symmetrical cables, cable assemblies ad coectors. Colloquium o screeig eectiveess measuremets, Savoy Place Lodo, 6 May 998, Reerece No:998/45 [4]. Häher, B. Mud: EMC-perormace o balaced (symmetrical) cables. Colloquium o screeig eectiveess measuremets, Savoy Place Lodo, 6 May 998, Reerece No:998/45 [5] D. Wilhelm: Measuremets o coupled oise atteuatio with special respect to requecies below 3 MHz. Colloquium o screeig eectiveess measuremets, Savoy Place Lodo, 6 May 998, Reerece No:998/45 [6] E. Bech: Improvemet o the absorbig clamp couplig ad screeig atteuatio measuremets ad stadards or symmetric ad coaxial cables ad coectig hardware. Colloquium o screeig eectiveess measuremets, Savoy Place Lodo, 6 May 998, Reerece No:998/45 [7] IEC 46A/3/CDV Shielded Screeig Atteuatio est Method [8] pren , Commuicatio Cables Speciicatio or test methods Part -6: Electrical test methods Electromagetic perormace Appedix A: traser impedace, triaxial method Appedix B: traser impedace; lie ijectio method Appedix C: screeig atteuatio, triaxial method Appedix D: couplig atteuatio, absorbig clamp method [9] Häher, : IEC C46/WG5, Ivalo 6,(Mai 997): Modiied triaxial set-up. [] W. Klei: Die heorie des Nebespreches au Leituge. Spriger Verlag 955 [] Halme, L./Szetkuti, B.: he backgroud or electromagetic screeig measuremets o cylidrical screes. ech. Rep. P(988) Nr. 3 [] H. Kade: Die elektromagetische Schirmug i der Fermelde- ud Hochrequeztechik. Spriger Verlag 95 [3] H. Kade: Wirbelströme ud Schirme i der Nachrichtetechik. Spriger Verlag 959 (Germa) [4] B. Deutsch, S. Mohr, A. Roller, H. Rost: Elektrische Nachrichtekabel. Siemes 998 [5] IEC 696-, Radio-requecy cables Part : Geeric speciicatio- Geeral Deiitios, requiremets ad test methods. [6] H. Schmid: heorie ud echik der Nachrichtekabel. Hüthig Verlag 976 [7] pren , Commuicatio Cables Speciicatio or test methods Part -9: Electrical test methods ubalace atteuatio [8] IEC C46/WG5 & SC46C/WG7 (Housto/Japa ): Proposal or modiied deiitio o CL. S. Hioshita Oct. 6, 998 [9] IEC 46C/WG7 adhoc/häher/ (998--):estig program or ubalace atteuatio [] Mud, B.: Messe der Schirmwirkug vo Kabelschirme. KM Verlag ud Kogress, EMC-Kompedium 997 S [] IEC 46/86/NP ( ): coupled oise atteuatio, ijectio clamp method [] Merz, C: Utersuchug a geschirmte symmetrische Kabel bei höhere Frequeze. Diplomarbeit FH Gieße-Friedberg, Fa. Bedea Berkeho ud Drebes GmbH, Asslar996 [3] CLC C 46X/WG3/Nürberg 8 ( ). Häher: Ivestigatio o the absorbig clamp method [4] Herrma, R: Weiteretwicklug vo Meßverahre zur Messug des EMV-Verhaltes koaxialer ud symmetrischer Leituge.Diplomarbeit FH Gieße- Friedberg, Fa. Bedea Berkeho ud Drebes GmbH, Asslar 997