High frequency cable connector for twisted pair cables

642 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 26, NO. 3, SEPTEMBER 2003

High Frequency Cable Connector for TwistedPair Cables

W. Peter Siebert

Abstract—A new cable connector for twisted pair cable usablefor high frequency applications is presented in this paper. Anelastic conductive matrix as an interface between cable andprinted wiring board (PWB) is pressed against the ends of thecopper wires of the cable, and a land grid array on the PWB,thereby making the connections. The shielding braid of the cableis lengthened by a tube structure up to the same plane as the endof the copper wires, where the shielding is connected to an earthplane on the PWB. This not only gives a sound basis for goodelectromagnetic interference (EMI) behavior, but can also serve asan adequate structure for a dc barrier of common-mode currentsin the shielding of twisted pair cables. A washer-formed capacitorbetween the earth of the PWB and the shielding tube structurewould probably be the only addition needed.

Measurements, performed on two connectors and the connectedtwisted pair cable, confirmed the hypothesis of how the perfor-mance of the new cable would be improved in the high frequencyrange compared to the SOFIX cable connector.

Index Terms—Elastomer interconnect, high frequency, MPI,signal connector, twisted pair cable.

I. INTRODUCTION

T WISTED pair cables are commonly used for the con-nections between the different parts of the system in the

telecom field. In a single cable there can be several twistedpairs, transmitting one differential signal on each of the pairs,thereby reducing the space required for the connections. Highquality twisted pair cables such as Category-5, are specifiedup to 100 MHz (defined by EIA/TIA); nevertheless, they canbe used up to significantly higher frequencies. Gotohet al.[1] have demonstrated a 625 sMb/s signal transmission overa 20 m twisted pair cable. At still higher frequencies the riskof crosstalk, as well as poor signal quality increases, insidethe cable and especially in the connector. However, excellentresults have been obtained by Hong [2], for a 2 Gb/s signaltransmission over a 20-meter Category-5 twisted pair cable,equipped with standard SOFIX cable connectors from FCI(Framatome Connectors International) [3]. This was achievedby the use of a standard adaptive equalizer with a maximaldata rate of 3.2 Gb/s. One in this series of adaptive equalizersis tailor-made for 10-Gb/s Ethernet, and as the need for evenhigher frequency applications appears, there will most likely besuitable equalizers for these higher frequencies.

Manuscript received February 27, 2003; revised May 28, 2003. This work wassupported by Ericsson UAB. This work was recommended for publication byAssociate Editor M. Swaminathan upon evaluation of the reviewers’ comments.

W. P. Siebert is with the Department of Information Technology and Media,Mid Sweden University, Östersund S-831 25, Sweden.

Digital Object Identifier 10.1109/TCAPT.2003.817656

In the datacom sector small sized connectors are common, butnot sufficient for use in telecom. For telecom applications, au-thorities and operators demand, a very high reliability as well asa high electro magnetic compatibility (EMC) for all the equip-ment. New downsized cable connectors, with better high fre-quency performance, are currently being developed. However,the SOFIX cable connector [3] is still an excellent choice fortwisted pair cable connectors on printed circuit board (PCB)front edges, combining good electrical performance in low tomedium frequencies, with superior mechanical robustness, andsufficient EMC-performance. This cable connector is presentlybeing developed for higher frequency applications, this indi-cates the potential of the connector. Although, as above men-tioned report [2] shows, already the SOFIX connector of todayhas a relatively good high frequency performance.

Regardless of design efforts, one major obstacle is stillpresent in cable-connectors for twisted pair cables in general.The two wires in a pair are impedance-matched, as long thatthey are at their correct distance to each other. If the wiresare to be connected to different contact pins inside the cableconnector, this distance has to be increased at some point.Therefore, the impedance matching is also broken at thispoint. The length of these mismatched wires is considerable.For the SOFIX cable-connector, the total mismatched lengthdown to the printed wiring board (PWB) is between 34 and38 mm, there is a potential risk of resonance from frequenciesas low as 1.5 GHz, slightly higher than the approved 1.25 Gb/s.Additional to the risk of resonance from the wires, there isthe danger of cavity resonance from the connector housing,according to measurements by Martenset al. [4].

On top of the threat of resonance effects, a number of otherfactors need to be addressed. In the untwisted part of the cableas well as in both parts of the cable-connector, the escalation ofcrosstalk at high frequencies is considerable; a similar case hadbeen analyzed by Kimet al.[5]. Inside the cable connector casethe wires can be close to each other in a random order, and thecrosstalk between each of the single wires cannot be predicted.Another important phenomenon is a conversion of differentialmode voltage to common mode voltage, thereby damping thesignal. The common mode voltage is also a source for electromagnetic interference (EMI), as well as crosstalk.

Moreover, downsized cable connectors have generally poorshielding arrangements. Though the SOFIX cable connector isbetter than average, there is room for improvement. The termi-nation of the shielding braid is accomplished by squeezing thebraid between two washers. They are in turn connected to theconnector housing. Finally, the connector housing is connectedto earth by means of connecting springs. The outcome of this

1521-3331/03$17.00 © 2003 IEEE

SIEBERT: HIGH FREQUENCY CABLE CONNECTOR 643

arrangement is that the housing is connected between two con-tact-resistances; there is a risk that the cable connector housingcan behave as an antenna at high frequency.

Besides the importance of good connections to earth, thereis the question of whether the earth potential is reliable enoughat different points in a system, placed in different cabinets. Intelecom systems the distance between the different connectionscan be rather large; in such cases it is doubtful if the earth po-tential of both these connected parts is on the same level. If not,the voltage differences will give rise to common-mode currents,circulating on the shielding of the cable; these currents can beconsiderable and dangerous. In fact they can be so large thatthere is a risk of fire. However, the current loop can be brokenwith the help of a DC-barrier.

Despite these problems with the available connectors, a datarate of 10 Gigabits per second (Gb/s) should not be entirely outof reach, given the following combinations.

1) A cable with excellent high frequency characteristics.2) Transceiver and receiver circuits tailor-made for very high

frequency use.3) Possibly both pre-emphasis and equalizer.4) The optimal coding algorithm.5) Cable connectors with outstanding performance also in

the high frequency region (at present probably the mostcritical).

This data rate is feasible if a short cable length is sufficient. Itwould be of immense value if we could increase the frequencyat which signals can be transmitted on copper cable.

In this study, the SOFIX cable connector [3] approved up to1.25 Gb/s, will be compared with the new cable connector devel-oped by the author, as a method of verifying the new approachto connector design.

II. NEW CABLE CONNECTOR

A superior cable connector could be designed, if the physicalinterface between the twisted pair cable and the printed wiringboard (PWB) could be removed.

The twisted pair cable might in some way be pressed againstthe receiving part of the PWB, thus forming the connec-tions both for the signal-connectors and the connections forthe shielding. Such a design would remove essentially allimpedance mismatches, except the connecting surface itself.Even the related crosstalk would not increase much over thelevels generated by the cable and the PCB.

The mechanical limitations of such a configuration would betwofold.

1) It does not guarantee a proper contact and a simultaneousbalanced pressure on all the connections, because of me-chanical tolerances and tractive force on the cable.

2) It does not dictate where the different parts of the cableare placed in the connecting area.

Searching for a compromise for the first problem, an elasticconductive matrix as an interface was chosen. This way themechanical tolerances could be balanced. One solution for thesecond problem was to fix the different parts of the cable into amatrix. The shielding braid was to be expanded with the aid ofa tube, creating an outer part of the matrix, to be connected to

earth on the PWB. A contact spring as ground connection hadto be manufactured for the prototype, but in future it should bepossible to get the ultimate connection to earth with the helpof a special elastic contact matrix for both signal and earthconnections.

On top of these speculations about a better cable connectorcomes the problem of how to assemble it. The SOFIX cableconnector and other cable connectors are assembled by hand.This leads to very high costs and to varying quality. A new, bettercable-connector, should be possible to assemble automaticallyor at least semi-automatically. In spite of these compromises,a design like this should hopefully still provide an acceptablesolution for a cable connector for high frequency solutions.

These considerations resulted in the design of a prototypebased on a standard twisted pair cable TEL426 001/004 [6], andthe use of the contact matrix metalized particle interconnect(MPI) from Tyco Electronics. This selected twisted pair cable[6] is only specified up to 100 MHz, though measurements havebeen done up to 600 MHz. Fig. 1 illustrates the design of the newcable connector (filed patent).

The chosen length of the test cable was 500 mm, includingthe cable-connectors, one on each side of the cable. It was as-sumed that such short cable would not noticeably influence theresult of the measurements of the cable-connectors. Besides thechosen elastic contact matrix, there are numerous other possibil-ities, but MPI had passed the Bellcore test [7] for telecom appli-cations and was ready for use. Inside the twisted pair cable, thecopper wires surrounded by the dielectric material are spaced0.8 mm from center to center, whereas the contact matrix has1 mm spacing. As each cable with different spacing betweenthe wires and a different cable diameter, needs special connectorparts, there is also a need for different spaced contact matrixes.Fortunately, different MPI matrixes with different spacing areavailable.

According to Tyco Electronics a contact force of 0.5 N percontact should be applied on each of the contact columns ofthe MPI, compressing them to a height of 0.7 mm. If, as onthe prototype, a solid distance-bar height of 0.7 mm is used,a larger pressure than necessary can and should be applied onthe cable contact structure. As a result, the distance-bar wouldabsorb all pressure exceeding 0.5 N per contact. In this way theconnections are secured, even if there is a strain on the cable.

Studies on the MPI have been carried out by Liuet al. [8]and by Xieet al.[9], showing that the silver-particle-filled elas-tomer connectors have the tendency to decrease in height whileexposed to pressure, heat and time. Thus, the effect of this re-laxation is a reduction of the contact pressure and consequentlyan increase of the contact resistance. The results indicate thatthe contact resistance due to interactions between the silver par-ticles embedded in the elastomer is highly dependent on thetemperature. Hence, in the temperature range of 20 to 50C,the contact resistance is close to 10 . This is in good agree-ment with the results in the qualification report [7] from TycoElectronics.

A nut on the cable provides the pressure, forcing the cableconnector in the direction of the PWB. The nut is screwed intothe cable-connector’s counterpart, a housing for the contactmounted on the PWB, where a land-grid array is used as the


Fig. 1. New cable connector.

contact area. An important requirement is that the cable con-nector is perpendicular to the board, and therefore the surfacesof the PWB and the cable-interface will be parallel. Thus thecable entry hole in the housing has to be perpendicular to thePWB, and the contact area of the cable must to be perpendicularto the tube structure mounted on the cable.

The grouping of the different parts of the cable into a matrixis accomplished with the help of a nonconducting spacing disc.This spacing disc was manufactured so that the matrix holeswere cut open to the surrounding surface (Fig. 2). Hence thewires need not be threaded through the matrix-holes; instead thewire can be inserted sideways. The cable wires are only strippedon the part of the wire that is fixed into the matrix, because of theneed to identify the surplus cable parts by the different colors oftheir insulation material (Fig. 3).

Using super glue, the partially stripped cable-wires are con-nected to the spacing disc. This operation is a delicate matterbecause of the risk that the glue may flow onto the cable, due tothe capillary effect. The impedance between the wires is a resultnot only of the dielectric material, but also of the relative dielec-tric constant of the air near the wires. If the super glue replacesthe air, the effective dielectric constant is dramatically changed,as was learned from the first prototype.

Before this step, the shielding braid is squeezed betweenboth the metal holder for the spacing disc and a tube formedmetal sleeve, embracing the shielding braid. Both these partsare cone-shaped, assuring a large contact area, and a lowcontact resistance between the shielding braid and its expandedtube-structure. As these parts should be transformed into onerigid section, the parts are carefully soldered or glued with anelectrical conductive adhesive, as in the tested prototype. Inthe prototype phase the soldering is not an easy task becauseof the risk of deforming the dielectric material inside the cable,but it has been done successfully. However, if soldering ischosen in production, laser soldering is preferable.

Subsequently, the spacing disc with the assembledcable-wires is pressed and glued into the tube-structuredescribed above. Next, the surplus cable-wires are removedand the surface of the spacing disc, including the assembledcable-wires, is machined perpendicular to the tube-structuremounted on the cable. At this point the copper-wires appearclearly as a matrix in this grinded surface. In the prototypethis was the final step, but in an industrial cable connector the

Fig. 2. Cable matrix.

Fig. 3. Stripped wire before assembly.

visible surfaces of the copper-wires would be plated with gold.Of course, between copper and gold there should always be adiffusion barrier.

III. M EASUREMENTSET-UP

The basic idea of the measurement was to compare theSOFIX cable-connectors [3] with the newly developed cableconnector (Fig. 4).

As pointed out earlier, the chosen length of the measuredtwisted pair cable TEL426 001/004 [6] was 500 mm. This lengthincluded the cable-connectors, one on each side of the cables.The two cables with SOFIX connectors were connected in twodifferent ways, the differential pairs were either connected di-agonally between the connector column A and B or in parallelinside the same column (Fig. 5).

For the purpose of stable test conditions, a four-layer test-board was manufactured (Fig. 6). Two SOFIX connectors wereplaced in one region, as the counterparts for the cable-connec-tors, along with two receiving parts for the new connector onanother section of the test-board. SMA-connectors were dis-tributed around these connectors for easy and controllable con-nection to the measurement equipment. The strip-lines betweenthe tested connectors and the SMA-connectors were measuredto ensure equal length and to avoid skew.

Two strip-lines with connected SMA-connectors were lo-cated at the center of the test-board. This allowed us to measureseparately the influence of the measurement cables betweenthe measurement equipment and the SMA-connectors, togetherwith the test-board with strip-lines and vias.

The time domain reflectometer measurement equipment usedwas a sampling oscilloscope Tektronix 11 801A with a TDRsampling-head SD-24, and measurement cables from Tektronix


Fig. 4. SOFIX and new connector (right).

Fig. 5. SOFIX connections inside the connector.

with a length of 0.5 m. The twisted pair cable with the con-nectors to be measured was attached to their counterparts onthe test-board, and the measurement cables connected to thesamplings-head as well as the proper SMA-connectors. All theother SMA-connectors both on the opposite side of this connec-tion and all the parallel connections were terminated with 50to ground, that is 100 between the differential pairs of theconnections.

As equipment for the eye-diagrams measurements, a sam-pling oscilloscope Tektronix CSA803 with a signal analyzersamplings-head SD-22 and pulse pattern generator AnritsuMP1608A was used. For both the S-parameter-measurementsand the crosstalk measurements, Hewlett Packard NetworkAnalyzer 85 047A and 8753C were used.

All the measurements were performed at Ericsson AB inÄlvsjö Sweden.

IV. TDR MEASUREMENT

All TDR measurements are relative; the instrument sendsa pulse through all the connected objects and compares thereflections from these objects to those created by a standardimpedance. On the screen the reflection coefficient is displayedas the ratio of the reflected pulse amplitude to the incident

Fig. 6. Test-board.

pulse amplitude; this is done continually all the way throughthe measured objects. By choosing the proper horizontal offset,the reflection coefficient from any section in the measurementchain can be shown.

The rise time (at 30 ps) and the settling of the incident pulse,is degraded by the connection length through the measured ob-jects. As the measurement cables were of good quality and nottoo long, the measurements were not significantly degeneratedas a result of these cables. Furthermore, the measurement of theimportant parts at the near end cable connector and the majorpart of the twisted pair cable were correct enough. However, itcould be observed that the measurement at the far end of thetwisted pair cable was not reliable.

The measurements displayed clearly the impedance mis-match of the SOFIX-connector and somewhat surprisingly theimpedance mismatches of the SMA-connectors; in fact themismatch amplitude was even larger than from the SOFIX-con-nector. As the electrical length of the mismatched section dueto the SMA-connectors was short compared with that of theSOFIX-connector, the influence on the signal should be lessimportant. All evidence indicated that this was because of thelarge connecting pins of the SMA-connectors, as well as theplated through-hole in the PWB. Subsequently, the connectingpins were shortened; the length of the mismatch was shortenedat the same time.

The result for the new cable connector was completelydifferent compared to the measurement result of the SOFIX-connector (Fig. 7). The impedance mismatch was small, almostas small as the impedance mismatch of the cable itself (Fig. 8).Fig. 7 furthermore shows that the influence on the systemextends over a shorter length, as should be expected from theimproved geometry of the connector.


Fig. 7. TDR measurements with nominally 100 between the differentialpairs. SOFIX-connector and new-connector displayed in the same diagram.

Fig. 8. TDR measurements as in Fig. 7, the new connector, and most of thetwisted pair cable.

The end of all the connectors was easy to distinguish in theTDR measurements. Bending the cable produced a noticeableshift in the impedance mismatch inside the cable. The pointat which the shift could no longer be detected was thereforethe inlet to the connector. This behavior in the twisted paircable is due to the cable design: the twisted pair is impedancematched not only by the differential coupling between thewire pair; the distance to the outer shield is also a part of thecapacity matching. When the cable is bent, the differentialparts move both with respect to each other and relative to theother shielding. The outer shielding is supposed to be circular,but as the twisted parts slide over and between the other parts,the cable is often elliptical. This gives rise to slightly differentcharacteristic impedances for the twisted parts inside the samecable, and changes with the length and the twisting pitch aswell.

As mentioned in Section II, “The new cable connector,” theconnecting copper-surface of the new connector was not coatedwith gold; therefore these measurements were repeated after aperiod of four months. These new TDR measurements displayedin Figs. 7 and 8, confirmed that the oxidation of the copper-sur-face during this period had not led to any noticeable differencescompared with the earlier measurements.

V. EYE-DIAGRAM

Eye-diagrams are not to be seen as a tool to reveal exact mea-surements; it works on a statistic level and gives an impressionof how the data-communication would behave. The significanceof this impression is how safe the identification of the true valueof the data will be. All the disturbances the imposed data pulsesare exposed to, by traveling through this connection, leads todifferent deformations of the signals. A consecutive number ofrandom data pulses are plotted on the oscilloscope screen ontop of each other, after that these signals have gone trough allthe connections. As all those random zeroes and ones are piledupon each other, a pattern of closed eyes can be seen.

The result of these eye-diagram measurements exposed thedifferences in how the signal was distorted when traveling allthe way through the measurement cables, the SMA-connectors,the test-PWB and finally through the connectors to be tested.At high frequency, the importance of all these connected parts,beside of the device under test, is very large. In order to distin-guish the influence of these parts, one measurement was madeat the control connection at the center of the test-PWB; here theconnectors under test were excluded.

At 5 Gb/s this result was not much different from the bestoutcome of the measurements with two times the new connec-tors, one on each end of the cable. The handmade prototype hadslightly different performance at each part of the connector, butalso with the worst combination of the parts in the two con-nectors, the result of the measurements of the new connectorwas satisfactory (Fig. 9), whereas the result with two times theSOFIX connector was quite different, the eye was almost closed(Fig. 10).

As shown by FCI, Teradyne and Thomas&Betts, already onevia alone on the test-PWB as a part of these connections, has atremendous influence at high frequencies on the Eye-diagram.Thus there was no point to use higher frequencies in these mea-surements; there would not be a chance to distinguish the sourceof the disturbances of the signal.

VI. S-PARAMETER-MEASUREMENT

Mixed mode S-parameters are important parameters for thecharacterization of balanced circuits; a theory on this had beenworked out by Bockelman and Eisenstadt [10]. With help ofmathematical conversion, the mixed mode S-parameters can betransformed from the different single ended S-parameters mea-surements. The mathematical model for the conversion of thesingle mode and the mixed mode S-parameters is developedand implemented as software in a Labview platform. In thismanner the calculated mixed mode S-parameters, using a mea-suring system developed by Tang [11], can directly be presentedfrom the measured data from the device under test.

Totally 16 mixed mode S-parameters can be achieved for atwo-port differential circuit, however in this study only two ofthese parameters were chosen as the most valuable for the val-idation of the performance of the connectors. During the mea-surement process the influence of the measurement cables wasestimated and removed.


Fig. 9. Eye-diagram with the signal-frequency of 5 Gb/s. On top only themeasurement cables and test-traces on the PCB. Below the best and the worstconnection of the new connector.

The parameter is defined as the differential transferloss and represents the differential signal losses, as the signalis transmitted from port one to port two. This parameter is re-lated to the signal quality for a differential transmission line forsignal integrity issue. From the measurements of the transferloss it can be seen that the attenuation of the signal with the newconnector is far less than with the SOFIX connectors, especiallyin the high frequency range (Fig. 11).

The parameter defines the differential transfer con-version loss and represents the differential signal convertedto common mode signal during its journey from port one toport two. The differential transfer conversion loss increasesnormally with frequency because of the common mode currentgiven by the unbalances in the connection pair. This is relatedto the electromagnetic interference issues (EMI), due to thetransmission of differential signals. However, for differentdevices under test, the parameter is not comparablebecause the attenuation parameter will also affect the

. If the value of the parameter is high enough,the reduction of the signal amplitude is significant. Thus asthe lower amplitude is the numerator for the calculation of

Fig. 10. Eye-diagram of the SOFIX-connector, with the signal-frequency of5 Gb/s.

Fig. 11. Transfer Loss for SOFIX-diagonal, SOFIX-parallel, and newconnector.

the parameter, the parameter will be smaller. In dB-scalethe corrected parameter can somehow be calculated by

(Fig. 12). At zero dB, the entire differentialsignal is converted to common mode signal and largerparameters than zero dB cannot be achieved in reality.

The measurements of the conversion loss are not so easy to in-terpret. It looks like all connectors are quite similar to each otherand as if the new connector is not good at all at low frequen-cies. But the most important thing to realize is that the SOFIXconnectors reaches zero at 3.8 GHz and the new connector isnot even close to zero even at the highest measured frequen-cies. At zero there is, as mentioned above, not any differentialsignal left to transmit. Therefore, if the only limitations wouldbe these corrected parameters, the SOFIX con-nectors cannot be used at higher frequencies than approximately3.5 GHz, whereas the new connector has no such restrictions upto the highest measured frequencies. The peak of the transferconversion loss at very low frequencies for the new connectorwill be further investigated, but this should not be a limitationfor this connector developed for high frequency use.

VII. CROSSTALK MEASUREMENT

The crosstalk measurements were performed with the samemeasuring equipment as with the S-parameter measurements


Fig. 12. Corrected conversion loss for SOFIX-diagonal, SOFIX-parallel, andnew connector.

except that two balun-transformers were used. Fig. 13 illus-trates the measuring circuit. With the help of one balun a singleended signal was transformed into a differential signal and fedto the active wire-pair. The victim wire-pairs were connectedto the other balun-transformer. Using the balun-transformers,the induced common mode current is blocked, thus the differ-ential crosstalk is measured. As receiver circuits for differen-tial signals only react on differential signals, the common modecrosstalk is of no interest for the signal transmission.

In the network analyzer the induced signal, as well as thetransmitted signal, is processed and the result is either theFEXT (far end crosstalk) (Fig. 14) or the NEXT (near endcrosstalk) (Fig. 15) depending on, whether the victim wire-pairsis connected close to the signal input, or the other side of theconnection (Fig. 16).

The amount of crosstalk in the diagram due to the connectorsis not easy to distinguish from the different sources for crosstalk,as the connectors and both the cable and the test-PWB gen-erate a substantial part of the crosstalk. Therefore, the level ofcrosstalk of the cable connectors cannot be discerned from thetotal measured crosstalk. According to measurement data on thecable [6], the NEXT value is 41 dB, measured on a cable lengthof 500 m and at 100 MHz. The corresponding FEXT value is,measured on a cable length of 50 m and at 100 MHz, 35 dB.The manufacturer performed these measurements on long ca-bles (50 m and 500 m each), and the reflections because of the

Fig. 13. Crosstalk measurement with two balun transformers.

Fig. 14. Far end crosstalk for SOFIX-diagonal, and the new connector.

Fig. 15. Next end crosstalk for SOFIX-diagonal, and the new connector.

Fig. 16. Near-end and far-end crosstalk.

impedance mismatch at the ends of the cables were quenchedby the attenuation of the cable. In these measurements the ratio


of attenuation is low because of the short length of the cable,therefore the ripple can be seen. The results of the resonancephenomena on both the active pair induced to the victim pair, ason the victim pair itself, are half-wavelength reflections in thecable. Since the frequency range is higher than the equivalenthalf-wavelength, all the multiples of this basic frequency span,up to the highest measured frequency can be seen as ripple.

Simple considerations give a relation between delay time andfrequency as

According to Ericsson Network Technologies [6], the delay onthe test cable is 4.8 ns. With a cable length of 500 mm, thefrequency span between each of the ripple should be

This calculated frequency corresponds to the ripple in Figs. 14and 15.

Superimposed on this undulation, the connector and testboard reflections cause an arc-shaped influence. Consequentlythe interpretation of the measurement diagrams should be:there is only the difference between the connectors that couldinfluence the result significantly. The result for the NEXTmeasurements appears, due to the logarithmic scale, as ifthere is no noticeable difference between the connectors,nevertheless the new connector should be between 5 and 10 dBbetter. Conversely, the FEXT crosstalk for the new connectoris reduced significantly compared to the measurements on theSOFIX connectors.

VIII. C ONCLUSION

The TDR measurements confirm the hypothesis that the newcable connector should reduce the impedance mismatches; theyindicate that the cable connector no longer constitutes a bottle-neck for high frequency transmission.

Both the eye diagram and the crosstalk measurements aremainly portraying the difference between the measured connec-tors. However, the crosstalk measurements have exposed someinteresting features that should be investigated in a future study.

Using an equalizer the eye diagram of both the testedconnector configurations will be immensely enhanced, i.e.,the equalizer gives significant improvement both with respectto the signal attenuation and the dispersion, due to the cable.However, the S-parameter measurements reveal that thisimprovement for the SOFIX connector is not valid for muchhigher frequencies. If the corrected parameter, describingthe differential transfer conversion, reaches zero, there is nolonger any differential signal to transmit. In the case of theSOFIX connector, this point is already reached at 3.8 GHz.On the other hand, the poorest result measured on the newconnector is reached at 5 GHz; where the differential signal isdown to 54% of the initial value.

One of the limitations of this study is, that it was not possibleto discern the contribution of the different parts in the connectionto the total result. As had been revealed in several reports,

the influence of the PWB as a whole and in particular thevia-technology is substantial. In this case the via-stub on theconnection pad of the new connector was 1.2 mm in length.Moreover, the connection pins on the SMA connectors whereprotruding with additional 1.5 mm, even after they had beenshortened. The plated trough hole for the connection of theSMA connectors where also of a significant size, the hole was1.5 mm in diameter with a pad size of 2.5 mm.

On account of the temperature dependence of the contact re-sistance on the MPI, studied by Liuet al. [12] and by Xieet al.[13], it should be noted that the measurements had been under-taken at room temperature. If low contact resistance is the mainissue, the connector should be used in areas where the temper-ature is neither too cold nor too warm; a connection field on abackplane would be the optimal place.

Further studies should also address the electro magnetic com-patibility (EMC) behavior of the new connector.

If the new cable connector is further developed from this pro-totype version into an industrial product, it should be possible toassemble in an automatic or at least semiautomatic form. Thisshould lead to lower costs and stable quality.

It should be noted that MPI has turned out to be fragile and iseasily damaged by any improper manipulation. A rugged designshould always conceal the MPI inside a protective structure inthe future connector design, the MPI has to be placed at thebottom of the cable entry hole of the connector housing.

The height of the distance-bar was assumed to be 0.7 mm inorder to achieve a contact load of 50 g per contact point. Inves-tigations by Xieet al. [14] led to the conclusion that the con-tact load should be at least 60 g per contact point. This far, theresults of the measurements have been excellent, but in futurestudies, the question of the optimal distance-bar height shouldbe addressed.

As the structure of the new connector is also suitable forinclusion of a dc-barrier of the common-mode currents in theshielding of twisted pair cables, a new study on this matterwould be appropriate.

ACKNOWLEDGMENT

The author wishes to thank A. Anton, J. Irebro, J. Fredriksson,I. Karlsson, M. Wilhelmsson, Dr. H. T. Ericsson AB, and H.Hesselbom, for their valuable contributions and comments, andC. Köhler, Tyco Electronics, for his support, samples of MPI, aswell as Measurement reports on the MPI.

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[8] W. Liu, M. G. Pecht, and J. Xie, “Fundamental reliability issues associ-ated with a commercial particle-in-elastomer interconnection system,”IEEE Trans. Comp. Packag. Technol., vol. 24, pp. 520–525, Sept. 2001.

[9] J. Xie, M. Pecht, D. DeDonato, and A. Hassanzadeh, “An investigationof the mechanical behavior of conductive elastomer interconnects,”in Microelectroncs Reliability 41. New York: Elsevier, 2001, pp.281–286.

[10] D. E. Bockelman and W. R. Eisenstadt, “Combined differential andcommon-mode scattering parameters: Theory and simulation,”IEEETrans. Microwave Theory Tech., vol. 43, pp. 1530–1539, July 1995.

[11] H. Tang, “Determination of mixed mode S-parameters for bal-anced lines: Theory and practice,” Tech. Rep., Ericsson Int. Rep.ERA/DST/SE 00:0086, 2000.

[12] W. Liu, M. G. Pecht, and J. Xie, “Fundamental reliability issues associ-ated with a commercial particle-in-Elastomer interconnection system,”IEEE Trans. Comp. Packag. Technol., vol. 24, pp. 520–525, Sept. 2001.

[13] J. Xie, M. Pecht, D. DeDonato, and A. Hassanzadeh, “An investigationof the mechanical behavior of conductive elastomer interconnects,”in Microelectroncs Reliability 41. New York: Elsevier, 2001, pp.281–286.

[14] J. Xie, C. Hillman, P. Sandborn, M. G. Pecht, A. Hassanzadeh, andD. DeDonato, “Assessing the operating reliability of land grid arrayelastomer sockets,”IEEE Trans. Comp. Packag. Technol., vol. 23, pp.171–175, Mar. 2000.

W. Peter Siebertreceived the M.Sc. degree in elec-trical engineering from the Royal Institute of Tech-nology, Stockholm, Sweden, in 1992 and is currentlypursuing the Ph.D. degree at Mid Sweden University.

Since 1993, he has been a PCB Designer at Eric-sson, Stockholm. His main area of interest lies in theelectronic packaging area.

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