DesignCon 2008

Mode Conversion and EMI Performance of Shielded Cable Assemblies for 10 Gbps Data Transmission Jim Nadolny, Samtec jim.nadolny@samtec.com Julian Ferry, Samtec julian.ferry@samtec.com Cesar Arroyo, Samtec cesar.arroyo@samtec.com

Abstract Differential signaling has become the defacto standard for high speed (>1 Gbps) data transmission. Differential crosstalk in high density connectors, footprints, and packages is typically 1/3 or lower than single-ended crosstalk due to field cancellation. A second reason is that differential signaling reduces radiated emissions and improves immunity. Balance, as characterized by mode conversion, is considered to be the metric for the degree of reduction in radiated field strength in differential systems. In this paper, we will look at measured results showing the reduction in emissions due to differential signaling in shielded cable assemblies. Author(s) Biography Jim Nadolny – Samtec Senior SI/EMI Engineer - Jim has as a MSEE from the University of New Mexico and has more than 10 years experience in the connector industry. He has worked on EMI performance issues of cables and connectors in the past including the development of test chambers specifically for passive interconnects. His other major area of focus has been using S-parameters for signal integrity simulation and analysis. Currently, he is involved with developing cable assembly models for SPICE simulations. Future efforts will include the EMI characterization and analysis of products. Jim is a former chair of TC-10 a technical committee on Signal Integrity within the IEEE EMC Society and is a frequent presenter within the IEEE and at DesignCon. Julian Ferry – Samtec High Speed Engineering Manager - Julian earned a BSEE with an emphasis in RF and Microwave Engineering from Penn State University, University Park, PA. He has more than 20 years experience in the high speed interconnect industry focusing on test and simulation, product design and development, and team management. Julian has been granted 12 US and numerous foreign patents covering products and processes for improved signal integrity and EMC performance. Cesar A. Arroyo - Cesar is a Senior Electronics Technician specializing in High Speed Signal Integrity and EMC with 6+ years experience in the connector industry and 13 years experience in the electronics industry. Cesar also has a 2-year degree in Computer Networking Technology with a strong background in computer and networking hardware. His current area of focus is EMI/SI characterization of products spanning CAT5 mod/jack connectors to 10 Gbps backplane connectors.

Introduction The demand for increased data throughput is evident in the marketplace and is reflected in industry standards. Fibre Channel, which has a strong presence in the data storage market, has seen an increased serial data rate from 1.0625 Gbps in the late 1990’s to 8.5 Gbps today. In the desktop PC market, SATA has seen a similar data rate increase from 1Gbps to 6 Gbps. Digital computing systems which use these higher data rates must still meet regulatory requirements for radiated emissions and testing to the 5th harmonic of the highest clock frequency is required [1]. This translates to 21.25 GHz for binary encoded 8.5 Gb/s Fibre Channel signals. Differential signaling is the preferred method to transmit high speed serial data, and it is specified in the aforementioned industry standards. Crosstalk in high density packaging is greatly reduced using differential signaling and EMI is reduced. Mode conversion and intra-pair skew are two metrics of the balance, or symmetry, in differential interconnects such as connectors, PCB traces, and cables. It is generally accepted that the lower the skew, the lower the radiated emissions [2], [3], and [4]. In [2] and [3], the common mode current was shown to increase 10-20 dB as delay skew was increased from 10 ps to 200 ps. Cable assemblies are part of the passive channel in high speed data links. Cable assemblies rely on shielded cable to reduce the radiated emission profile and are not perfect shields. In [5], an extensive treatment of the theory of shielded cable is presented, but it is generally limited to frequencies less than 1 GHz. In this regime, the common mode current is assumed to be longitudinal. Aperture coupling tends to dominate due to connector effects in a cable assembly and degradation in shielding performance is observed. At higher frequencies, circumferential resonances occur characterized by the axial transfer impedance [6] and [7] leading to an increased leakage. In summary, high speed digital computing systems have EMI requirements in the 10-20 GHz range and rely on shielding and differential signaling for EMI compliance. Maintaining low skew and shielding in this frequency range is not trivial and is the subject of this paper. In this paper, we characterized cable from three manufacturers for mode conversion and for shielding performance. To examine the shielding performance, a novel measurement approach is used to derive the differential shielding performance. We will begin with a description of the measurement system followed by a presentation of the results from which we will draw several conclusions. Measurement System For this investigation, we will use two different measurement systems to characterize the shielding performance of devices. The High Frequency (200 MHz – 20 GHz) Method relies on a mode stirred chamber to illuminate the device in an electromagnetic field and record the received power. The Low Frequency (100 MHz-1 GHz) Method relies on an absorbing clamp to detect common mode signals from the device. High bandwidth splitters and baluns will be used to look at common mode or differential components in both systems.

Mode Stirred Chamber Method The Mode Stirred Chamber Method is documented in IEC 61000-4-21 and was used in this testing. The method relies on exposing a device to electromagnetic energy in a large resonant cavity (shielded room). An electrically large tuner perturbs the boundary conditions of the cavity resulting in different standing wave patterns and a randomized excitation of the device. Multiple device measurements are made at different tuner positions, and the results are averaged. Shielding effectiveness is defined to be relative to an in-band reference antenna for IEC 61000-4-21. If the shielding effectiveness is 20 dB, it means that the received power with the sample in place is 20 dB lower than the received power when a reference antenna is in place. A log periodic antenna serves as the reference from 200 MHz to 2 GHz, and a double-ridge guide horn antenna is the reference from 2 GHz to 20 GHz. This method has a practical high frequency limit determined by the instrumentation used, in this case 20 GHz. The low frequency limit is determined by the size of the chamber, which in this case is 200 MHz. The system used for this testing is a SMART 200 system by ETS-Lindgren and is shown in Figure 1.

Figure 1. Mode Stirred Chamber Method

VNA

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Absorbing Clamp Method The Absorbing Clamp Method is specified in CISPR 16-1-3 and relies on a current probe with absorbing ferrite cores. The current probe acts as the pickup device, and the ferrites dampen standing waves on the cable. For the cable testing herein, a network analyzer is configured to measure S21 where port 1 is used to drive the cable and port 2 detects the output from the current probe. Figure 2 shows the test setup used.

Figure 2. Absorbing Clamp Method Balun and Splitter Performance To investigate the role of balance on shielding performance, baluns and splitters are required. Baluns transform a ground referenced signal to a differential signal and have typically been limited to 1 to 2 GHz. Picosecond Pulse Labs has recently introduced a balun (PPL 5310) with a 4 GHz rating, and it is used in these experiments. Splitters are used to drive and receive common mode signals and are available with a more broadband performance. The PPL 5310 has a phase balance of 2 degrees up to 3 GHz and degrades to 4 degrees at 4 GHz. The amplitude balance between the inverting and non-inverting output is matched to within 0.5 dB to 4 GHz. Graphs of the amplitude and phase balance are shown in Figure 3.

COM-POWER CLA-150 Absorbing Clamp, 30M-1GHz

PPL 5310 Balun, 100M-4GHz HP 11667B power splitter, DC-26.5GHz

SR diff probes w/ 50-ohm loads

Agilent E5071C VNA, 9K-4.5GHz

P1

P2

Data acquisition PC Shielded Room

Figure 3. Balun Performance These devices are not perfect, and a simple check was performed to quantify the mode separation. The balun is connected to port 1 of a VNA, and the splitter is connected to port 2. Two precision phase matched cables are used between the balun and splitter. The balun generates a differential signal, and the splitter measures the resultant common mode. If the devices were perfect, there would be no transmission; instead, we measure a level of at least -40 dB up to 3 GHz as shown in Figure 4. Note that the insertion loss of the balun and splitter are included in this measurement, but these combined are less than 6 dB up to 10 GHz. We conclude that these components will allow us to separate modal components on our differential pair transmission lines to at least 40 dB up to 3 GHz.

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Figure 4. Mode Separation using Balun and Splitter

Fixturing For the Mode Stirred Chamber measurements, the objective is to measure the cable shielding performance per IEC 61000-4-21. Three factors complicate the measurements: 1. The shielding performance of the cable is relatively high > 50 dB. 2. The frequency range is very broad (20 GHz). 3. The cable has multiple differential pairs.

Connection to an individual differential pair was performed with phase matched semi-rigid probes. Because this connection provides insufficient isolation, a well shielded enclosure is used to prevent the probe connection from leaking. This shielded enclosure is roughly 18” x 12” x12” and is lined with RF absorber to reduce enclosure resonances. The outer shield of the multi-pair high speed differential cables were terminated to the chassis using a brass plate. The brass plate was drilled slightly larger than the cable diameter, and the cable shield was soldered to the plate. In this way, the cable braid is not degraded in the termination process. Figure 5 (left) shows the shielded enclosure and the brass plate; Figure 5 (right) shows a close up of the cable shield terminated to the chassis via a 3600 solder joint. In all cases, only one differential pair of the assembly was terminated to probes and excited during the test. The remaining pairs were not terminated as they are very well isolated (typically >50 dB) from each other due to the cable construction.

Figure 5. Shielded Enclosure and Shield Termination

Sample Description Cable samples were provided from three different manufacturers and targeting external PCIe 4x applications. There are 8 differential pairs, 1 clock pair, and 5 discrete lines in a cable. The 8 differential pairs were 26 AWG and used a drain wire and conductive tape construction. Samples A and B used a spiral wrapped foil and woven braid for the outer shield. Sample C used only a woven braid for the outer shield construction. Photographs of the samples and a cross section of the cable are shown in Figure 6 A, B, and C.

Figure 6A. Cable Sample A

Braid

Foil

Alulaminate foil

Figure 6B. Cable Sample B

Figure 6C. Cable Sample C

Measurement Results The High Frequency Common Mode test results are shown in Figure 7. “Common mode” means that the common mode output power was recorded while the mode stirred chamber was illuminated. The dynamic range curve is a measure of the system with no cable in place; notice that it degrades as the frequency increases. This is due to the increased losses in the walls, ceiling, and floor of the reverberation chamber. No measurable leakage is detected due to the instrumentation cables or the shielded enclosure, rather it is system loss degrading the dynamic range. Cable C shows a degradation in shielding performance above 3 GHz compared to cables A and B, and this was somewhat expected. Cables A and B used a foil and braid outer shield construction whereas cable C relied on only a braid. While the shielding performance is close to the dynamic range of the measurement system above 10 GHz, it is believed that it is credible data. The circumferential resonance of the cable occurs at roughly 5 GHz, and it is expected that an additional leakage mechanism should occur. Cables A and B appear to show this behavior.

Braid

Aluminized poly foil

Braid

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Aluminized poly foil

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Aluminized poly foil

Shielding Effectiveness vs Frequency

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Dynamic Range Cable B Cable C Cable A

Figure 7. High Frequency Common Mode Results The high frequency differential mode test results are shown in Figure 8. “Differential” means that the PPL 5310 balun was used to measure the differential signal on one pair of the cable. Note that the loss associate with the balun was removed so that it does not artificially increase the recorded differential shielding performance and that the measurement is considered valid up to about 6 GHz where the phase match of the balun becomes greater than 4 degrees. It was expected that the differential shielding performance would be greater than the common mode performance, and that was indeed the case.

Shielding Effectiveness vs Frequency

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Figure 8. High Frequency Differential Mode Results

The improvement in shielding (and hence EMI improvement) due to differential signaling is more clearly portrayed in Figure 9. All three cables are carefully designed and manufactured to provide low intrapair skew and have low mode conversion. It was expected that the cable with the lowest mode conversion would have the greatest change in shielding. Cable C had relatively little improvement in shielding due to differential signaling compared to Cable A and B.

Difference Between CM and DM Shielding vs Frequency

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Figure 9. Difference in Shielding Performance The differential insertion loss for the three cables is shown in Figure 10. Notice that all three cables have a “suckout” in the 8 to 9 GHz frequency range, and this is typical for cables using a foil and drain wire construction. Note that neither the differential return loss (Figure 11) nor the mode conversion (Figure 12) exhibits a peak in this frequency range.

Figure 10. Differential Insertion Loss

Figure 11. Differential Return Loss

Figure 12 shows that the cable with the best mode conversion is Cable C. Time domain skew measurements confirmed this behavior as Cable C had the lowest intrapair skew when tested with high performance TDR. In general, the measured mode conversion (Figure 12) was much lower than the improvement in shielding performance due to differential signaling (Figure 9).

Figure 12. Common to Differential Mode Conversion

It has been shown that the improvement in shielding due to differential signaling is not directly related to the differential to common mode conversion term. An improvement in mode conversion results in improved EMI performance for a given cable type, but it is difficult, if not impossible, to use mode conversion as a metric to compare between different cable types and constructions. To further illustrate this, a similar set of experiments were performed over a lower frequency band using Cat5E cable and an individual shielded parallel pair cable. Figure 13 (left) shows the Cat5E cable, and Figure 13 (right) shows the shielded parallel pair configured with semi-rigid coax test probes.

Figure 13. Cat5E and Shielded Parallel Pair Cable for Low Frequency Test The shielded parallel pair uses a braided shield construction while the Cat5E cable does not have a shield. The differential insertion losses of these two cables are shown in Figure 14.

SDD21 (1.5 meter sample length)

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Figure 14. Differential Insertion Loss

Figure 15 shows the mode conversion for these two cables in the low frequency range (100 MHz to 1 GHZ) and it has a value better than -30 dB.

SDC21 - 1.5 meter sample length

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Figure 15. Mode Conversion of Cat5E and Parallel Pair Cables

The cables were fixtured with semi-rigid probes, and the assemblies were measured using the absorbing clamp method. During this procedure, the absorbing clamp is rolled along the cable length and the maximum value was recorded so that the peak standing wave current is captured. The output of the absorbing clamp relative to the input drive level is screening attenuation. Screening attenuation is proportional to, but not the same as, shielding effectiveness. Figure 16 shows the screening attenuation for the parallel pair. Notice that the differential screening attenuation is roughly 15 to 20 dB lower than the common mode screening attenuation whereas SDC21 for this cable was 30 to 40 dB. To show the impact of source induced skew on this cable, a very large phase shift was introduced to one leg (“differential with phase shift” on Figure 16) 1.26 nS of skew was added which translates to complete mode conversion at roughly 400 MHz. The skew was added by using different length coax cables from the balun to the semi-rigid probes which connect to the cable. Notice that this path length difference equates to 360 degrees of phase shift at roughly 800 MHz which causes the deep null in the “differential with phase shift” curve.

Screening Attenuation - parallel pair cable

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Figure 16, Screening Attenuation of Parallel Pair Cable

Figure 17 shows the screening attenuation for the Cat5E cable. Notice that this cable had better mode conversion (refer to Figure 15) than the parallel pair, but the difference between common and differential mode screening attenuation is not as great as the parallel pair (10 to 15 dB).

Screening Attenuation - Cat5E cable

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Figure 17. Screening Attenuation of Cat5E Cable

Conclusions 1. Mode conversion (SDC21) is an unreliable predictor of the reduction in radiated

emissions using differential signaling. If SDC21 is 40 dB, it does not mean that the radiated emissions will be 40 dB lower with differential signaling vs. common mode signaling.

2. Using mode conversion as a metric to compare different cable types for EMI performance is problematic. Cable construction with very good mode conversion performance can have relatively minor improvements in differential shielding compared to their common mode shielding.

3. At higher frequencies, shielding performance is predicted to degrade at frequencies where the cable circumference is greater than λ/2, and the data appears to supports this. Circumferential resonance occurs at or near 5 GHz for these cables which have a diameter of about 0.35”.

4. For the 3 PCIe 4x cable constructions studied, the foil and braid shield had superior performance than the braid only shield at 3 GHz and above.

References [1] R. Gubisch, “Inside FCC Part 15 and Canada’s Corresponding Standards”,

Compliance Engineering 1999 Annual Reference Guide. [2] J. Knighten, N. Smith, J. Dibene, L. Hoeft, “Experimental Analysis of Common

Mode Currents on Fibre Channel Cable Shields due to Skew Imbalance of Differential Signals Operating at 1.0625 Gb/S”, IEEE International Symposium on Electromagnetic Compatibility, Seattle, WA, August 2-6, 1999.

[3] J. Nadolny, M. Fogg, “Radiated Emission Issues in a Fibre Channel Environment”,

Proceeding of DesignCon 1998, February 1998. [4] J. Nadolny, S. Sercu, “Characterization and Impact of Skew on Differential

Connector Systems” 18th International Zurich Symposium on Electromagnetic Comatibility, Zurich, Switzerland, February 20-22, 2001.

[5] A. Tsaliovich, “Cable Shielding for Electromagnetic Compatibility”, Van Nostrand

Reinhold, 1995. [6] F. Broyde, E. Claverlier, “Definition, Relevance and Measurement of the Parallel

and Axial Transfer Impedances” IEEE International Symposium on Electromagnetic Compatibility”, Atlanta, GA, August 14-18, 1995.

[7] L. Hoeft, “A Simplified Relationship Between Surface Transfer Impedance and

Mode Stirred Chamber Shielding Effectiveness of Cables and Connectors”, IEEE International Symposium on Electromagnetic Compatibility, Sorrento, Italy, September 2002.

[8] J. Broomall, C. Ericksen, “Meaningful Measurement of Differential Transmission

Line Skew at 10 Gbps and Above” Proceedings of DesignCon 2007, Santa Clara, CA, February 2007.

[9] R. Fluke, “Tackling SE Testing on Microwave Cables”, Test and Measurement,

March 2005. [10] G. Zhou, L. Gong, “An Improved Analytical Model for Braided Cable Shields”,

IEEE Transactions on Electromagnetic Compatibility, vol. 32, 1993. [11] C. Balanis, “Antenna Theory, Analysis and Design”, J. Wiley and Sons, 1982. [12] Picosecond Pulse Labs, Product Specification Model 5310 Phase Matched Balun. [13] IEC 61000-4-21, Testing and measurement Techniques- Reverberation Chamber

Test Methods.