Shielding 4K2K Display Graphics Connector Solder Tails Mitigates EMI

Transcription

1 Shielding 4K2K Display Graphics Connector Solder Tails Mitigates EMI Gregory A. Young I-PEX Connectors A Division of Dai-ichi Seiko Ltd. Co. of Japan greg.young@ipex-us.com Abstract: 4K2K Display Resolution requires high-datarate (HDR) differential signaling of 4-lanes of edp at 5.4 Gbps and the emerging 8K resolution displays that require the transfer of 8.1 Gbps data rates. If the embedded display graphics connector is placed in close proximity to the wireless radios and antennas carrier signals, then the radiated electromagnetic interference (EMI) energy of the high-data-rate being transferred across the graphics connector solder tails causes interference with the wireless carrier band frequency bands and causing the communications links to fail to operate properly. Key words: embedded, HDR, graphics connector, 5.4 Gbps, 8.1 Gbps, edp, wireless radio, antenna, EMI, shielding Introduction When working with high-data-rate (HDR) signals, covering the connector solder tails will help stop EMI radiating from the HDR display graphics connectors. Today solutions usually require some metallized tape or metal cover to be added over the connector solder tail area. There is no need to add tape to the I-PEX Cabline -CAII Connector shielding cover (Fig. 1). with solder until a solid grounding bar is formed along the span of the coaxial wires (Fig. 2). Figure 2. Solid Grounding Bar for Coax The first view (Fig. 3) depicts the internal connector plug housing construction with the grounding bar soldered to the shield braids of the coaxial wires where the H_Ground between the differential signals is connected with a wire. The second view shows where a ground finger was used to interconnect the ground bar to the ground contact in the housing. This technique reduces MCX bundle diameters. Figure 1. I-PEX Cabline-CAII with Integrated Shielding The Cabline-CAII Connector has an integrated solder tail cover that prohibits EMI from radiating when HDR signals are being transferred. I. Faraday Wire Termination Area I-PEX HDR graphics connectors provide an internal Faraday caged wire termination area. The coaxial wires are attached to the cable side connector plug housings. The housings and wires are then covered by a metal shell. The shell is soldered to a grounding bar that is part of the wire preparation assembly. During the wire preparation, the shield braids of the coaxial wires are soldered to the grounding bars which are applied to the top and bottom of each coaxial wire. The void between the wires is filled Figure 3. MCX Attachment to Housing Page 1

2 The shell completes the shielding encapsulation of the wire to connector transition area mitigating EMI leaking from the wire attachment area (Fig. 4). Figure 6. Cabline-CA Connector Grounding Under Receptacle Figure 4. Faraday Caged Wire Attachment Area When the cable plug shell connects to the receptacle shell, shielding currents are taken to ground through the receptacle shell that is attached to ground pads on the PCB (Fig. 5). The Faraday cage shells shielding the currents are routed to grounds along the shortest pathway-to-ground across the span of the connector. II. Two MCX Equal a Balanced Line The differential signals (Figures 3 and 7) are equal and opposite. They require a balanced transmission line to avoid common mode voltage that is a major contributor to EMI. Figure 7. Differential Signals Need Balanced Line For more than a decade, millions of display cables per month have used the two coaxial wires terminated in a balanced line configuration (Fig. 8). The grounding bar construction (Fig. 3) provides a natural path for the negative and positive shielding currents to cancel their capacitance (voltage) along their pathways to ground. Figure 5. Grounding the I-PEX Cabline -CAII Connector Shells Data rates continue to increase and it was apparent that shorter grounding paths provide better signal quality I-PEX developed the Cabline -CA Connector, a 0.4mm pitch connector that added grounding contacts under the receptacle, which provided a shorter shield path-to-ground that is desired in the transfer of the higher data rates (Fig. 6). Figure 8. Balanced with Two Coaxial Wires Page 2

3 Differential to Common Mode Conversion, known in s-parameters as Scd, has caused EMI radiation issues for years. It has become a parameter specified by USB, VESA and PCI-SIG organizations. The Scd21 is the leakage from the differential lane to the common mode at the receiver end. It is formally called Transverse Conversion Transfer Loss (TCTL). If the Scd21 is near or below -25dB, then the Scd mode conversion (Fig. 9) has negligible contribution to the system noise. The connector transfers the 4 edp lanes at the HBR2 5.4 Gbps data rate through the connector with good signal quality. All performed acceptably. However, when an LTE wireless radio signal using a carrier frequency near 2.7 GHz was implemented in the device and was located near the Cabline-CA Graphics Connector, the LTE 2.7 GHz carrier signal failed to communicate properly. IV. Graphics HDR and LTE Frequency Interference Data rates are related to frequency by the term Nyquist frequency. The Nyquist frequency of different protocol data rates can be seen in Figure 11. The HBR2 5.4 Gbps Nyquist is 2.7 GHz which is the same frequency as some LTE carrier bands. Figure 9. Diff. to Common Mode Conversion Scd21 III. Cabline -CA Connector Used for 4K2K Display With the Faraday cage shielding and the grounding bar balanced line construction, the Cabline-CA Connector was selected as the graphics connector used in many 4K2K display assembles. A major display company released their K2K display (Fig. 10) in 2014 and I-PEX purchased one to use as a demonstration. The rear of the panel is shown in Figure 10. Figure 11. Nyquist Frequency for Data Rate It was surmised that when located in close proximity to the LTE radio/antenna ~2.7 Ghz carrier signal, the unshielded solder tails of the Cabline-CA Connector (Fig. 10), radiated a Nyquist frequency of ~2.7 GHz. Interference between the HDR Nyquist radiation from the connector solder tails and the LTE wireless radio/antenna carrier frequency ~2.7 GHz may have caused the LTE communications to fail. V. Shielding the Solder Tails: Cabline -CAII Connector The electric field (E-field) measurement of intensity is in volts per meter. For E-field strength analysis, converting the V/m to decibels, which is referenced to microvolts, where dbµv=20log(v)+120µv, then the E-field color gradients are used to indicate the field strength levels. Electrostatic flux is impeded or blocked by metallic objects, so the more complete the shielding the better the E- field shielding effectiveness. Figure 10. 4K2K Panel Uses Cabline-CA Connector 40p I-PEX obtained E-field simulation modules from CST Microwave Studios. Analysis was performed on different designs which evolved into the Cabline CA-II Connector. Page 3

4 The Cabline -CAII Connector design includes a shieldinglocking cover lid over the solder tails that does not increase the mating height of the thin display connector. The E- fields were simulated for the differential signals through positions 20 and 21 while stressing the shielding performance at 20 GHz through the connectors (Fig. 12). Figure 14. Cabline-CAII Connector Springs Grounding Lid Figure 12. EMI of Cabline -CA and CA-II 20 GHz The Cabline-CAII Connector design created a fence of grounding contacts outside the receptacle solder tails (Fig 13). The shielding cover locks into place after full engagement of the mated set. The cover connects the fence grounding posts outside the receptacle solder tails. The Cabline-CAII Connector side view cross-section and bottom view of the grounding contacts (Fig. 15) clearly depict the complete 360 degree shielding and ground paths provided through the entire connector interface. The shield braids are connected to the grounding bar. The grounding bar is connected to the plug shell. The plug shell connects to the receptacle shell which is connected to ground. Figure 15. Cabline-CAII Connector 360⁰ Shielding/Grounding Figure 13. Cabline-CAII Connector Grounding Fence The connection between the Cabline-CAII Connector shielding lid and the receptacle shell, as well as between the shielding lid and the grounding fence post, are springloaded contacts that engage mechanically (Fig. 14). The tiny opening between the cover lid and board outside of the solder tail area is less than 3mm to ensure the high frequency signals with very short wavelengths are not allowed to escape and radiate from the transfer caged area. Customers requested analysis across the span of the contacts at different positions to insure the proper shielding of the connector design. Again, using CST Microwave Studio, I-PEX simulated the E-field strength of the Cabline-CAII Connector 40p at HDR Nyquist frequencies along different locations of the connector. Page 4

5 The Cabline -CAII Connector 40p 3-lane simulation was performed near one side at the locations of Pin 2 and Pin 3, and also near the middle of the 40p connector at Pin 20 and Pin 21. A third location was analyzed between the shield cover ground spring fingers near Pin 8 and Pin 9. The setup conditions for the simulation are shown in Figure 16 using the GSSGSSGSS signal assignment pattern. Figure 18. E-Field at 5.0 GHz Customers have already selected the 20p, 40p and 60p Cabline-CAII Connector sizes to be used in their devices since the Internet of Things (IoT) has expedited the proliferation of LTE wireless applications, therefore, the proliferation of shielding. Figure 16. EM Simulation Conditions The E-Field simulation for the three locations Positons 20-21, 2-3 and between the ground springs for the connector are shown in Figure 17 at 2.4 GHz frequency. The built-in solder tail shielding feature of the Cabline- CAII Connector does isolate the E-fields of the HDR graphics signals from the wireless carrier frequency signals. This saves assembly time and trouble shooting of EMI interference issues while transferring the highest data rates producing the best display resolutions in a thin profile assembly. Figure 19. Cabline-CAII Connector: Opened and Closed Figure 17. E-Field at 2.4 GHz No emissions were reported for the same three connector positions at the higher 5 GHz frequency passed through the connector (Fig. 18). Page 5

6 References 1. I-PEX, IER , Cabline -CAII 40p, February 4, I-PEX, IER , Cabline-CAII 40p Grounding Bar Design, May 29, Poole, Ian, LTE Frequency Bands & Spectrum Allocations Radio-Electronics- Cellular/Mobile Telecommunications, September 8, 2015 I-PEX and CABLINE are trademarks of DAI-ICHI SEIKO Co., Ltd. All other trademarks are owned by their respective companies. Disclaimer All specifications of the products shown here are subject to change without notice. DAI-ICHI SEIKO Co., Ltd., assumes no responsibility for any inaccuracies or obligation to update information on these documents. I-PEX Connectors 2305 Donley Drive, Suite 110 Austin, TX DAI-ICHI SEIKO Co., Ltd All rights reserved. Page 6