Case Study – Cloud based telecoms system

Background

We were approached by a company specialising in cloud-based telecoms to develop a revolutionary comms device for virtual call centres. The PCB would need to interface with other bespoke and off-the-shelf devices. All with their own functions and requirements. Our PCB would have to be the “brain” to control all of these devices. 

Requirements

All of these peripherals have their own high speed signal requirements: PCI-E connector, Ethernet connector, USB connector, QSFP+ connector, DDR4 SODIMM connectors, DDR4 RAM 8Gbit. If any one of the interactions between PCBs does not perform correctly, costly reworks would be required. Time would also be lost to identify the problem, this could also lead to our customer losing ground to competition. The PCB would also be the power supply for most of these interfacing PCBs. 60 A is required, flowing through various SMPS (switch mode power supply) to provide power to each connector. The high speed requirements for this PCB are extremely complicated, adding SMPS as well can cause problems with the high frequency electrical noise. For the high speed signals, we would need to route the signals at 50 ohms (DDR), 90 ohms (USB), 100 ohms (PCI-E, DDR diff pairs, Ethernet connector). The QSFP connector would need to use 100 ohm tracks at 25 Ghz. Most complex microprocessors have rules to follow. Chip manufacturers supply these rules as well as guidelines and reference designs to follow

Using the numerous tools at our disposal, we were able to adjust the PCB stack up, the gap and track thickness to obtain the required impedance for each peripheral to operate correctly.

Rules

Nets are assigned to differential pairs, buses and classes to for complex rules to be assigned. These rules control the track thickness, track length and clearances to other nets and classes. Assigning the rules accurately allows the PCB to be routed without reworking and unnecessary mistakes. As the majority of these signals need to run at 50 ohms, we decided to organise the PCB stack for all of the signals to run at 50 ohms. 

PCB Layout

QSFP connector – TE Connectivity 2299940-5 16 diff pairs 100 ohm 25 GHZ, tracks with no 45 corners. The diff pairs are connected using 100 ohm diff pair coplaner signals. As the tracks are running at 25 GHz, there can be no 45 degree bends in the tracks. Track lengths are within 25ns of each other. Ethernet connector – Amphenol RJMG214034430NR The signals are connected to the Ethernet connector via 4 Ethernet transceivers (DP83867IRPAP). The signals are routed into the transceivers and out as 100 ohm diff pairs into the Ethernet connector. Each bus is routed within 0.5mm of each other.DDR4 SODIMM connector – TE Connectivity 2309409-1 Memory card connector DDR4. There are 8 buses DDR4_SODIMM-BYTE0-7 (11 signals in each) 50 ohm. All track lengths are within 8ps of each other PCI-E connector – Amphenol FCI MDT420M02001 2 x 100 ohmdiff pairs are routed from the BGA to connector  USB connector – Wurth 692122030100 2 x 90 OHM diff pairs are routed from BGA to USB connector USB Interface Transceiver – Microchip USB3320C-EZK-TR USB bus from BGA via USB transceiver into USB connector. Diff pairs run at 90 ohms into USB connector  Infineon Technologies TDA21240 is a SMPS with a maximum load current of 40A. Decoupling capacitors are recommended to reduce voltage spikes. Multiple GND and PWR planes are used. Visit our page for more details on SMPS https://dl-designs.co.uk/blog/switch-mode-power-supplies Infineon Technologies IR38064TRPBF. Another SMPS with unusual pad configurations. Input capacitors, inductors and output capacitors should be kept as close the IR38064 as possible to reduce EMI radiating. Feedback components should be close to the Fb and Comp pins. Max output current is 35A

Conclusion

Spending time researching and developing a practical solution to design this PCB allowed us to deliver a PCB design that surpassed our customers expectations. Even after prototypes were built and tested, the PCB design did not need any major modifications and is currently being mass produced. We are satisfied to be a major part of this process to deliver a leading edge application aimed at reforming the communication industry Components used in this project Microprocessor – Xilinx XCZU11EG-1FFVC1760E PCI-E connector – Amphenol FCI MDT420M02001 DDR4 SODIMM connector – TE Connectivity 2309409-1 QSFP connector – TE Connectivity 2299940-5 Ethernet Transceivers – Texas Instruments DP83867IRPAP Ethernet connector – Amphenol RJMG214034430NR USB Interface Transceiver – Microchip USB3320C-EZK-TR USB connector – Wurth 692122030100 Infineon Technologies TDA21240 Infineon Technologies IR38064TRPBF

Click on the link to jump to a 16 layer PCB with 90 & 100 OHM diff pairs. Xilinx Ultrascale FBGA & DDR4 memory

Click on the link to jump to a 26 layer PCB with 75, 90, 100 & 120 OHM diff pairs Zynq Ultrascale+ FBGA & DDR4 memory