Cases & enclosures
THE TECHNICAL CONSIDERATIONS OF BOARD-TO-BOARD CONNECTIVITY Splitting a complex control system over separate PCBs is an ideal solution to increasing the overall system density. However, engineers must account for several technical considerations when adopting this approach. Many feature-rich industrial control systems today rely on compute-intensive SoCs and processors such as GPUs and neural processing units (NPUs). These high- throughput devices achieve stunning processing performance and typically interface with high-speed memory, optical transceivers, and vision sensors. Working with these high-performance processors demands careful PCB design. The layout of power supply rails, high- speed data lines, analogue sensor inputs, and data converter ICs all require specialist attention. The separation of ultra-low-voltage analogue tracks from electrically noisy data lines and the placement of decoupling capacitors close to loads are all familiar topics for PCB design engineers. However, incorporating separate PCBs and the associated connectors into the layout adds further board complexity. The selection of suitable board-to-board connectors warrants a detailed investigation; the main topics include: Connector EMC/EMI capabilities:
High-speed data lines can create considerable high-frequency noise and create significant electromagnetic interference (EMI) radiating from PCB tracks and connector terminations. This EMI may interfere with signals on adjacent connector pins, resulting in erratic system behaviour. Using shielded connector assemblies and dividing up the internal connector layout to incorporate ground pins helps minimise the impact of EMI. Likewise, sensitive analogue signals and high-bandwidth digital data lines are susceptible to EMI, so electromagnetic conformity (EMC) measures are crucial for maintaining signal integrity. Connector insertion loss: Another connector characteristic, particularly relevant for high-frequency data transmission, is the insertion loss. This metric measures the attenuation a signal experiences passing through the connector arrangement. The connector's geometry, materials, and pin layout impact the loss, typically increasing with frequency. Insertion loss is quoted in dB, with a -3dB loss equating to a 70 per cent signal attenuation. Connector pin-to-pin crosstalk: The crosstalk characteristics of a connector indicate the transfer of signals from one pin becoming induced to an adjacent pin(s). Crosstalk, measured in dB, occurs at both ends of the connector link, the near-end NEXT (transmitting end) and the far-end FEXT (receiving end). Spacing contacts sufficiently apart and placing ground pins between helps minimise the impact of crosstalk.
Connector impedance characteristics: Impedance matching of any high-speed data connection is essential to maintaining signal
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Figure 2: The insertion loss characteristic of the Phoenix Contact FP 0.8/…-FV-SH 7,85 connector in combination with the FP 0.8/…-MV-SH 2.65 with a 12mm stacking height. (Source: Phoenix Contact)
integrity and quality of service. Wired data networking such as Ethernet employs two differential pairs of signals and requires a 100 Ohm line impedance. Impedance is frequency dependent, and if the impedance differs from specification, signal reflections occur with the resulting loss of link integrity.
FUTURE-PROOFING YOUR HIGH-SPEED BOARD INTERCONNECT Figure 1 illustrates several PCB interconnect configurations achieved using the FinePitch (FP) 0.8 connector series from Phoenix Contact. This versatile connector portfolio employs an innovative contact method (ScaleX) that is robust and tolerant of misalignment. Offering multiple connection configurations, stacking heights from 6mm to 21mm, and shielded and unshielded versions, the FP 0.8 family is ideal as an all-round board-to-board interconnect series for high- speed data applications.
As highlighted in the previous section, there are several key criteria for selecting a board-to-board connector. Figure 2 illustrates the typical insertion loss of the FP 0.8 family, with a -3dB cut-off frequency of 26Hz, showcasing its suitability for high-bandwidth data use cases. The innovative double-contact ScaleX contact technology rovides a highly robust, flexible, and vibration-proof interconnect. The combination of male and female contacts provides a 1.5mm wiping length, and the product line-up consists of connector iterations with minimum stack heights of 6mm, increasing by 1.5mm, up to 21.0mm. The ScaleX contacts accommodate tolerance compensations of +/- 0.7mm and up to five degrees of inclination offset, providing a robust and reliable connection even if a slight mismatch occurs during assembly.
Another selection criterion highlighted is the connector’s EMC characteristics. The FP 0.8 shielded connectors have excellent EMC credentials with multiple female and male shield connection points.
Figure 3 illustrates the shield’s effectiveness in isolating internal contacts from an external interference signal applied to the shield. An optimal shielding pattern is achieved with both female and male shields connected to the ground. Figure 4 showcases the crosstalk attributes of an FP 0.8 connector. Crosstalk is typically better than - 30dB across most of the operating frequency range up to 20GHz. Increasing the contact spacing and assigning adjacent contacts as ground pins helps minimise the impact of crosstalk.
IMPLEMENTING ROBUST AND RELIABLE HIGH-SPEED BOARD-TO- BOARD INTERCONNECT
This short article has highlighted some of the challenges associated with designing high- speed, highly integrated industrial automation applications in space-constrained control cabinets. The creative use of available space using an innovative board layout requires board-to-board connectors that meet the requirements of such applications. The Phoenix Contact FP 0.8 series is an excellent all-round solution for high-speed interconnect challenges and is available from Mouser Electronics, an authorised Phoenix Contact distribution partner.
Mouser Electronics
www.mouser.com
August 2023 Instrumentation Monthly
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