This article discusses the initial stages of design and verification of a high-speed shield concept for Arduino form-factor boards. Testing with our FPGA-based Darsena card shows that 625 MHz LVDS signals can be reliably routed across high-quality 100-mil connectors. On Darsena, we re-define four pairs of adjacent pins to route two bi-directional channels of high-speed data (e.g., SGMII Ethernet) between shield and an FPGA resident on Darsena.
Figure 1 below shows a differential probe placed across a pair of pins on a Samtec TSW/SSW connector pair. Note the long posts of the TSW connector to accommodate the RJ-45 jack on the base and to provide for air flow & visibility.
Figure 2 shows the resulting waveform from the measurement shown above. We were encouraged enough at this point to build our first shield for further testing.
Figure 3 below shows our first-article high-speed SMA Shield for Darsena. The TSW connector shown in Figure 1 has been soldered to the card, and the SMA connectors are Samtec SMA-J-P-H-ST-TH1. Each connector routes either the positive or negative signal of an LVDS pair. As shown, we have TX2 / RX2 and TX3 / RX3. The 0 and 1 pairs are utilized on the Darsena base.
Figure 4 shows a close up of our new SMA Shield on Darsena with six of the SMA connectors being utilized.
Figure 5 below shows the SMA shield in action. Darsena's FPGA is configured to switch one of the Ethernet port's traffic onto the shield. A pair of SMA cables connect into our lab bench scope, and the decoded Ethernet stream is shown being successfully decoded.
The final figure below shows our SMA shield now being used to switch two different Ethernet streams simultaneously. One full-duplex channel (2 pairs / 4 cables) is routed to a prototype Ethernet card we developed, which in turn connects to a Raspberry Pi. Another pair of cables on the SMA shield continues to stream Etherent into our scope, so we can observe the traffic in real-time in a fully deterministic manner.
We have several other shields in development, and we'll be posting about these soon.
- LVDS: Low Voltage Differential Signaling
- SGMII: Serial Gigabit Media Independent Interface