As many of you that work with high-speed signals may know… physics is not your friend – especially if you’re trying to design on lower-cost board materials, such as FR-4. When moving data at 10 Gbps and above, all kinds of phenomenon – including dielectric loss; skin effect; transmission line impairments, such as connectors and ground plane stack discrepancies; and more – can affect channel performance. All of these increase the jitter in the channel, ultimately degrading the bit error rate (BER).
Thankfully there are a few tricks to work around these issues. For example, you can use active devices to improve the signal transmission (improved amplitude as well as pre-emphasis or de-emphasis) or to equalize the channel at the receiving end. Both have benefit, and in combination, can work around channel loss and various impairments.
When things get really bad due to non-deterministic jitter, you’ll need to use a re-timer (often called a re-clocker) to resample the data and produce a new, clean data stream. These devices can greatly improve signal quality and are often used just prior to optical modules where the jitter specification is extremely strict. So unless you’re one inch away from the source, you’ll probably need a re-timer.
Example re-timers include devices such as the DS100RT410, which includes the re-timer, receive equalizer and de-emphasis driver for a highly integrated solution for severe jitter conditions. Also, if you’re like me and want to know in detail how a channel is operating, select a re-timer (like the DS100RT410) that includes an “eye-monitor” function built in. This allows you to read out the eye pattern directly from the re-timer to get a real-time image of the signal quality – it also helps tune the channel as well. Another trick is to move slower signals that are less than 5 Gbps utilizing careful layout techniques and then, when the signals need to be transported, further serialize them into a higher-speed stream. You can then transmit this stream via high-performance cables or by fiber via optical modules.
At the other end, you can de-serialize the serialized data to reconstruct the original slower-speed serial links. To simplify this, you can use devices that contain two complete two-way channels that can run up to 10 Gbps, such as the dual-channel TLK10002 multi-rate transceiver (see Figure 1). By using one of these devices at each end, you can have two high-speed serial channels. This enables you to use one and have the second as a fail-over, or use them together to double the through-put.
Figure 1 – Using the TLK10002 to consolidate low speed connections can simplify signal integrity issues and box-to-box interconnections.
I hope this helps you with your next high speed interconnection challenge! Till next time…
