Redrivers and retimers maintain satisfactory signal quality

Article By : Zhihong Lin

Signal conditioners such as redrivers or retimers maintain satisfactory signal quality and system performance in high-speed Ethernet networks serving data centers and cloud storage.

The increasing amount of video consumption on the internet and the proliferation of cloud applications are driving data centers and cloud storage toward 400-Gigabit Ethernet networks and beyond to satisfy network bandwidth demands. As data usage increases so too does the challenge for maintaining signal integrity for high-speed networks on Gigabit Ethernet transmission lines in communication and data-center equipment.

Signals can suffer severe degradation at multigigabit data rates when traversing through PCBs, connectors, and cables. This signal distortion will cause systems to fail Ethernet standard compliance testing and create poor interoperability with other network equipment. Designers often need to use signal conditioners such as redrivers or retimers to maintain satisfactory signal quality and system performance.

Root cause of signal degradation

The ways in which a signal degrades will vary based on transmission media, including PCBs, copper or optical cables, passive components on the signal line, and connectors. Signals can suffer distortion in both the time and frequency domains.

The most common cause of signal degradation comes from insertion loss, which is the loss of signal power from any device or media in the data path. Figure 1 shows examples of insertion-loss plots from different PCB traces. High-frequency components suffer more losses than low-frequency components; so do longer traces or cable lengths.

graph showing PCB insertion loss

Figure 1 Examples of PCB insertion loss over different trace lengths. Source: Texas Instruments

Indications of loss in the time domain include a received signal amplitude drop and pulse spreading, resulting in intersymbol interference (ISI), where each transmitted pulse interferes with its neighbors. This can result in eye-diagram closure at the receiver. Figure 2 shows signal degradation over a 15-m cable, where the signal distortion is proportional to the length of the cable.

four graphs showing signal degradationFigure 2 These diagrams show signal degradation over a 15-m cable, where the signal distortion is proportional to the length of the cable. Source: Texas Instruments

Additional factors may also compromise signal integrity:

  • Connector impedance mismatches, causing signal reflections.
  • Adjacent high-speed signals interfering with one another, resulting in crosstalk.
  • Thermal or other noise introducing random jitter, affecting duty cycles and causing phase and timing errors on the signals.

Signal conditioning solutions

So, how do you resolve signal-integrity challenges for high-speed interfaces? Ideally, the signal loss should be 0 dB over a transmission medium across all frequency components. In reality, however, any transmission media will add insertion loss to the signal.

If the signal loss is affecting system performance, signal conditioners are effective in helping preserve signal integrity for high-speed designs by restoring signal strength and achieving an equalized frequency response.

There are two types of signal conditioners: Ethernet redrivers and retimers. Which one you choose will depend on the severity of the degradation.

Redrivers

A redriver, as shown in Figure 3, is an analog component used to restore an attenuated input signal through equalization and gain adjustment that then retransmits the signal based on signal standard specifications. Redrivers perform signal conditioning primarily through equalization. They are the simplest and most cost-effective way to combat signal degradation caused by intersymbol interference, while also overcoming the insertion loss introduced by long PCB trace and cable lengths.

 

diagram of an analog redriverFigure 3 Redrivers can compensate for channel losses as high as 20 dB. Source: Texas Instruments

Taking a closer look inside a redriver, a continuous time linear equalizer (CTLE) is a circuit often implemented in the receiver side of a redriver. A CTLE provides more gain for high-frequency signals than low-frequency signals in order to compensate for larger losses in higher-frequency components. This enables the equalized signal to have a more uniform frequency response over the channel.

Optionally, a redriver’s transmitter can include de-emphasis or pre-emphasis functions to provide preemptive signal distortion to compensate for the channel loss. De-emphasis attenuates the low-frequency components of the signal, while pre-emphasis boosts up the high-frequency component of the signal to achieve an equalized channel response. Figure 4 shows the effect of the redriver equalizer on a distorted input signal.

diagram showing how a redriver can help open an input eye diagramFigure 4 This diagram shows how a redriver can help open an input eye diagram. Source: Texas Instruments

A redriver can be a linear redriver if the output signal amplitude is a linear function or directly proportional to the input signal amplitude. Otherwise, it is a limiting redriver. Linear redrivers will faithfully pass through all electrical characteristics of a signal such as pre-shoot, de-emphasis, or pre-emphasis, with the added frequency-dependent gain made possible through CTLE.

A linear redriver is particularly useful when systems need to use link training to establish the best signal-conditioning settings for each channel. Linear redrivers will pass through link training without blocking the signal wave shape or intentional distortions created by the transmitter.

Retimers

A retimer, as shown in Figure 5, is a more complex signal conditioner than a redriver and usually includes an equalization function, as well as a clock data recovery (CDR) function. These features compensate not only for intersymbol interference but can also clean up random jitter, crosstalk, and reflections.

block diagram for a retimerFigure 5 Retimers can compensate for channel losses as high as 35 dB. Source: Texas Instruments

The clock data recovery component inside the retimer will recover the data and extract a clean clock. CDR can compensate for phase-delay variations and random jitter, and eliminate additional deterministic jitter from the input channel in order to provide the best output signal quality. Figure 6 shows the effect of a retimer’s CDR.

 

diagram shows the effect of a retimer’s CDRFigure 6 A retimer CDR removes jitter, resulting in a clearer eye diagram. Source: Texas Instruments

Redrivers are typically used for compensating channel losses as high as 20 dB. If more severe signal degradation or channel losses are present due to timing and phase jitter, a retimer is more appropriate because it can compensate for 30 dB to 35 dB of channel loss with jitter removal.

In some cases, designers may consider more costly PCB materials to improve signal quality as an alternative to using signal conditioners. These PCB materials are often very expensive, and they only solve intersymbol interference induced from insertion loss to a certain degree. If the PCB trace length is long, you will still need a redriver or retimer to compensate for additional losses. Also, PCB materials cannot solve crosstalk, reflection, or other random jitter from the connector or cable, so an additional redriver or retimer will be beneficial in such systems to clean up the jitter.

Redriver and retimer applications

Redrivers and retimers are commonly used in Gigabit Ethernet networks for data-center switches, network interface cards (NICs), and wired and wireless networking equipment, as well as data and storage server networks. They can be placed between the switch application-specific integrated circuit (ASIC) and front port, or along the path between the mid-plane and the backplane to enable better signal integrity and system performance.

Zhihong Lin is product marketing engineer for the interface business of Texas Instruments (TI).

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