“The growing number of video and cloud applications on the Internet is driving data centers and cloud storage toward 400G Ethernet networks to meet bandwidth demands. As data usage increases, so does the challenge of maintaining high-speed network signal integrity on Gigabit Ethernet transmission lines in communications and data center equipment.
The author of this article: Lin Zhihong, is a product marketing engineer for Texas Instruments (TI)’s interface business.
The growing number of video and cloud applications on the Internet is driving data centers and cloud storage toward 400G Ethernet networks to meet bandwidth demands. As data usage increases, so does the challenge of maintaining high-speed network signal integrity on Gigabit Ethernet transmission lines in communications and data center equipment.
As signals pass through PCBs, connectors, and cables, their data transfer rates can be severely degraded. This signal distortion will cause the system to fail Ethernet standards compliance tests and cause poor interoperability with other network equipment. Designers often need to use signal conditioners such as redrivers or retimers to maintain signal quality and system performance.
The root cause of signal attenuation
The way a signal is attenuated varies with the transmission medium, including PCBs, copper or fiber optic cables, passive components and connectors on the signal lines. Signals are distorted in both the time and frequency domains.
The most common cause of signal attenuation is insertion loss, the loss of signal power from any device or medium in the data path. Figure 1 shows examples of insertion loss from different PCB traces. High-frequency components lose more than low-frequency components; so do longer wiring or cable lengths.
Figure 1 Example of PCB insertion loss for different trace lengths.Source: Texas Instruments
Losses in the time domain include amplitude dips and pulse spreading of the received signal, resulting in inter-symbol interference (ISI), where each transmitted pulse interferes with its neighbors. This causes the receiver’s eye to close. Figure 2 shows the signal attenuation over a 15-meter cable, where the signal distortion is proportional to the cable length.
Figure 2: These graphs show signal attenuation over a 15-meter cable, where signal distortion is proportional to cable length.Source: Texas Instruments
Other factors can also compromise signal integrity:
Connector impedance mismatch, causing signal reflections.
Adjacent high-speed signals interfere with each other, resulting in crosstalk.
Thermal or other noise can cause random jitter, affect the duty cycle, and cause phase and timing errors on the signal.
Signal Conditioning Solutions
So, how to solve the signal integrity challenges of high-speed interfaces? Ideally, on the transmission medium, the signal loss for all frequency components should be 0 dB. In practice, however, any transmission medium will increase the insertion loss of the signal.
If signal loss affects system performance, signal conditioners effectively help maintain signal integrity in high-speed designs by restoring signal strength and achieving an equalized frequency response.
There are two types of signal conditioners: Ethernet redrivers (redrivers) and retimers. Which one you choose depends on the severity of the recession.
As shown in Figure 3, a redriver is an analog component that restores an attenuated input signal through equalization and gain adjustment, and then retransmits the signal according to the signal standard specification. Redrivers perform signal conditioning primarily through equalization. They are the easiest and most cost-effective way to resist signal attenuation caused by intersymbol interference, while also overcoming insertion loss from long PCB traces and cables.
Figure 3 The redriver can compensate up to 20db of channel loss.Source: Texas Instruments
Looking closely inside the redriver, a continuous-time linear equalizer (CTLE) is a circuit that is usually implemented on the receiving end of the redriver. CTLE provides more gain to high frequency signals than low frequency signals to compensate for larger losses in high frequency components. This allows the equalized signal to have a more uniform frequency response across the channel.
The redriver’s transmitter can optionally include de-emphasis or pre-emphasis to provide signal distortion to compensate for channel loss. De-emphasis attenuates the low-frequency components of the signal, while pre-emphasis boosts the high-frequency components of the signal to achieve a balanced channel response. Figure 4 shows the effect of a redriver equalizer on a distorted input signal.
Figure 4: This figure shows how a redriver helps open the input eye.Source: Texas Instruments
The redriver can be a linear redriver if the output signal amplitude is a linear function or proportional to the input signal amplitude. Otherwise, this is a restrictive redrive. A linear redriver will faithfully pass all electrical characteristics of the signal, such as pre-transmission, de-emphasis or pre-emphasis, made possible by the frequency-dependent gain added by CTLE.
Linear redrivers are especially useful when the system needs to use link training to establish optimal signal conditioning settings for each channel. Linear redrivers will train through the link without blocking the signal waveform or intentional distortion from the transmitter.
As shown in Figure 5, retimers are more complex signal conditioners than redrivers and typically include equalization and clock data recovery (CDR) functions. These features not only compensate for intersymbol interference, but also eliminate random jitter, crosstalk, and reflections.
Figure 5 Retimer can compensate up to 35dB of channel loss.Source: Texas Instruments
The clock data recovery component within the retimer will recover the data and extract a clean clock. CDR compensates for phase delay variation and random jitter, and removes additional deterministic jitter from the input channel to provide the best output signal quality. Figure 6 shows the effect of the retimer’s CDR.
Figure 6. The retimer CDR removes jitter, resulting in a clearer eye diagram.Source: Texas Instruments
Redrivers are typically used to compensate for 20dB of channel loss. If there is more severe signal degradation or channel loss due to timing and phase jitter, the retimer is more suitable as it can compensate for 30dB to 35dB of channel loss by removing the jitter.
In some cases, designers may consider using more expensive PCB materials to improve signal quality as an alternative to using signal conditioners. These PCB materials are usually very expensive, and they can only solve the intersymbol interference caused by insertion loss to a certain extent. If the PCB trace is long, you’ll still need a redriver or retimer to compensate for the extra loss. Also, the PCB material does not account for crosstalk, reflections, and other random jitter from the connector or cable, so adding a redriver or retimer to such a system will help eliminate jitter.
Redriver and Retimer Applications
Redrivers and retimers are commonly used in Gigabit Ethernet in data center switches, network interface cards (NICs), wired and wireless networking equipment, and data and storage server networks. They can be placed between the switch application-specific integrated circuit (ASIC) and the front ports, or along the path between the midplane and backplane, for better signal integrity and system performance.