“The continuous increase of video and cloud applications on the Internet is driving the development of data centers and cloud storage towards 400G Ethernet networks to meet bandwidth requirements. With the increase in data usage, the challenge of maintaining high-speed network signal integrity on Gigabit Ethernet transmission lines in communications and data center equipment has also increased.
The author of this article: Lin Zhihong, is a product marketing engineer in the interface business of Texas Instruments (TI).
The continuous increase of video and cloud applications on the Internet is driving the development of data centers and cloud storage towards 400G Ethernet networks to meet bandwidth requirements. With the increase in data usage, the challenge of maintaining high-speed network signal integrity on Gigabit Ethernet transmission lines in communications and data center equipment has also increased.
When the signal passes through the PCB, connector and cable, its data transmission rate may be severely reduced. This signal distortion will cause the system to fail the Ethernet standard compliance test and cause poor interoperability with other network equipment. Designers usually 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 of signal attenuation differs depending on the transmission medium, including PCB, copper or optical cable, passive components and connectors on signal lines. The signal is distorted in both the time domain and the frequency domain.
The most common cause of signal attenuation is insertion loss, which is 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 are more lossy than low-frequency components; the same is true for longer wiring or cable lengths.
Figure 1 Examples of PCB insertion loss for different wiring lengths.Source: Texas Instruments
Loss in the time domain includes the amplitude drop and pulse spread of the received signal, resulting in inter-symbol interference (ISI), where each transmitted pulse interferes with its neighboring pulses. This will cause the receiver’s eye diagram to close. Figure 2 shows the signal attenuation on a 15-meter cable, where the signal distortion is proportional to the cable length.
Figure 2: These graphs show the signal attenuation on a 15-meter cable, where the signal distortion is proportional to the cable length.Source: Texas Instruments
Other factors can also compromise signal integrity:
The impedance of the connector does not match, causing signal reflection.
Adjacent high-speed signals interfere with each other, causing crosstalk.
Thermal noise 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 problem of high-speed interfaces? Ideally, on the transmission medium, the signal loss of all frequency components should be 0 dB. However, in fact, any transmission medium will increase the insertion loss of the signal.
If signal loss affects system performance, signal conditioners can effectively help maintain the signal integrity of high-speed designs by restoring signal strength and achieving 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, the redriver is an analog component used to restore the attenuated input signal through equalization and gain adjustment, and then retransmit the signal according to the signal standard specification. The redriver mainly performs signal conditioning through equalization. They are the simplest and most cost-effective method to resist the signal attenuation caused by inter-symbol interference, while also overcoming the insertion loss caused by long PCB wiring and cables.
Figure 3 The redriver can compensate for channel loss up to 20db.Source: Texas Instruments
Take a closer look inside the redriver. Continuous time linear equalizer (CTLE) is a circuit usually implemented at the receiving end of the redriver. CTLE provides more gain for high-frequency signals than low-frequency signals to compensate for larger losses in high-frequency components. This makes the equalized signal have a more uniform frequency response on the channel.
The transmitter of the redriver can choose to include de-emphasis or pre-emphasis (pre-emphasis) to provide signal distortion to compensate for channel loss. De-emphasis weakens the low-frequency components of the signal, while pre-emphasis increases the high-frequency components of the signal to achieve a balanced channel response. Figure 4 shows the effect of the redriver equalizer on the distorted input signal.
Figure 4: This figure shows how the redriver helps to open the input eye diagram.Source: Texas Instruments
If the output signal amplitude is a linear function or proportional to the input signal amplitude, the redriver can be a linear redriver. Otherwise, this is a restrictive redriver. The linear redriver will faithfully pass all electrical characteristics of the signal, such as pre-transmission, de-emphasis or pre-emphasis, and it is possible to increase the frequency-dependent gain through CTLE.
Linear redrivers are especially useful when the system needs to use link training to establish the best signal conditioning settings for each channel. The linear redriver will be trained through the link without blocking the signal waveform or distortion deliberately caused by the transmitter.
As shown in Figure 5, a retimer is a more complex signal conditioner than a redriver, and usually includes an equalization function and a clock data recovery (CDR) function. These features can not only compensate for inter-symbol interference, but also eliminate random jitter, crosstalk, and reflections.
The retimer in Figure 5 can compensate up to 35dB of channel loss.Source: Texas Instruments
The clock data recovery component in the retimer will recover the data and extract a clean clock. CDR can compensate for phase delay variation and random jitter, and eliminate additional deterministic jitter from the input channel to provide the best output signal quality. Figure 6 shows the effect of the CDR of the retimer.
Figure 6 Retimer CDR eliminates jitter, so that the eye diagram is clearer.Source: Texas Instruments
The redriver is usually used to compensate for the 20dB channel loss. If more serious signal degradation or channel loss occurs due to timing and phase jitter, retimer is more appropriate because it can compensate for the 30dB to 35dB channel loss by removing 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 inter-symbol interference caused by the insertion loss to a certain extent. If the PCB wiring is very long, you still need a redriver or retimer to compensate for the extra loss. In addition, PCB materials cannot solve crosstalk, reflections, connectors or other random jitter of cables, so adding a redriver or retimer to such systems will help eliminate jitter.
Redriver and retimer applications
Redrivers and retimers are commonly used in data center switches, network interface cards (NICs), wired and wireless network equipment, and data and storage server networks in Gigabit Ethernet. They can be placed between the switch application-specific integrated circuit (ASIC) and the front port, or along the path between the midplane and the backplane to achieve better signal integrity and system performance.