Overview of SERDES channel equalization techniques for serial interfaces

The newer industry-standard SerDes protocols such as PCIe Gen6, USB4, and the 100G per-lane Ethernet and OIF/CEI standards offer an increasing challenge for PCB designers on multiple fronts. On the one hand, the speeds are approximately doubling for each generation. At the same time, the circuit board material used is often the same as the previous generations in order to keep costs down. To compensate for the increased loss at higher data rates, complex equalization techniques are employed.

Equalization, solution to the intersymbol interference

The maximum data rate at which a serial interface can operate is limited by various impairments such as reflections, losses and crosstalk. The frequency dependent attenuation of the channel has a low-pass filtering behavior causing “smearing” of a symbol into the adjacent symbols from a data stream. This phenomenon, which is the biggest source of noise for serial interfaces, degrades the performance of the link and is known as intersymbol interference (ISI).

How can ISI be solved? Equalization is a signal conditioning technique that counters the effects of the channel, helps reduce ISI while increasing the signal to noise ratio and potentially increases the maximum achievable data rate.

Different types of equalization techniques

Equalization can be applied at different locations within a link and it can be of different types, including:

• Transmitter emphasis
• Transmitter finite impulse response (FIR) equalization
• Receiver continuous time linear equalizer (CTLE)
• Decision feedback equalizer (DFE)
• Receiver Feed-forward equalizer (FFE)

Discover the advantages and disadvantages of different equalization types

• Tx FIR is immune to noise enhancement and can cancel ISI in pre-cursor and beyond filter span. But it has limited output power, attenuates low-frequencies, and amplifies crosstalk. Back channel is required for adaptation.
• CTLE is cost effective and simple to implement, provides gain and equalization with low power and area overhead, long post-cursor reach and does not require back-channel for adaptation. Though CTLE's equalization is limited to 1st order compensation, very sensitive to PVT and hard to tune. It has no pre-cursor cancellation and amplifies high frequency noise and crosstalk. Linearity can be challenging.
• DFE is noise and crosstalk neutral and does not require back-channel for adaptation. However, DFE is nonlinear, can only cancel post-cursor ISI, and has error propagation, feedback loop latency with critical timing path. DFE also requires complex CDR design.
• Rx FFE does not have limited output power as Tx FIR and does not require back-channel for adaptation. But it amplifies noise and precision. Setting coefficients require adaptive algorithms. Tuning delays for analog implementations.

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