A 32Gb/s wireline receiver with a low-frequency equalizer, CTLE and 2-tap DFE in 28nm CMOS

Standards such as OIF CEI-25G, CEI-28G and 32G-FC require transceivers operating at high data rates over imperfect channels. Equalizers are used to cancel the inter-symbol interference (ISI) caused by frequency-dependent channel losses such as skin effect and dielectric loss. The primary objective of an equalizer is to compensate for high-frequency loss, which often exceeds 30dB at fs/2. However, due to the skin effect in a PCB stripline, which starts at 10MHz or lower, we also need to compensate for a small amount of loss at low frequency (e.g., 500MHz). Figure 2.1.1 shows simulated responses of a backplane channel (42.6dB loss at fs/2 for 32Gb/s) with conventional high-frequency equalizers only (4-tap feed-forward equalizer (FFE), 1st-order continuous-time linear equalizer (CTLE) with a dominant pole at fs/4, and 1-tap DFE) and with additional low-frequency equalization. Conventional equalizers cannot compensate for the small amount of low-frequency loss because the slope of the low-frequency loss is too gentle (<;3dB/dec). The FFE and CTLE do not have a pole in the low frequency region and hence have only a steep slope of 20dB/dec above their zero. The DFE cancels only short-term ISI. Effects of such low-frequency loss have often been overlooked or neglected, because 1) the loss is small (2 to 3dB), 2) when plotted using the linear frequency axis which is commonly used to show frequency dependence of skin effect and dielectric loss, the low-frequency loss is degenerated at DC and hardly visible (Fig. 2.1.1a), and 3) the long ISI tail of the channel pulse response seems well cancelled at first glance by conventional equalizers only (Fig. 2.1.1b). However, the uncompensated low-frequency loss causes non-negligible long-term residual ISI, because the integral of the residual ISI magnitude keeps going up for several hundred UI. As shown by the eye diagrams in the inset of Fig. 2.1.1(b), the residual long-term ISI results in 0.42UI data-dependent Jitter (DDJ) that is difficult to reduce further by enhancing FFE/CTLE/DFE, but can be reduced to 0.21UI by adding a low-frequency equalizer (LFEQ). Savoj et al. also recently reported long-tail cancellation [2].