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To achieve high bit rates link designers are using more sophisticated communication techniques, often turning to 4PAM transmission or decision-feedback equalization (DFE). Interestingly, with only minor modification the same hardware needed to implement a 4PAM system can be used to implement a loop-unrolled single-tap DFE receiver. To get the maximum performance from either technique, the link has to be tuned to match the specific channel it is driving. Adaptive equalization using data based update filtering allows continuous updates while minimizing the required sampler front-end hardware and significantly reduces the cost of implementation in multi-level signaling schemes. A transceiver chip was designed and fabricated in 0.13pm CMOS process to investigate dual-mode operation and the modifications of the standard adaptive algorithms necessary to operate in high-speed link environments.

The backplane environment presents a serious challenge to signaling rates above 5Gb/s. Previous 10Gb/s transceivers [1] are not designed for this harsh environment. In the raw single bit response of Fig. 4.6.1, a single 200ps pulse undergoes serious loss and dispersion and initiates reflections that may be a significant percentage of an equalized eye. Figure 4.6.1 (inset) shows a zoom-in of the reflections plotted on a scale equivalent to a single 4-PAM equalized eye. The total usable amplitude after equalization is slightly smaller than 3d which is the distance between the peak sample point and the next sample point of the raw pulse response. While the use of multiple signaling levels and transmit equalization help minimize the effects of dispersion [2,3], transmit-only equalization is an expensive way to combat the effect of reflections which are more destructive to multi-level signaling. Decision feedback-based receive equalization (DFE) is effective when dealing with configuration dependent reflections. The design of both transmit and receive equalizers and clock recovery circuits are described for operation in this type of backplane environment.Since dispersion is a function of many properties in backplanes, flexibility in the transmit equalizer, both in number of taps and their settings is highly desirable. One completely flexible approach involves the use of a digital filter and DAC [4], while the simplest approach is two-tap pre-emphasis [5]. Any technique must be evaluated for both additional insertion loss as well as for power and complexity.The five-tap merged differential transmitter/equalizer shown in Fig. 4.6.2 leverages the fact that the total transmitted current is limited to less than the sum of the maximum taps to reduce pad parasitics. It achieves this by using large segments that can be individually allocated to any tap position. The transmit equalizer is partitioned into a shared section and a dedicated section. The shared section consists of 7 large sub-drivers, each driving 16i current, where each shared sub-driver can select from any of the 5 equalization tap streams A -E. The dedicated portion consists of five binary weighted drivers, one for each equalization tap, and each capable of driving up to 15i current. This combination of shared and dedicated drivers allows each equalization tap to have the same current range, 127i, and resolution, 1i, of a non-equalizing 7b transmitter with only 50% additional parasitic overhead. Comparatively, a 5-tap transmitter/equalizer with the same range and resolution implemented by replicating the primary driver has a 400% parasitic overhead. A pure digital filter and DAC implementation requires a FIR filter running at the symbol rate and consumes more than twice the power.For receive equalization, the linearity and high bandwidth of the transmission line environment were leveraged by adding and subtracting currents directly at the input pads. The receive equalizer is equivalent to a 1/5th scaled transmit equalizer. High-latency reflections are effective...

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