VDD Channel Stage 1 Stage 2 VDD (a) 10 12 14 -2 2 4 6 8 108 109 Frequency (b) 1010 1011 -30 -25 -20 -15 -10 -5 5 108 109 Frequency (c) FIGURE 8: (a) The two-stage CTLE, (b) its frequency response, and (c) the channel-CTLE cascade response. 1010 1011 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4 0.5 -0.4 -0.3 -0.2 -0.1 05 10 15 20 Time (ps) (a) -0.5 -0.4 -0.3 -0.2 -0.1 25 30 35 05 10 15 20 Time (ps) (b) FIGURE 9: The output eyes of (a) the channel and (b) the CTLE. reveal that the eye suffers further closure in this case. Conversely, R 600 S = S = assumption at this point is that the DFE will further improve the eye. X and C 100 fF increase the boost factor but do not improve the output eye. Whether the eye opening depicted in Figure 9(b) is adequate for a BER < 10 -12 or not is ultimately de termined by the DFE design. Our The Need for Programmable Boost Broadband Rxs must operate properly with different channel responses. The CTLE developed thus far is optimized for a loss of approximately 20 dB at f ,Nyq and " overequalizes " the IEEE SOLID-STATE CIRCUITS MAGAZINE data if the loss is lower. As a result, significant intersymbol interference appears at the CTLE output. For this reason, the boost must be variable. This is typically accomplished by implementing the degeneration capacitors as programmable units. A lower (continued on p. 160) FALL 2021 11 25 30 35 VCTLE (V) Magnitude (dB) VCTLE (V) Magnitude (dB)