A 60 MBd, 480 Mb/s, 256 QAM decision-feedback equalizer in 1.2 μm CMOS

Using a combination of architecture optimization techniques and unconventional circuit designs, a 60 MHz decision-feedback equalizer (DFE) chip is presented for high-bit-rate digital modem applications. The equalizer can accommodate quaternary phase-shift keying (QPSK), and 16, 64, and 256 quadrature amplitude modulation (QAM) and achieves a peak throughput rate of 480 MB/s. The chip contains four complex-valued programmable filter taps and incorporates coefficient updating circuitry for implementing the LMS adaptive algorithm with user-selectable adaptation step sizes. Cut-set retiming architecture techniques were used so that the chips could be cascaded to implement longer equalizer lengths without any speed degradation, and a circuit design technique called adaptively biased pseudo-NMOS logic (APNL) was adopted to reduce on-chip critical-path delays. The fully parallel chip architecture achieves a computational throughput of 1.44 billion operations per second (GOPS)