Accurate Characterization for Continuous-Time Linear Equalization in CMOS Optical Receivers

Recently published CMOS optical receivers consist of a limited-bandwidth first-stage transimpedance amplifier (TIA) followed by an equalizer. Limiting the TIA’s bandwidth improves the gain and reduces the noise but introduces a significant inter-symbol interference (ISI) that is dealt with by the subsequent equalizer. Continuous-time linear equalizer (CTLE) is a commonly used equalizer in both electrical and optical links. However, recent research reported different findings about CTLE-based optical receivers. Some research papers concluded that CTLEs boost high-frequency noise compared to a full-bandwidth design. Other publications reported that high-frequency noise remains unaffected while white noise is significantly reduced. This work aims to solve this discrepancy by presenting an accurate analysis for CTLE-based optical receivers considering noise, gain, and jitter. We show that the noise performance depends on the pole/zero locations of the limited-bandwidth (LBW)-TIA and the follow-on equalizer. A properly designed CTLE-based receiver achieves a
$2.5 \times $
higher gain and a
$1.74\times $
better noise than the full-bandwidth design. The CTLE is also compared to the well-known decision feedback equalizer (DFE). The noise performance of the CTLE-based receiver lies between that of finite and infinite impulse response DFE-based receivers but achieves better gain than both architectures.