Equalization Techniques for CMOS Optical Receivers with Integrated Silicon Photodiode
Optical links are progressively penetrating into short-range communication applications largely due to the advancement in silicon (Si) photonics technology. Si photonics provide a small footprint and cost-effective solutions. However, improving the speed of the photodiode remains as one of the biggest challenges in Si photonics. In this thesis, we present a finite impulse response (FIR) filter based on segmented photodiodes for Si photodiode equalization. The photodiode-based FIR filter utilizes the segmented photodiodes which serve as current sources and capacitors, and realizes a high-pass FIR that partially compensates for the frequency-dependent gain loss due to the physical and the electrical bandwidths of Si photodiode. This technique is well-suited for low-power and low-cost applications since it uses only passive components available in standard Si CMOS technology. We initially implemented the FIR filter utilizing discrete components to check the feasibility of the proposed design technique. After checking its feasibility in sub-GHz range, we built the FIR filter in a standard 65 nm CMOS technology for multi-gigabit applications. The measurement results demonstrate that the proposed FIR filter developed for optical receivers with on-chip Si photodiode relaxes the equalization requirements of the following active equalizer circuit, and consequently helps to achieve an energy-efficient multi-gigabit data transmission. The second part of the thesis covers a Si photodiode optical receiver with a dual-loop adaptive equalizer. The proposed dual-loop adaptive equalizer improves the accuracy of the equalization, and hence the eye-opening, compared to that of the conventional single-loop approach. The adaptation block utilizes the spectral properties of the random bit sequence to automatically generate the optimal control voltages for the equalizer. In order to overcome the DC gain reduction issue in typical source degeneration type equalizers, the optical receiver adopts a variable gain amplifier (VGA) based equalizer that has been optimized for the proposed dual-loop adaptation block. The simulation and measurement results of the implemented optical receiver demonstrate the effectiveness of the proposed dual-loop adaptation approach.
Jung, Purdue University.
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