This chapter presents optical receivers that intentionally limit the bandwidth so as to achieve higher gain and better sensitivity. This has the consequence of introducing that ISI that must be removed by a suitable equalizer. Important aspects of the noise analysis of these receivers are clarified. Various approaches to equalization in these reduced bandwidth receivers are presented by way of recently published examples.

A central challenge in the design of electrical links is to compensate for frequency-dependent loss in the channel that introduces inter-symbol interference (ISI). This chapter presents the overall objectives of joint Tx/Rx equalization. The system-level operation of transmitter-side feed-forward equalizers (FFEs) is discussed. Circuit details are presented in Chapter 5. Receiver-side continuous-time linear equalizers (CTLEs) and finite-impulse-response (FIR) filters are discussed next, followed by decision-feedback equalizers (DFEs). DFEs differ from FFEs, CTLEs and FIRs in that they only remove ISI rather than attempt to invert the low-pass channel characteristic. With the growing trend toward ADC- based receivers, the implementation of DFEs and Rx FFEs is discussed in the analog domain and the digital domain. The topics in this chapter are also a relevant background for the sections in Chapter 10 that discuss TIAs for reduced bandwidth systems, where equalization is used to remove ISI from an intentionally bandwidth-limited optical receiver front-end.