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Broadband Circuits for Optical Fiber Communication phần 3

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rằng tất cả các BSS nên được thời gian đồng bộ hóa với một tín hiệu thời gian thông thường. Bởi vì họ thường là không tương quan, chúng ta có thể viết (5,36) thu tốc độ cao, đóng góp tiếng ồn front-end thường là lớn hơn so với sự đóng góp của các điện trở phản hồi. Tuy nhiên, nhận tốc độ thấp, | BANDWIDTH 77 the bit estimation technique used and so forth. Figure 4.14 shows a decision tree distinguishing the most important cases. For the following discussion we assume that each bit is estimated independently by comparing the sampled output voltage to a threshold voltage as indicated in Fig. 4.3.9 1 B severe broadening Fig. 4.14 Decision tree to determine the optimum receiver response. If the NRZ pulses at the input of the receiver are well shaped in particular if the pulses are broadened by less than 14 of the bit interval 1 B a matched-filter response or a modified matched-filter response is the best choice 83 . In the case of white input-referred noise and the absence of sampling jitter the matched-filter response gives the best results. The matched filter is defined by its impulse response h t which must be proportional to or matched to a time-reversed copy of the received pulses x t more precisely h t x T r where T is the duration of the received pulses. This definition implies that the matched-filter frequency response matches the spectral shape of the input pulses but not generally the phase . It can be shown that in the absence of ISI the matched-filter response maximizes the the sampled signal-to-noise ratio and thus results in the lowest BER 42 83 . In the case of an undistorted NRZ signal the matched filter is given by h t x T t x t where x t is a rectangular pulse starting at Í 0 and ending at T 1 B and hence this filter is known as the rectangular filter. We discuss this case as well as a possible implementation integrate and dump in a moment. If the input-referred noise spectrum is not white or if the decision circuit exhibits sampling jitter the concept of matched filtering can be generalized to take these effect into account 19 . This case is indicated by the box labeled Modified Matched Filter in Fig. 4.14. In long-haul transmission systems the NRZ pulses at the input of the receiver usually are severely broadened for example as a result of