Transimpedance Amplifier
02XXX-WTP-001-A March 28, 2003
NRZ Bandwidth (-3db HF Cutoff vs SNR) How Much Bandwidth is Enough?
White Paper
Introduction
A number of customer-initiated questions have arisen over the determination of the optimum bandwidth for any transimpedance amplifier and subsequent filter employed in a fiber optic receiver module using NRZ coding. When asked what the optimum bandwidth for such a system, most engineers will respond with a number between 0.7 and 0.75 times the NRZ bitrate. The real answer is: It depends on the rolloff rate of the receiver. There is a relationship between rolloff rate and the needed sensitivity, given by the formula in Figure 3. Since fiberoptic receivers are broadband noise limited, their sensitivity is reduced when bandwidth is any greater than absolutely required. The misconception that the “magic number” is between 0.7 and 0.75 times the bitrate comes from industry standards and other manufacturers application notes taken out of context. These standards and application notes either make no mention of the roll-off rate or assume that a single-pole (3db/octave) rate is used. Once the roll-off rate is incorporated, it can be seen that the faster the roll-off rate, the lower the bandwidth required, and the greater sensitivity realized due to the narrower bandwidth over which broadband noise is amplified. Calculations indicate that a TIA bandwidth of 0.7 to 0.75 times the NRZ bitrate is too high to be optimum, and the reasoning that leads to this number is based on at least two simplifying assumptions. When these assumptions are corrected, the optimal bandwidth is seen to be nearer 0.56 1,2 times the bitrate, which is in agreement with simulation results presented in this paper. A transimpedance amplifier (TIA) can be designed with excess bandwidth on the assumption that the user will follow it with a precision filter set at the optimum frequency and with the ideal phase response. However precision filters tend to be expensive, with the
NRZ Bandwidth (-3db HF Cutoff vs SNR) How Much Bandwidth is Enough?
White Paper
Introduction
A number of customer-initiated questions have arisen over the determination of the optimum bandwidth for any transimpedance amplifier and subsequent filter employed in a fiber optic receiver module using NRZ coding. When asked what the optimum bandwidth for such a system, most engineers will respond with a number between 0.7 and 0.75 times the NRZ bitrate. The real answer is: It depends on the rolloff rate of the receiver. There is a relationship between rolloff rate and the needed sensitivity, given by the formula in Figure 3. Since fiberoptic receivers are broadband noise limited, their sensitivity is reduced when bandwidth is any greater than absolutely required. The misconception that the “magic number” is between 0.7 and 0.75 times the bitrate comes from industry standards and other manufacturers application notes taken out of context. These standards and application notes either make no mention of the roll-off rate or assume that a single-pole (3db/octave) rate is used. Once the roll-off rate is incorporated, it can be seen that the faster the roll-off rate, the lower the bandwidth required, and the greater sensitivity realized due to the narrower bandwidth over which broadband noise is amplified. Calculations indicate that a TIA bandwidth of 0.7 to 0.75 times the NRZ bitrate is too high to be optimum, and the reasoning that leads to this number is based on at least two simplifying assumptions. When these assumptions are corrected, the optimal bandwidth is seen to be nearer 0.56 1,2 times the bitrate, which is in agreement with simulation results presented in this paper. A transimpedance amplifier (TIA) can be designed with excess bandwidth on the assumption that the user will follow it with a precision filter set at the optimum frequency and with the ideal phase response. However precision filters tend to be expensive, with the
References: 1) Personick, S.D. Bell System Technical Journal # 52, pp. 843; 1973 2) Personick, S.D. Bell System Technical Journal # 52, pp. 875; 1973 3) Smith, R.G and Personick, S.D “Semiconductor Devices for Optical Communications” Chapter 4, Springer-Verlag; 1980 4) Williams, G.F Bell System Technical Journal pp 105 (1983) 02XXX-WTP-001-A Mindspeed Technologies™ 11