Abstract
This paper presents the investigation on the design of automatic gain
control (AGC) circuits assumed to be robust against both bias and swing variations
in the received unipolar digital signal. We have derived a small signal AGC
general model that includes the impact of all relevant AGC components on the
AGC transfer function. This model has been used for the calculation of AGC
loop parameters of the specified AGC transfer function. The AGC robustness
against signal level transients has been improved by introducing the second-order
AGC loop. In ac-coupled receivers, the order and corner frequency of high-pass
filter (HPF) placed in front of the AGC have been investigated. The HPF has
been designed so that the input signal bias variations caused by signal level
transients are compressed in acceptable limits, and so that bit-pattern bias
variations do not significantly decrease the receiver performance. Signal
swing and bias variations become very critical in optical systems using unipolar
modulation format. Since the AGC design depends on its application, as an
example, we have presented the AGC design concept in dispersion compensating
10.7 Gb/s intensity-modulated direct-detection (IM-DD) nonreturn-to-zero (NRZ)
optical receiver using maximum-likelihood sequence estimation (MLSE).
© 2008 IEEE
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