Abstract:A low cost embedded OTDR method is presented for monitoring the fiber plant of TDM optical networks without extra optical components nor performance penalty. It localizes abrupt changes in link attenuation without disturbing network operation.
This paper describes a digitally controlled on-chip monotonic Reference Current Generator (RCG) with 8-bit resolution and a LSB (Least Significant Bit) current as low as 100 nA. It was designed as a building block of a generic DC-coupled Burst Mode Laser Diode Driver (BM-LDD) for GPON (Gigabit Passive Optical Network) applications and acts as an on-chip RCG with a settling time of 18 ns for fast and accurate optical level monitoring with guaranteed monotony. The proposed architecture of the on-chip RCG is based on an 8-bit segmented current-steering Digital-to-Analog Converter (IDAC) and a bandgap voltage reference. The (3 + 5) segmented architecture of the 8-bit IDAC is an optimum combination of a 3-bit MSBs (Most Significant Bits) unit-element sub-DAC and a 5-bit LSBs binary-weighted sub-DAC offering good DNL (Differential Nonlinearity) performance. The bandgap voltage reference deviates only 0.6% of the nominal value over temperature and power supply variations. A cascade current mirror with a super beta helper circuit is used to guarantee monotony and high accuracy. The linearity errors caused by systematic influences and random variations are reduced by the proposed 2-D double centroid symmetrical architecture. Experiments confirm a DNL of +/- 0.5 LSB for the proposed RCG. The tested performance of optical level monitoring and APC (Automatic Power Control) algorithm is compliant to ITU-T GPON standards. The design was realized in a 0.35 mu m SiGe BiCMOS technology with 3.3 V power supply. The technology choice was made by heavy requirements of the innovative DC-coupled 1.25 Gbit/s BM-LDD chip. Although the proposed structure was designed as a building block for a BM-LDD chip, the design concept can be applied for developing other high linearity on-chip RCG for a wide junction temperature range (-40 to 110 degrees C)
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