NTT Research and Development 2008 Review of Activities

The explosive growth in Internet traffic continues, and as of December 2007, there were more than 11 millions FTTH subscribers. A standardization initiative for 10G-EPON^*1 is currently defining the physical specifications to attain broader bandwidths (IEEE^*2 802.3av). NTT Laboratories have developed a burst-mode receiver for 10G-EPON systems that gives a quick response to data packets from subscribers to a central office. A burst-mode CDR^*3 used in the receiver has two issues: how to achieve a large enough margin for variations of PVT^*4, and how to reduce the external devices in order to boost yield and reduce costs. The conventional architecture needs two VCOs^*5 with an oscillation-frequency error within several MHz, but this causes a low yield.

The figure shows a block diagram of the burst-mode CDR we developed. The frequency of the recovered clock is adjusted in the circuit by comparing it with the frequency of the reference clock. The CDR uses digital counters to compare frequencies directly instead of using a conventional phase detector. This enables the CDR to use only 1 VCO and removes the error in the oscillation frequency. The CDR uses a ΔΣ D/A converter^*6 in which almost all components are digital circuits for frequency adjustment, and this increases tolerance to PVT variations. The converter also reduces the number of external devices by implementing the filter in the IC itself, instead of outside of it in the conventional CDR.

We will push ahead with further miniaturization and power reduction and continue to improve the waveform quality.

• *1 10G-EPON: 10-Gigabit Ethernet Passive Optical Network
• *2 IEEE: The Institute of Electrical and Electronics Engineers, Inc.
• *3 CDR: Clock and Data Recovery
• *4 PVT: Process, Voltage, and Temperature
• *5 VCO: Voltage Controlled Oscillator
• *6 ΔΣ D/A converter: The digital-to-analog converter achieves high accuracy using oversampling and noise-shaping techniques.