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How to add value to MSA optical transceivers

Posted: 16 Dec 2003 ?? ?Print Version ?Bookmark and Share

Keywords:msa? optical transceiver? optical communication? dac? adc?

Optical fiber is the transport medium that has emerged to accommodate the expected long-term growth in the communications industry.

Taking advantage of the wide bandwidth - the principal advantage in optical communications systems - requires stringent control and monitoring of various optical fiber related parameters such as temperature, average laser power and extinction ratio.

The evolution of today's optical transceivers and transponders is driven by multi-source agreements (MSA), which establish a standard for optical and electrical characteristics, module form factor, and pin functions of optical transmitters and receivers. In more recent years, the emerging MSAs have stipulated that optical system designers incorporate various diagnostic functionalities into transceivers, as well as the traditional monitoring and control functions. These requirements are in addition to an overall drive to minimize size and cost of the optical transceiver design.

Engineers whose core competence and background is in optical fiber systems now struggle to understand how to best implement analog and digital control loops, and provide diagnostic information via a 2-wire I2C-based communications bus. Furthermore, they must ensure that their designs continue to adhere to the fast moving and evolving optical transceiver specifications.

The ongoing development and evolution of optical MSA specifications such as XFP, XENPAK, XPAK and SFF-8472 present today's optical system designers with a unique set of challenges which must be addressed in an increasingly competitive market. The design, test and manufacture of optical transceivers can be greatly simplified by a single chip, reprogrammable DAQ solution that can provide control, monitoring, and diagnostic functions in MSA optical transceivers.

To date, the primary challenge for designers has been maintaining constant optical power (Pav) and constant extinction ratio (Er) with changes in temperature - these parameters are a measure of signal integrity and transmission distance. In addition, designers must include digital diagnostic monitoring to comply with MSA specifications.

A reprogrammable, single-chip precision DAQ system with microcontroller in a small footprint can simplify this design challenge. Addressing both the optical monitoring and control design task, this approach allows designers the flexibility to easily incorporate additional features, thus adding value to their optical transceiver or transponder module.

Let us examine a typical optical module. On the receive side, data is fed directly from the trans-impedance amplifier to an external ASIC or Serdes. On the transmit side, data is fed to a high-speed laser diode driver, which drives the laser and modulates the data signal onto it. A single-chip, such as Analog Devices' MicroConverter, can take care of Pav and Er control, and also feed digital diagnostic information back to the host.

In this case, the ADCs, 8052 controller and DACs are integrated on a single mixed-signal device, the ADuC832. The ADCs and DACs are used to control Pav and Er. This approach takes full advantage of the analog peripherals, while offering users the flexibility to implement either conventional closed-loop control via a proprietary algorithm or feed-forward control via a look-up table.

A typical MSA requires that five parameters be available from a module as follows:

7 TX bias current (accuracy >= 2?A);

7 TX optical power (accuracy >= 0.1?W);

7 RX optical power (accuracy >= 0.1?W);

7 Transceiver supply voltage (accuracy >= 100?V);

7 Transceiver temperature (accuracy >= 3C).

This can easily be achieved by using four 12bit ADC channels to measure Vrxp, Ibias, Imod and Vdd, and a sensor to measure temperature. Results can be scaled or offset by the 8052 microcontroller before being transmitted to the host system via the I2C serial port. Furthermore, users can compare these values to preset alarm and warning threshold levels to detect fault-conditions. Users have the option of monitoring additional parameters using the remaining four ADC channels. For more sophisticated transceivers the PWM outputs may be used to provide extra control channels.

The same principles apply in all optical transceivers and transponders, whether single-channel or D-WDM; direct or external modulation; 2.5G or 10G; SONET or Ethernet.

- Helen Stapleton & Brian O'Mara

Analog Devices Inc.

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