Low-Cost Driver Powers OC-192 Modulators To 12.5 Gb/s

This evaluation board combines a limiting amplifier, PHEMT MMIC driver, RF power detector, high-frequency choke, and dither circuit, and allows driver and modulator to be matched for optimum performance.

High-capacity OC-192 optical transmitters and receivers often require optimized components, such as modulator drivers, to achieve the best performance. Due to high levels of integration, such components are typically difficult to modify. Fortunately, iTerra Communications (Fremont, CA) offers a solution with its iT6120 modulator driver evaluation board. The board allows the driver to be matched to an optical modulator for 6.5 V peak-to-peak maximum output voltage at data rates to 12.5 Gb/s. The iT6120 modulator driver board provides on-board RF power detection, dither input, and a high-frequency output choke.

The iT6120 is designed to drive electro-absorptive, Mach-Zehnder, and LiNbO3 modulators. It consists of a model iT3012 limiting amplifier cascaded with a model iT5061 modulator driver, a high-frequency choke, and dither input circuitry (see figure). Components are mounted on a two-layer printed-circuit board (PCB) measuring 1.9 × 2.0 in. (4.8 × 5.08 cm). The top layer of the board is a polytetrafluoroethylene (PTFE) laminate capable of supporting microwave data rates while the FR4 bottom layer is suitable for distributing DC.

Among the company's goals in creating the iT6120 were to extend its maximum data rate to 12.5 Gb/s for forward-error-correction (FEC), to allow impedance matching adjustments to be made, to employ a two-chip rather than single-chip solution for better thermal management, and to achieve low manufacturing cost by housing the chips in plastic rather than ceramic packages.

The iT6120 (see table) was chosen to satisfy the needs of long-haul applications, in which the greater bandwidth for FEC is a much-requested capability. The evaluation board provides an open environment for designers, who can modify component values to provide optimum performance with specific modulators. For example, the output impedance of most drivers is fixed for a standard eye at 50 Ω. However, modulator input impedance varies from manufacturer to manufacturer and sometimes even from part to part. If there is no convenient way to raise or lower impedance to match the impedance of the modulator, it is difficult or impossible to achieve optimum performance. The iT6120 uses low-cost surface-mount-technology (SMT) resistors for drain-load resistance that can be changed by the user to optimize performance for a specific modulator.

The two-chip approach taken by iTerra allows better heat dissipation without significantly increasing cost since the components are packaged in plastic. Although plastic presented significant design and packaging challenges, the resulting cost advantage was deemed great enough to warrant the additional effort, such as passivation of the iT5061 amplifier. Both devices are housed in 4 × 4-mm QFPN packages.

The iT3012 limiting amplifier allows single-ended signals ranging from 350 to 800 mVpp at the input to be limited at a constant output voltage of 1.8 Vpp. Differential-to-single-ended conversion allows signals up to 1.6 Vpp to be limited to 1.8 Vpp. The iT3012's output voltage is insensitive to variations in input voltage, so dynamic range at the input is maintained with negligible effect on the output. The bias supply accepts voltages ranging from −5.2 VDC and +5 VDC. It has a 3-dB bandwidth of 11 GHz, and provides external offset correction.

The iT5061 modulator driver acts as the output stage for the evaluation board. The traveling-wave MMIC amplifier is fabricated with a 0.15-µm GaAs PHEMT process that supports bit rates from 9.95 to 12.5 Gb/s. The amplifier operates from DC through 11 GHz with 14 dB gain, and has a 1-dB compression point of +21 dBm, and +23 dBm output power at saturation. The iT5061's integral power detector allows the amplifier's output to be monitored and used as a reference for temperature compensation. The power detector's own temperature dependence is minimized by means of an on-chip reference diode. The iT5061's dither input provides a low-frequency signal from 1 kHz to 1 MHz to enable closed-loop control of the modulator's DC biasing. For those with specialized physical requirements, custom versions of the iT6120 modulator driver evaluation board are also available. iTerra Communications, 1585 Reliance Way, Fremont, CA 94539, (510) 657-1751, FAX: (510) 657-1313, Internet: www.iterracommunications.com.

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