Vehicle buses get the light

Automotive-grade and consumer-grade plastic fiber-optic transceivers designed for automotive optical serial buses such as Intelligent Transportation System Data Bus-1394 (IBD-1394) and Media Oriented Systems Transport (MOST). The devices send and receive light over plastic optical fiber at data rates up to 250 Mbps. Their primary use is for carrying digitized audio and video.

Test the transceivers during production for para-meters such as average optical output power and current consumption. On the receiver side, measure output voltage levels and pulse timing.

• Agilent Technologies: bit-error-rate tester, high-speed oscilloscope. www.tm.agilent.com.
• Keithley Instruments: source-measure unit. www.keithley.com.
• National Instruments: graphical programming language. www.ni.com.
• Newport: optical power meter. www.newport.com.
• Singlewell Industrial: test handler. www.singlewell.com.tw.
• Stanford Research Systems: clock generator. www.thinksrs.com.
• Tektronix: oscilloscope. www.tektronix.com.

Firecomms (Cork, Ireland, www.firecomms.com) manufactures fiber-optic transceivers used in IBD-1394 and MOST automotive optical serial buses. Each transceiver consists of a resonant-cavity LED (RCLED) and an LED driver IC for transmitting data. The device’s receiver section consists of a pin diode and a combination transimpedance amplifier and limiting amplifier. The IBD-1394 transceivers use low-voltage differential signaling (LVDS), while the MOST devices use TTL-level electrical signals.

A production test system performs functional checks on all devices. The completed parts arrive at a device handler in tubes that hold about 50 devices. The handler moves the DUT to a test head that makes connections to test equipment.

A production test system measures optical power, differential voltage swing, and timing.

To perform an average-optical-power test on a transmitter, a clock generator produces a clock signal that excites the transmitter at the device’s maximum bit rate. An optical splitter divides the DUT’s optical output, with 50% going to a plastic fiber that connects to an optical power meter. The system measures average optical power over a –40°C to 9°C temperature range for a known input current. The remaining optical power goes to a 650-nm optical detector that connects to an oscilloscope, which measures the rise time, fall time, pulse width, and pulse amplitude of the optical pulse.

Production tests also include measurements on the receiver circuits. Here, a “golden” Firecomms RCLED transmitter sends a calibrated light level through a plastic fiber to the DUT. A bit-error rate (BER) tester generates a PRBS7 data pattern for the RCLED to send to a receiver under test to simulate bus traffic. An oscilloscope measures the differential output voltage swing and timing parameters of the receiver’s electrical output.

The transceivers have a so-called “wake-on-LAN” feature. When not in use, the device enters sleep mode to conserve power, returning to full power when it receives data. The test system’s source-measure unit measures “sleep current” drawn through the DUT’s power pins. Maximum allowable sleep current is 10 µA.

Firecomms also makes consumer-grade IEEE 1394 optical transceivers that receive the same tests as their automotive cousins, but over a smaller temperature range: –°C to 70°C. For commercial-grade components, Firecomms tests transmitters with a 40-MHz clock instead of the 250-MHz clock used in automotive-grade devices. “A 40-MHz clock is fast enough to give us a high degree of confidence in the part,” said CTO John Lambkin.

“Signal integrity is important in testing these devices,” commented senior engineer Mike O’Gorman. “Sending 250 Mbps through cables and test pins to probe a part requires careful design. It has to be correct every time because of the hostile production environment with vacuum pumps running on the next bench, and molding presses hammering out parts close by.” O’Gorman uses high-quality cables and BNC and SMA connectors to maintain signal quality. “Don’t skimp on the cables” he advised.