Paul Brusenbach, manager of test engineering at Pico-light (Boulder, CO), has turned to PXI hardware for his company's latest tester. "We test optical transceivers," Brusenbach said, "and for both the transmitter (TX) and receiver (RX) tests, we need a digital pattern.

"We're currently using VXI," he explained, "which is more difficult to work with, much larger, and more expensive. Otherwise, they [PXI and VXI] work similarly. A bit error rate test is similar with all equipment, but you'd like it to be smaller while being fast to execute and, of course, accurate."

Brusenbach could not get all the functions that Picolight needed in VXI, but he has found all the required functionality in PXI, and the modules are smaller and less expensive. The company's new tester includes a host PC connected to a PXI chassis via MXIbus. MXI-bus is a high-speed extension of the PCI bus that allows the desktop computer to act as if it were a slot-0 controller directly connected to the PXI bus.

Both TX and RX test involve optical connections (made through the two yellow pins in the foreground) and electrical connections (made through the printed wiring board). Courtesy of PicoLight.

For the TX test, these electrical signals go directly into the transmitter input. The transmitter generates optical pulses based on the bit pattern and launches them into an optical fiber.

That fiber leads to a second PXI module—an optical-to-electrical (O-E) converter. The resulting electrical signals go back to the host computer over the MXIbus. The computer then closes the test loop by comparing the received bit pattern to that which it sent to the BERT in the first place.

The RX test is similar except that the BERT's electrical signal goes to a previously characterized transmitter. This transmitter is a special unit selected from the company's regular production for its outstanding performance. The engineers then carefully characterize the unit and put it aside for use in RX tests.

The signal leaves this test transmitter as an optical signal sent through a fiber. At the fiber's end, these light signals go into the DUT's optical input port.

The receiver converts the light signals back to electrical signals, which go back to the PXI backplane via an electrical-to-electrical (E-E) module, which is essentially an O-E module without the "O."

Once the electrical signals get back into the PXI chassis, they go back to the host computer via the MXI bus. The host computer once again closes the test loop by comparing what it sent out with what it receives.