Programmable Digitizer for Military Avionics Test System
A Line Replaceable Unit (LRU) is an avionics component designed for quick replacement at the operating location. A military customer relies on a legacy avionics test system to certify LRUs for a jet aircraft platform. Originally developed in the 1980s, this test system remains in use because several compliance tests are dependent on it.
One critical instrument in the system was a 2-channel, 8-bit, 200 MegaSample/second (MS/s) box digitizer dating back to the 1980s. This now-obsolete unit used cathode-ray tube-based digitizing elements that required calibration multiple times per year. The test system was originally controlled by a 1980s-era master computer, later upgraded in the 1990s to a newer PC—but still using the original software. Communication between the master computer and the digitizer was maintained through a GPIB (General Purpose Interface Bus) cable.
The customer sought a modern, solid-state replacement for the aging box digitizer unit—one that would not require frequent calibration. However, they required a surgical replacement: the new digitizer had to behave identically to the original in order to maintain full compatibility with the legacy software and other instruments.
The Gage solution was a 2-channel, 8-bit, 200 MS/s Gage digitizer card, installed in a modern, rugged rackmount Windows PC with front-accessible connectors and a standard GPIB card. The Gage digitizer was modified to match the original unit’s sampling rates and input ranges. The system was designed for headless operation (no keyboard, mouse, or monitor), which required a customized Windows boot-up sequence.
To ensure full compatibility, Gage engineers developed a LabVIEW shell that emulated the behavior of the original digitizer. The new system communicates using the same GPIB text command protocol as the legacy box unit. On startup, the Gage digitizer initializes to the same state as the original, processes incoming text commands, and returns data through the GPIB interface exactly as before.
Multiple Gage-based systems have been deployed with no need for routine calibration. Calibration is verified via an existing test procedure performed prior to each LRU test. Gage digitizer units are only sent for repair if they fail this built-in verification. These Gage systems have been in reliable service for many years and have tested thousands of LRUs.
