Piecing Together Boeing's Cable Test System

Boeing Vertol's automated coaxial cable test system consists of a scalar network analyzer from Anritsu Co. (Morgan Hill, CA) operating under the control of a personal computer (PC) with custom test software. The system measures the insertion loss and/or VSWR of every coaxial cable within an aircraft. An option supports testing of the distance to a fault for any cable found to be defective. The SNA, which incorporates a dedicated test signal source, is supported by two sensors and an autotester

Typically, an SWR/return-loss bridge is connected between the output of a stable test source and the sensor being calibrated, with two different calibrations performed. First, a short and open are connected to the autotester's test port. This reflects all of the power from the source to the sensor in the autotester. The load is then connected to the test port of the bridge for distance-to-fault measurement calibration.

In addition to the open/short calibration, a through calibration is made to the B sensor through the autotester. This provides a reference power level for insertion-loss measurements, with the measured level serving as a 0-dB reference.

The return loss of a cable under test is measured by inserting it between the autotester's test port and the B sensor. The SNA's internal generator applies RF/microwave stimulus through the autotester to a cable under test. Any impedance mismatch, caused by the connector at either end or the cable itself, produces a reflected signal that returns to the autotester sensor where it is measured in relation to the 0-dB calibration reference. Results are shown on the computer display.

Cable insertion loss is measured using the same test setup as used for the return loss measurement. The SNA's internal signal generator applies an RF/microwave stimulus through the autotester and cable under test to the B sensor connected to the other end of the cable being measured. The insertion loss is analogous to the attenuation of the signal through the actual cable and connectors being tested.

Distance-to-fault measurements are based on frequency-domain reflectometry (FDR). The technique utilizes vector addition of the SNA's internal source output signal with the reflected signals from any faults (impedance mismatches) within the cable under test. This vector addition of the signals creates a ripple pattern at the A sensor of the SNA. The number of ripples is directly proportional to the distance to the mismatch or reflective point on the transmission line.

The software engine that operates the measurement equipment is a LabVIEW based, graphical-user interface (GUI) test executive developed by In-Phase Technologies The test executive provides both text and photographic instructions that prompt the operator in performing system calibrations and measurements. Integrated within the software are GPIB commands that control the SNA, printer and, in the case of a multiple input system, the coaxial switches. Each instrument command is sent over the GPIB bus and the SNA's response data is received and analyzed.

Cable selection is usually made from a directory listing that contains all of the cables on the aircraft, sorted by part number. Once a cable is selected, a calibration is performed. Test results are displayed on the system monitor and a clear pass/fail indication is provided. Test results can be printed or forwarded to the engineering department/customer for analysis or saved to hard disk. Software updates can be accomplished via Ethernet port or a CD-ROM supplied by In-Phase Technologies.

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