SAE/Ford Triplate (TPL) RF Immunity

This is a test for components or subassemblies intended for use in automobiles. The components are subjected to very high field strengths which might be found inside the structure of an automobile.

A Triplate is an open sided transmission line consisting of three plates. The outer plates are at ground potential and are connected to the shield of the coax cable that feeds the signal. The center plate, or septum, is connected to the center conductor of the coax cable. The DUT (device under test) is placed between the bottom plate and the septum. The field strength may be as high as 200v/m. At lower frequencies, a tranvese wave is generated between the plates, and the E-field is vertical in polarity. At higher frequencies the TPL will go modal and the field generated will be less homogenous, but the field can still be used to determine susceptibility to RF interference.

The current version of the test standard (based on SAE J1113-25 Feb 1999) uses mid-point power to level the field at the prescribed field strenth. A calibration is performed to determine the impedance of the triplate at each test frequency. This is done by measuring the field strength at five different positions along the centerline of the triplate, the net-power (forward minus reverse) going into the triplate and the power coming out of the other end of the triplate. The impedance values are used later during the actual test run.

During a test run, the three power levels are measured to determine the power level at the mid-point of the triplate. This, along with the impedance of the triplate, is used to determine just how much power is needed to reach the prescribed field strength.

The software to perform this test has to do a lot of things, namely controlling multiple instruments and constantly making adjustments to the power levels to reach the required test level. In order to save time leveling at each frequency, the software I wrote only takes forward power readings until the forward power exceeds the required mid-point power. Then, only the forward and reverse power readings are taken until that exceeds the mid-point power required. Then the output power is included to finally achieve the desired test level.

The current version of the software is written in Turbo-C running under Dos, although I have plans to convert this to a VB program in the near future.

TPL-Software Documentation.