In Compliance

From the time when automotive vehicles were essentially mechanical with spark ignition the only electrical system, through the many decades that brought the development of electrical, electronic and computer controlled automotive systems, the need for and methods of automotive testing have evolved along with the vehicles. At one time, electrical testing was sufficient. But with the dawn of the digital computer era, compatibility became a major issue. Those pesky clocked systems are inherently noise producers and are also subject to immunity issues.

Low power, license-exempt FM transmitters are used in vehicles to transmit a weak signal onto a vacant channel in the FM Broadcast Band (88-108 MHz) for the purpose of receiving Sirius-XM broadcasts on the vehicle’s FM Radio. The only method for determining compliance of these transmitters according to the FCC Rules and Regulations is to measure the field strength from the transmitter installed in three typical vehicles. This article describes an alternative method for determining compliance by measuring conducted RF power output and using a transfer function derived by measuring the radiated field strength from transmitters installed in a number of vehicles.

A case study describing the influence of TFT LCD graphics on automotive radiated emissions testing and its impact to the FM band receiver. The underlying root-cause of the EMI issue is determined using a novel technique that decodes the display’s graphics into the transmitted RGB data and predicts the data’s impact to radiated emissions. The countermeasure implemented to resolve the issue is equally novel and does not involve any hardware optimizations. The use of test images to be used in component level EMC testing is also discussed.

In the rush to complete RF immunity testing on schedule, it is not all that unusual to overlook inherent test equipment limitations. While some test equipment characteristics such as power amplifier harmonics are obviously a limiting factor, the broadband characteristics of antennas, directional couplers, power meters and isotropic field probes can hardly be considered a limitation for most applications. However, when used with power amplifiers exhibiting significant harmonic distortion in Immunity test systems, the broadband characteristics of these devices can result in measurement uncertainty and unacceptable errors.