June 26, 2020
With the rise in connected cars and autonomous vehicles, the need to accurately test automotive radar has taken on greater importance. Engineers must characterize materials used in bumper manufacturing to ensure they do not affect radar transmissions in these mission critical applications. It is such a concern that the major automobile manufacturers are requiring contractors to measure the material characteristics in the E-band frequency range on all related components used in radars.
Different materials, paint coatings, plastic injection methods, and material widths must be tested in a manufacturing environment to accurately characterize their behavior. To accomplish this within the time and cost constraints associated with high-volume production, a vector network analyzer (VNA) system must be simple to operate. It must be nearly plug-and-play, have a calibration method that takes a few minutes, and have a repeatable and easy-to-perform measurement method.
Production Testing Procedures
A radar placed behind an emblem or bumper of a car must transmit its signal with minimal interference for automotive safety systems to operate properly. At the output of the production line, all bumpers and emblems must be measured to ensure that they comply with safety standards. Results should be saved to maintain traceability.
To test the emblem or the bumper, a VNA with E-band support, such as the ShockLine™ MS46522B, can provide the necessary frequency range coverage. It can also measure all S-parameters (S11, S12, S21, and S22) and characterize the behavior of the material. For these measurements to be conducted accurately and efficiently, the configuration consists of two antennas attached to both ports of the VNA to transmit and receive the signals.
Factors Influencing Testing
When conducting this test, the shape of the material under test (MUT) must be considered. Width, paint, and manufacturing process are all factors that will influence results. Another determining factor is if it is better to use a single, large beam on the whole surface to be tested or a smaller beam requiring two or three tests.
The most important variable is reflections. In an open environment, reflections can adversely affect the measurements. Engineers can avoid them in one of two ways. The first is to place an absorbent material all around the room, however, this can complicate the setup. The second is to use a VNA, such as the ShockLine, that has a gating process.
Conducting Tests
Figure 1 shows a system to conduct radar measurements on automotive emblems and bumpers. Test results using this configuration were precise and consistent, with measurement accuracies in the order of a few hundredths of a dB. When conducting these measurements, the ShockLine VNA did not seem to be the most critical element affecting the measurement accuracy; it was the actual position of the tested sample. The measurement is influenced by the angle between the direction of the transmission and the plane of the sample.
Traditional VNA E-band measurement systems can yield sufficiently accurate results. They are not suited for a manufacturing environment, though, because of their cost and size. The economical and compact “plug-and-play” form factor of the ShockLine MS46522B, however, is perfectly suited for this type of production use.
LRM Calibration
The calibration method used for these radiated measurements is the Line, Reflect, and Match (LRM) algorithm, in which a reflection plane is used for reflective calibration, a line for transmission calibration, and a perfect match. For the LRM algorithm, the Reflect for the ports is performed with a metal plate perpendicular to both antennas. The Line is done without the plate. For the Match, the metal plate is positioned at a 45° incline so there is no reflection back to the ports to create the same behavior of a perfect match.
Once the system is calibrated, the measurement of the “Line” will give the flat trace of both magnitude and phase. Figure 2 shows an example the results achieved after a good calibration procedure.
It is expected to have a magnitude ripple of less than ±0.25 dB and a phase ripple of less than ±0.2°. The calibration process, however, can be affected by parameters such as the position of the metal plate and external reflections.
The bumper or material sample is set at the reference plane, which is the middle point between the antennas. The distance between the antennas is dependent on the test requirements. Far field conditions need to be met. For far field, which is ten times the wavelength, the antennas are set at least 5 cm apart. To avoid dispersion, the radiation spot on the sample must not exceed or even be similar to the sample size to avoid dispersion. Figure 3 illustrates the situation if a dielectric antenna and horn antenna are used.
Other Testing Variables
Using dielectric antennas with a distance of 31 cm between them, the radiation spot diameter is less than 3 cm for a 3 dB attenuation and less than 5 cm for a 10 dB attenuation. Similar calculations with the horn antennas show a spot of 57 mm in diameter. Based on this data, a sample of 15x15 cm provides enough margin to have confidence the spot will fall entirely in the sample.
In the case in which absorbent material is not available, a gate can be configured to improve the measurement and avoid external reflections. Thus far, the material position has been established as the reference plane, which is the zero position. The gating should now be configured around the zero position.
Next, the width of the window to use in the setup needs to be determined to improve the results. This depends on the frequency span being used in the measurement. The higher the frequency span, the shorter the window size needed. Table 1 shows the relationship between the configured frequency span and the window size (Note: these values state the window size for reflection measurements; for transmission measurements the window size must be doubled):
To learn more about conducting E-band measurements for automotive radar, download the Anritsu application note entitled E-Band VNA for Automotive Market: Bumper and Emblem Measurements.
