VNA Reflections

How to Make Efficient E-Band Car Radar Emblem Measurements

February 26, 2020

Automotive radar systems operate in complex and dynamic environments that can have a significant impact on their performance. In particular, the material properties of the vehicle bumpers and location of the radar behind or within the bumper can negatively impact operation. An example is that insertion loss or phase change of the material in front of a car radar can influence the radar’s detection range.

Engineers designing these radars in systems such as collision avoidance and adaptive cruise control (ACC) must determine the impact materials in front of the radar will have on the radar detection range. The test tool of choice for most engineers is a vector network analyzer (VNA) to conduct transmission and phase measurements. In today’s labs, the VNA system must not only be accurate but make measurements as efficiently as possible.

Radar System Categories

Radar systems are the technology of choice for collision avoidance because of their inherent advantages. They operate in all-weather conditions, conduct better measurements, detect more objects, and are easily incorporated into the car design. Radar systems typically fall into three categories: short-range radar (SRR), ultra-wideband SRR (UWB SRR), and long-range radar (LRR) (Figure 1).

Figure 1: Typical automotive radar field of view (FOV) and range distance.

Originally operating in the 21.65 GHz to 26.65 GHz frequency range, these systems have transitioned to the 77 GHz to 81 GHz band. This creates new issues with mounting the SRR and UWB SRR sensors on the car chassis. Materials and coatings of radomes, emblems, and bumper skins can also interfere with the radar systems.

Car Radar Technology

Nearly all ACC systems use a 77 GHz LRR system that is typically mounted behind the car emblem. Transmit and receive patch antennas of the radar are focused by a dielectric lens and operate at a 4 mm wavelength range. Because the radar beam looks through the car emblem, the reflected signal from the target is exposed twice to the influence of the radome.

The advantage of a frequency-modulated continuous wave (FMCW) radar is that it measures the distance of other vehicles, as well as directly measures the vehicles’ speeds. Based on design principles, the range of a car radar can cover up to 200 m.

When using an FMCW radar for distance measurements, the output signal frequency continuously changes over the transmission time. Plus, it is linearly modulated to reach a maximum of 81 GHz over a given time period. This waveform is called a chirp. A frame consists of numerous chirps, each lasting for a given chirp time, known as  TChirp. The bandwidth and slope of each chirp is also crucial to the performance of the FMCW radar. These parameters have a direct influence on the maximum range and velocity and their corresponding resolutions.

An example of these factors is shown in Figure 2. A 76.15 GHz radar wave starts at 0 ms and 30 ms later a reflection is received that is influenced by the distance to the target and its speed (the frequency shifts are depicted by the red arrows for ∆f1 and ∆f2). The comparison between the transmitted signal generated by the radar to the reflected signal by the object gives an indication of the distance. The transmitted and received signals are mixed into an intermediate frequency signal where a Fourier transfer range (FFT) on this intermediate frequency signal reveals information on the distance of the object with high accuracy. Based on this, the frequency difference is a direct measure of the distance.

Figure 2: FMCW transmit and shifted receive frequency chirp.

A second effect has to be included in the calculation – the relative speed of the object. In Figure 2, the emitted frequency that is received is reduced, indicating that the object is moving away from the radar.

By selecting a triangle edge (i.e., increasing and decreasing frequencies), the distance and the velocity of an object after one cycle (in this case 200 ms) can be determined. The radar speed measurement considers the Doppler effect, which states that there will be an increase or decrease in frequency wave as the vehicle moves towards or away from an object. In the figure, the emitted frequency received is reduced, indicating that the object is moving away from the radar.

A New VNA Configuration

VNAs for mmWave applications such as these have been historically large, heavy, complicated, and very expensive. This scenario is less than ideal for automotive radar designs. Fortunately, a new approach that shows significant advantages for these applications can be seen with the Anritsu ShockLine™ MS46522B VNA, which can be configured as a dedicated E-band VNA. The ShockLine MS46522B E-band VNAs configuration consists of small tethered source/receiver reflectometer modules and a base chassis.

The remote, small form factor reflectometer modules have native WR12 waveguide interfaces for convenient connection to typical waveguide devices. They also have short/long-term thermal stability due to the vanishing thermal gradient across the modules, high amplitude and phase stability, and raw directivity. Most importantly, placing the sampling directional bridge closer to the AUT/DUT provides long-term amplitude and phase stability.

Attenuation Measurements

Benefits of this system can be seen in the following example. Figure 3a shows the attenuation at 77 GHz. The middle of the emblem is marked by a vertical and a horizontal line. The attenuation of both lines at 77 GHz is displayed in Figure 3b. In the region of interest, the attenuation is fairly flat (between 0 dB and –1 dB) and increases at the edges of the emblem due to the shape and characteristics of the material.

Figure 3: 1-way insertion loss of radome.

To give an overview of the attenuation of the whole frequency range, figure 4a has five marked positions. The corresponding lines are shown in the same color in figure 4b. The electromagnetic wave hits the central position orthogonally, and therefore, the signal has a higher attenuation. Each position over the whole bandwidth has attenuation between 0 dB and –1 dB. Based on this, the signal loss caused by the shape and material characteristics is minimal and does not affect the radar significantly.

Figure 4: 1-way insertion loss of radome vs. frequency.

Value of Phase Shift Measurements

Phase shift provides additional information. Figure 5a shows a phase map of the emblem at 77 GHz. Figure 5b shows that the vertical red line changes over the distance, which might be due to an unevenly positioned emblem. The horizontal blue line has an average of –40° except on the edges of the emblem. The constant phase over the horizontal length indicates a uniform emblem shape and characteristic over the observed distance, so the phase is not heavily affected.

Figure 5: Phase along radome at 77 GHz.

Figures 6a and 6b display an unwrapped phase to see continuous progress. Figure 6b displays the various frequency sweeps. Due to the dispersion, the phase changes with increasing frequency, so the shape of the emblem does not affect the phase of the transmitted signal. The red and yellow line have a different starting point because the emblem used for this measurement was uneven.

Figure 6: Unwrapped phase versus frequency.

To learn more about the challenges of conducting E-band car radar emblem measurements and how they can be overcome with the proper VNA-based test system, download this Anritsu application note.

Improving mmWave Device, Subsystem Design and Modeling Via New Broadband VNA Technology

February 11, 2020

Millimeter wave (mmWave) frequencies are now designed into more direct applications and device/subsystem modeling efforts require even higher frequency measurements. This poses a challenge, as limitations in conventional test equipment make conducting accurate measurements a problem. Engineers need a solution that can reduce design turns, lower cost of test, and give them greater confidence in their designs. New technology in vector network analyzers (VNA) addresses these issues and presents a solution to high-frequency design validation.      

A broadband VNA system is now available that covers low frequencies through 220 GHz and accompanying probes can help resolve some of these measurement complications. It eliminates such problems as stitching together data from multiple bands, power control, linearity, and data stability. The overall performance of the system improves measurement performance in a variety of categories.

High-frequency Broadband Measurement Challenges

A typical mmWave test setup may consist of a broadband system supporting up to 110 GHz or 125 GHz followed by measurements using banded waveguide modules. In mounting and demounting modules and probes from the probe station, there are several issues. Let’s discuss a few.

Multiple Touchdown Cycles. For broadband device model parameter extraction and IC performance verification tasks, product and specifications require the same device under test (DUT) to be measured at numerous touchdown cycles of the DUT contacts. 

Small DUT Contact Pads. Another issue is that the DUT contact pads suitable for sub-mmWave probing are much smaller since parasitic pad reactance increases with frequency. As small-sized pads can support only a few RF probe touchdown cycles, it can become impossible to measure the same DUT over the whole frequency range and all temperature points.  

Test Cell Downtime. Frequent system reconfiguration and changeovers are required for wide-frequency range measurements. This increases the test cell downtime and risk of damaging expensive system components, such as wafer probes and VNA extenders. All of these increase an already expensive cost of test.

Data integrity. Since different probes are being used in the separate bands and various measurement modules are used, the data is acquired under different circumstances. While the calibration will correct for many differences (though not linearity, noise, spectral purity, drift or repeatability differences), the uncertainties and distributions in the different bands will likely vary. This raises some question on how to handle the inevitable data steps such as those shown schematically in Figure 1. It is an example of data stitching anomalies along with a change in expected uncertainties at an equipment boundary. Since different hardware, probes and calibration elements are employed in the two bands, the measurement characteristics can change significantly.

Figure 1

Adventure Known as De-embedding

Once the data is collected, de-embedding to the desired reference plane can sometimes be an adventure. Particularly in more current BiCMOS and CMOS processes, there may be 5-9 metal layers involved. The desired transistor reference plane may be at the bottom layer while the probe pads are at the top. This leads to a “network to de-embed” consisting of many vias and transitions, significant insertion loss, and mismatch. Since such fabrication processes are increasingly of interest at mmWave frequencies, the stability and basic accuracy of the S-parameter data can be critical. 

RF Drive Level Considerations

Another aspect of the measurement process, RF drive level, can be important for bare transistors and certain amplifiers at higher frequencies. The drive power may need to be -40 dBm or lower and accurate control of that power may be important. Figure 2 is an example measurement of a 120 GHz LNA where initially drive level was not considered (labeled “nominal,” was not flat, and reached -15 dBm in places) along with a measurement where the power was leveled at -40 dBm. Not only was the gain highly compressed in the first measurement, but because of flatness issues, the amount of compression varied significantly with frequency.

Figure 2

Another consideration is the linearity of the instrument receiver, as correcting for receiver distortion can be difficult. Since many mmWave receivers use relatively high order harmonic mixing, management of the internal LO system is important to maintaining linearity at the test port.

Novel Approaches Improve Testing

One way to address some of the challenges is a broadband VNA with sufficient levels of integration and system control to allow for reasonable stability and relatively high linearity receivers. The ME7838G (figure 3) is composed of a base VectorStar™ VNA and mmWave modules. The modules handle receiving chores from 30 GHz - 226 GHz using broadband forward couplers and a high LO sampling system based on III-V nonlinear transmission lines.

The port-referred third order intermodulation product of these downconverters exceeds 30 dBm, which helps with the linearity requirements. The modules handle source multiplication above 54 GHz and use a series of four multiplexers to inject energy from the respective multipliers. Since progressively less power is available at the higher mmWave frequencies, the highest frequency multiplexer is last and has the tightest coupling. Leveling loops, for both RF and LO, extend out to the modules to help improve measurement stability.

The integration of the receivers, couplers and multipliers in a small space helps with thermal uniformity and stability, as does the close location of the couplers to the probe tip. The test port is a novel structure supporting a coaxial mode (0.6 mm outer conductor diameter) but uses a precision UG-387 flange instead of a threaded body to form the outer mate. The increased mating area of the flanged interface improves durability and significantly reduces axial forces and bending moments of the connecting device. Such an interconnect system allows a single on-wafer probe to make measurements from 70 kHz to 226 GHz.

Precision probes also contribute to well-matched probing for improved calibration and measurement results over a wide frequency. The 220 GHz MPI TITAN^TM Probes incorporate perfectly-matched 50 Ω MEMS contact tips to ensure optimum results over extreme frequency range. The design provides excellent visibility of the tip contacts during touchdowns. The result is consistent positioning of the RF probe on calibration standards and small DUT pads for better repeatability and reproducibility, even by inexperienced operators.

Figure 3

Stability is important to maximize the time between probe tip calibrations and to avoid adding data distortions that may complicate model fitting or other analyses. The measurement of a thru line over 18 hours at 25+/-3 C is shown in Figure 4. As might be expected with temperature variations affecting LO cables, there is some increase in drift with frequency since the LO multiplication factor increases along that axis. 

Figure 4

To learn more about the design of the new broadband VNA technology and how it improves mmWave testing, download this new Anritsu white paper.