A new generation of compact radar sensors with long range and high resolution is under development for automated driver assistance systems (ADAS) and future fully autonomous vehicles. Operating in the frequency range from 76 GHz to 81 GHz, these sensors use phased-array antennas to obtain location information. The accuracy of the obtained data is directly correlated to the accuracy of the relative phase angles of the emitted signals, making precise adjustment of the antenna system a crucial factor for precision.
Characterization of these sensors in the development phase requires sophisticated testing and measurement equipment due to the high frequencies. Oscilloscopes are ideal for this because they can simultaneously analyze multiple signals and precisely compare them. While some spectrum analyzers offer dynamic range and sophisticated analysis features, they have only one input channel and therefore are not able to measure the phase differences of multiple signals. Four-channel oscilloscopes have an advantage here, as they can act as a phase coherent receiver and simultaneously analyze and compare up to four signals.
A test setup
External mixers are used to down-convert the radar signals to the oscilloscope’s frequency range (Fig. 1). The mixers in this example (Rohde & Schwartz FS-Z90) use the sixth harmonic of a local oscillator (LO) to generate the desired output frequency. A signal generator (R&S SMA100B) serves as the LO, while the evaluation board of a commercial radar sensor acts as the radar signal source.
The radar system uses a chirp sequence signal consisting of several high frequency pulses in direct succession. Each of these pulses is a chirp with a bandwidth of approximately 4 GHz. The sensor is configured so that the frequency of the radar signal rises linearly from 77 GHz to nearly 81 GHz (up chirp). The end of the sequence is followed by a break of several milliseconds (interframe time). During this time, the radar processor calculates the locations and speeds of the detected objects.
The IF signals from the mixers are fed to the oscilloscope inputs. The attenuation and S-parameters of the individual components in the signal path can be taken into account by the hardware and software de-embedding functions of the oscilloscope. The impact of de-embedding is illustrated in Fig. 2. The received signal is attenuated over the entire frequency range and detected with decreasing amplitude as the frequency increases (upper screenshot). De-embedding compensates for these losses (lower screenshot), enabling the oscilloscope to analyze the actual signal.
Stable trigger conditions are essential for reliable signal analysis with an oscilloscope. Oscilloscopes usually offer advanced trigger options in addition to traditional edge triggering. However, these options can only be used up to a certain bandwidth, depending on the manufacturer. With its digital triggering, an oscilloscope such as the R&S RTP can use the entire range of trigger options up to the maximum bandwidth.
Simple edge triggering is not useful for these measurement tasks since the oscilloscope will trigger on virtually any point of the signal due to the nature of the radar pulse. A pulse width trigger, which can be used to trigger on the interframe time between pulses, is more useful because it allows individual pulses or entire pulse sequences to be detected and analyzed. The trigger condition can be configured for specific radar signal parameters, for example to only display pulses with a specific duration (see the application note at the end of this article).
For the best possible spatial resolution, current automotive radars operate with bandwidths up to 4 GHz. The RTP meets the associated test and measurement requirements. With its high sampling rate and large memory, it captures the downconverted radar signal with a sufficiently high sampling frequency. The analysis tools included in the base configuration are sufficient to check the modulation in the radar signal. The signal used starts at 1 GHz and rises linearly to 5 GHz. An initial check of these frequencies starts with a frequency measurement that is configured to perform many frequency measurements within one acquisition (frequency tracking). The result is a display of the downconverted frequency versus time fIF(t).
At higher frequencies, the data points are closer together, making the measurement more difficult. Noise often increases but can be filtered out by the oscilloscope’s lowpass filter math function. It is possible to change the scaling of fIF(t). (increase the frequency axis) to display the radar signal in its original frequency range fHF(t) (Fig. 3).
Other measurement functions help users quickly determine important parameters such as the rise time of the linear frequency modulation. For example, the oscilloscope’s FFT function creates a spectrogram that shows how the radar signal changes over time. These two analysis methods (Figs. 2 and 3) allow users to perform an initial check of the bandwidth and the modulation.
Measuring phase and amplitude
Many automotive radars are equipped with multiple transmit and receive antenna arrays. These determine the directivity of the antenna and allow beamforming and detection of the direction of the target. To specifically investigate the transmit properties, for example, multiple mixers can be operated simultaneously on the oscilloscope. The setup is similar to that for single-channel analysis; the LO signal simply has to be distributed to all the mixers (Fig. 1).
When used as a phase coherent receiver, the oscilloscope analyzes multiple signals relative to each other. Typically, the phase differences and difference between the two spectra are analyzed. The FFT function of the RTP is also helpful. It is used to calculate the amplitude spectra of the signals in the two channels. The difference is then calculated with another math function and displayed.
For the phase measurement, the analysis range is limited to a narrow time corridor, and the phase difference of the two input channels is calculated from the phase properties determined by FFT (Fig. 5). The advantage of the indirect method using FFT is the larger time analysis range. Whereas a single measurement of the phase difference in the time domain can be strongly dominated by noise, in the frequency domain multiple signal periods are compared with each other, resulting in a significantly smaller measurement uncertainty.
The RTP can measure the amplitude and phase differences of multiple antenna paths simultaneously and correlate the radar signals with other signals, such as the supply voltage or digital bus signals (Fig. 6 - top). Simultaneously acquiring CAN bus or automotive Ethernet signals together with radar signals is particularly helpful during development and debugging.
The analysis time of the radar sensor can be determined from the delay between the radar signal and the bus protocol signal. If the measured delay exceeds a specified time, deployment in autonomous vehicles is not acceptable.
Dr. Ernst Flemming is director of oscilloscope product management at Rohde & Schwarz in Munich, Germany. Dr. Andreas Ritter is R&S’s application engineer, oscilloscopes.Continue reading »