TEST, SAFETY & SYSTEMS Figure 3
and higher chirp frequency. Often all three need to be taken into account simultaneously. Chirp signals are usually measured
with a spectrum analyser, which is typically used to evaluate transmission characteristics for wireless communications equipment. There are two types of spectrum analysers — swept and real time. The swept spectrum analyser is based on superheterodyne technology where sampling and signal processing are sequentially repeated operations. Where the chirp frequency changes over an extremely short period, the swept spectrum analyser often cannot keep up due to sequential processing, failing to capture some of the chirps. Sections where the chirp is not captured are referred to as blind spots. The real-time spectrum analyser,
which leverages fast Fourier transforms, performs sampling and signal processing in parallel, enabling it to capture short-time changes in the chirp signal. However, the measurement frequency range or
Figure 4
analysis bandwidth of the real-time spectrum analyser is limited to the bandwidth of the instrument, which is typically a few tens to a few hundred MHz. This is inadequate compared to the FMCW chirp bandwidth for automotive radar, which ranges from a few tens of MHz to a few tens of GHz. To address this, it is necessary to measure multiple frequency ranges across the chirp bandwidth and ‘stitch’ the waveforms together. This method can capture the entire chirp bandwidth but the time taken to switch frequency ranges can give rise to a blind spot. For the most complete solution,
a combination of an oscilloscope and a spectrum analyser are often used for chirp measurement. The oscilloscope is fast enough to acquire the full-time and frequency response characteristics of the chirp as well as capture sinusoidal signals. The spectrum analyser is also used to analyse the waveforms acquired by the oscilloscope to evaluate frequency characteristics.
EVALUATING BASIC FMCW PERFORMANCE The compact and easy-to-handle ultra-wideband spectrum analyser MS2760A from Anritsu can measure the basic characteristics of automotive millimetre-wave radar signals such as FMCW chirp signal start/stop frequency, bandwidth, amplitude, frame time/period, and number of chirps per frame. Figure 4 shows the results of a 1GHz-band FMCW chirp measurement between 76 and 77GHz using the MS2760A. The instrument captures all the FMCW chirps in a single sweep. After the measurement, the data is processed using a PC. A key feature of the MS2760A
is its ability to cover a continuous frequency range from 9kHz to 170GHz for ultra-wideband applications. Furthermore, the pocket- sized instrument is easy to carry, install and perform measurements in production, test chamber and field test scenarios. These features are made possible by Anritsu’s patented non-linear transmission line (NLTL) technology which eliminates the need for a large mixer for down conversion. The Anritsu NLTL “Shockline” receiver can generate harmonics at very high frequencies and sample up to 170GHz. Due to its compact size, the MS2760A allows installing many spectrum analysers to improve testing and development efficiency and reduce the risk of project delays and expensive capital investment. As millimetre-wave automotive
radar becomes more advanced, the use of the dominant FMCW method creates test challenges due to rapid chirp frequency changes, ultra-wide bandwidth and higher frequencies. NLTL technology addresses these challenges with a single ultra- wideband instrument, the MS2760A, that is not only small but is ideal for basic FMWC radar testing to support ADAS and AD applications.
Tomohide Yamazaki is assistant manager at Anritsu Corporation.
www.anritsu.com
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