TEST & MEASUEREMENT FEATURE E-band testing of automotive radars


Laura Sanchez, product management of spectrum analysis at Rohde & Schwarz discusses how automotive radars can now be tested by using a signal and spectrum analyser that supports up to 2GHz analysis bandwidth internally and 5GHz using an oscilloscope as external A/D converter


A


utomotive FMCW radars typically operate in the frequency range from 76GHz to 77GHz, and some countries have additionally granted approval for operation between 77GHz and 81GHz. Since range resolution is proportional to signal bandwidth, manufacturers of these components need high bandwidths during the development process to achieve the maximum range resolution.


SPECTRUM MEASUREMENTS IN THE E BAND For development, production and verification testing, the R&S FSW85 signal and spectrum analyser has become the first choice for measuring radar sensors’ RF parameters such as frequency, effective isotropic radiated power (EIRP) and occupied bandwidth as well as spurious emissions. The analyser scans the range from 2Hz to 90GHz and analyses RF signals produced by radar sensors in the E band. No external harmonic mixers are required. For frequencies between 8GHz and 85GHz, the analyser’s narrowband YIG filter enables hardware preselection as a means of suppressing unwanted mixing products. Compared with solutions using


harmonic mixers, this approach has advantages beyond the basics of frequency range, dynamic range and suppression of unwanted mixing products. The built-in RF attenuator makes level setting more convenient and setup is simpler without the need for additional cabling. Signal-to-noise ratio can be further


improved with an optional preamplifier between 1GHz and 85GHz (Fig. 2). This is especially useful for over-the-air measurements of radar signals.


5GHZ ANALYSIS BANDWIDTH The Rohde & Schwarz FSW85 signal and spectrum analyser now provides up to 2GHz internal analysis bandwidth with the optional bandwidth extension FSW- B2001, so that wideband signals like those from radar systems can be analysed with one single instrument. Demodulation and analysis of automotive radar signals in the E band -


Figure 1:


The R&S FSW85 supports an analysis bandwidth of 2 GHz internally.


especially in research and development labs – require even wider analysis bandwidths up to 5GHz. The R&S FSW can also analyse bandwidths greater than 2GHz, by using the optional 5GHz bandwidth extension and an oscilloscope as an external A/D converter. The R&S FSW85 mixes the input signal


Figure 2:


Using the adjustable legs, the R&S HA-Z24E external preamplifier can be set to just the right connecting height


Figure 3: Signal path for


interconnection of the R&S FSW85 signal and spectrum analyser with the R&S FSW-B5000 option and the R&S RTO2064 oscilloscope for an analysis bandwidth of 5GHz


down to an intermediate frequency (IF) of 3.5GHz, which is digitised by the RTO and sent back to the analyser via the LAN (Fig. 3). The analyser equalises the signal and mixes it into the digital baseband. The equalised I/Q samples are subsequently fed to the measurement software. The signal path from the analyser inputs to the A/D converter in the oscilloscope has been fully characterised in terms of the amplitude and phase response. From the user’s perspective, this combination of instruments behaves like a single instrument. This instrument controls the oscilloscope and handles all the steps involved in transferring, processing, equalising and analysing data.


radar signal can influence the accuracy and performance of the radar system. The R&S FSW-K60 transient


measurement application can be used to analyse these CW radar signals. Extensions for this software support analysis of chirp signals and frequency hopping signals. The application sets the start and end of individual chirp or frequency hopping signals in the I/Q data acquired. Software then calculates performance parameters within a user- defined range, e.g. measurement bandwidth and time. Fig. 4 shows the R&S FSW-K60C


measurement application for chirp signals. In window 1 (Full Spectrogram), the complete content of the I/Q acquisition memory in the time domain (vertically downward) and frequency domain (horizontal) is visible. The colour indicates the power level. Here, six chirps were detected within a measurement interval of 100μs and a bandwidth of 5GHz. Window 2 (Full RF Spectrum) isolates


one line from the spectrogram, i.e. the line in the middle that is indicated with two white markers. At this instant, the chirp is just passing a frequency of 79.4GHz (at the right in the window). Window 5 (Region FM Time Domain)


shows the frequency modulation (FM) vs. time. Green and blue bars indicate the six chirps that were detected. A video filter with 1% of the demodulation bandwidth (50MHz) suppresses unwanted signals and the noise of the peak detector. Window 4 (Chirp (4) Frequency Deviation Time Domain) shows the frequency error for one of the detected chirps (4) vs. time. The table lists all relevant parameters for the measured chirps. The approach described offers a high


Figure 4:


R&S FSW-K60 transient measurement


application: analysis of chirp signals with the R&S FSW-K60C extension


/ ELECTRONICS


ANALYSING LFMCW SIGNALS Most automotive radars use chirp sequences consisting of very short linear frequency modulated continuous wave (LFMCW) chirps. A radar’s range and speed resolution are dependent on parameters such as the signal bandwidth, chirp duration, chirp rate and signal linearity. Unwanted effects within the


performance and compact solution for measurements on automotive FMCW radars and other frequency agile, very short pulse radars in the mmW. The R&S FSW-K60 transient measurement option; its extension for chirp signals and the pulse analysis option for pulsed radars provide versatile measurement functions.


Rohde & Schwarz


www.rohde-schwarz.com T: 0) 1252 818888


ELECTRONICS | SEPTEMBER 2018 9


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