Direct answer: Rohde & Schwarz’s 48-page application note Automotive Radar Sensors: Transmit Signal Analysis and Interference Tests (document 1MA267_1e) shows how to capture automotive-radar waveforms, inspect their time-frequency behavior, and expose interference effects such as noise-floor rise, reduced SNR, missed detections, and apparent targets. The source title says “Inference Tests,” but the document consistently discusses interference tests. It is a vendor application note and a practical measurement example—not a peer-reviewed benchmark, universal performance limit, compliance certificate, or vehicle-safety case.
Read the official PDF (1MA267_1e) or the hosted All About Circuits page.
What the application note addresses
Radar supports collision warning, blind-spot monitoring, adaptive cruise control, lane-change assistance, rear cross-traffic alert, parking assistance, and automated-driving functions. As vehicles add sensors, nearby transmitters and unrelated RF sources can enter a receiver’s band. Depending on frequency offset, waveform similarity, timing, power, filtering, polarization, and distance, interference can raise the receiver noise floor, hide a real reflector, corrupt a detection list, or produce an apparent target.
The note discusses examples around 24 GHz, 76 GHz, 77 GHz, and 79 GHz. Current automotive work commonly covers the 76–81 GHz region, including emissions, susceptibility, interference robustness, and OTA measurements; the older note is not a complete statement of worldwide requirements. See the current R&S automotive radar-testing overview.
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Radar waveform basics
CW and FMCW
Continuous-wave radar transmits a carrier; frequency-modulated continuous-wave (FMCW) radar sweeps it, usually in chirps. Mixing a received echo with a reference produces beat frequency, from which range and velocity are estimated. A chirp’s bandwidth, slope, duration, and repetition interval determine what the receiver can separate.
Chirp sequences and hopping
The note describes chirp sequences, including typical chirp lengths of approximately 10–50 µs, and contrasts them with a much longer example used in a 76 GHz measurement. Transient analysis also reveals stepped or hopped components that a single spectrum trace can conceal. Determine whether a waveform uses upchirps, downchirps, multiple slopes, idle intervals, or auxiliary transmissions before designing an interferer.
Measurement setup and signal chain
The historical setup measures a radar under test (RUT) over the air. A horn antenna receives the transmission; an R&S FSW signal and spectrum analyzer downconverts it to an IF; an R&S RTO2044 or RTO2064 oscilloscope digitizes that IF; and an R&S HMP programmable supply powers the RUT. LAN or direct Ethernet links the instruments, while a shared 10 MHz reference synchronizes the FSW and RTO. The FSW B2000 analysis-bandwidth and FSW-K60C/H transient-analysis options are used. FS-Z90 harmonic mixers support W-band generation or conversion in the interference setup.
- Transmit the RUT over the air.
- Collect it with the horn antenna.
- Downconvert the received RF to IF in the analyzer.
- Digitize the IF in the oscilloscope.
- Inspect spectrum, transient, spectrogram, and signal-description views.
- Complete the combined FSW–RTO alignment before treating the IF path as calibrated.
Alignment, common referencing, antenna pointing, conversion-loss correction, and known path loss are essential. A generator setting is not the same as power incident at the RUT.
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How to read the transmit measurements
Spectrum
A spectrum view shows occupied bandwidth and outlying emissions, but not when they occur. One example shows an approximately 198 MHz-wide chirp near 76.5 GHz and additional signals roughly 375 MHz from the displayed center. Those values describe one radar measurement, not a general radar specification.
Transient and spectrogram views
Transient analysis answers how long a chirp lasts, how often it repeats, what slope it uses, and whether unexpected segments occur. A spectrogram supplies the time-frequency relationship needed to identify hopping, idle periods, and overlap with a receiver’s active window. The note’s example uses a 1 ms acquisition to capture several chirps and transient-analysis bandwidths up to 500 MHz within a 2 GHz analysis-bandwidth workflow.
Detector and trace choices
In the 77 GHz example, Trace 1 uses Clear Write with an RMS detector and Trace 2 uses Max Hold with a Positive Peak detector. Max hold is useful for finding intermittent emissions but is not time-resolved. Center frequency, span, RBW/VBW, detector, trigger, acquisition time, and analysis bandwidth must be chosen from the actual waveform and test requirement rather than copied as universal settings.
Interference-test scenarios
24 GHz demonstration
An IMST RADAR SR-1200 observes a reflector about 12.2 m away. The comparison uses 50 FFT-capture cycles and mean values:
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| Stimulus | Levels and characteristics | Typical displayed effect |
|---|---|---|
| AWGN | 0 dBm and 10 dBm; 160 MHz bandwidth | Broad FFT contamination and higher apparent noise |
| FMCW | 0 dBm and 10 dBm; 200 MHz bandwidth; 6 ms duration | Timing- and waveform-dependent distortion |
| CW | 0 dBm and 10 dBm at 23.3 GHz | Localized response that can resemble a close target |
These are stimulus levels for this geometry, not immunity thresholds. A strong close-range bin can be leakage, a room reflection, or receiver processing—not proof of a physical ghost object.
76 GHz demonstration
An INRAS Radarbook is tested with an interference source built from an SMW vector signal generator and SMZ90 multiplier. The cases are a time-aligned chirp, a triggered downchirp, and CW near 76.23 GHz. In the displayed example, the aligned chirp raises the noise floor and reduces close-target echo power by approximately 9 dB. The downchirp leaves an approximately 14 m office-wall target detectable but at lower SNR, while CW raises the spectrum noise floor. Some targets disappear in the displayed processing. These observations depend on the RUT, power, geometry, trigger, and algorithms.
What interference can do to radar output
- Noise-floor elevation: small radar-cross-section targets lose detection margin.
- Structured artifacts: a similar chirp slope can create range-Doppler features.
- Close-range false response: CW or leakage can create a localized apparent target.
- Missed detections: corrupted or saturated processing can remove an otherwise visible reflector.
- Track instability: intermittent detection loss can break continuity or increase latency.
- Angle errors: spatially uneven interference can disturb antenna-array estimation.
A ghost target means an apparent target produced by interference rather than a physical reflector. It must be distinguished from a sidelobe, stationary room reflector, leakage spike, elevated noise bin, or tracker error.
How to design a meaningful modern test
Characterize the transmitter
- Frequency accuracy and phase noise
- Occupied bandwidth, chirp slope, duration, and repetition interval
- Hopping, spurious, and harmonic emissions
- Power or EIRP and antenna-pattern behavior
- OTA-versus-conducted repeatability
Measure receiver robustness
- Interferer offset, bandwidth, waveform, and incident power
- Relative timing, polarization, angle of arrival, distance, and path loss
- Simultaneous controlled target echoes
- Detection probability, false-alarm rate, range/velocity/angle error
- Track continuity, latency, saturation, and recovery
Use the right evidence level
RF spectra establish coupling or processing impact. They do not establish failure of emergency braking, adaptive cruise control, or another vehicle function. Vehicle-level claims require detection-output, tracking, system-response, and safety-oriented tests under documented scenarios.
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Limitations of the application note
- It is vendor-authored and demonstrates Rohde & Schwarz equipment, not an independent comparison.
- The interferers are not necessarily fully matched to a real second radar, synchronized in every case, or calibrated to a universal incident field.
- FFT comparisons provide limited statistical evidence and do not establish probability of detection, false alarms, or functional safety.
- Legacy product names and workflows require verification against current instruments and software.
- Demonstrated 9 dB loss, 198 MHz bandwidth, 12.2 m geometry, and other figures are example-specific.
Common failure modes
- Poor antenna alignment, uncontrolled room reflections, or unknown polarization
- Missing common reference or incomplete FSW–RTO alignment
- Insufficient analysis bandwidth or an acquisition shorter than a frame
- Unstable triggering, analyzer compression, mixer conversion-loss error, or unverified calibration
- Confusing generator power with OTA incident power
- Using max hold as though it were a time-resolved waveform
- Interference that overlaps only selected chirps or frame positions
A null result does not prove immunity: the stimulus may simply be too weak, poorly timed, differently polarized, or unlike the RUT’s active processing window.
Equipment for a current laboratory
| Equipment | Role | Relevant qualification |
|---|---|---|
| R&S FSW | Wideband RF capture, spectrum, transient, and spectrogram analysis | Specify frequency range, instantaneous bandwidth, phase noise, and options |
| R&S SMW200A | CW, AWGN, FMCW, and replayable interference | W-band work may require multiplication or millimeter-wave hardware; enterprise pricing is quote-based |
| R&S RTP | IF digitization and time-domain analysis | Current family is listed at 4–16 GHz bandwidth and up to 40 Gsample/s |
| R&S AREG800A | Controlled range, velocity, direction, and multi-object echoes | Unnecessary for transmitter-only characterization |
| R&S ATS1500C | Repeatable 77/79 GHz OTA measurements | Compact 1.3 m² footprint; installation and calibration are substantial |
| R&S NRPxxS/SN/SN-V | Power verification and RF-path calibration | Family reaches 90 GHz; power measurement does not replace waveform analysis |
Keysight, Anritsu, and NI offer alternative radar and modular-RF platforms, but model-level equivalence, availability, and pricing must be verified for the intended band and test standard. For regulatory work, use the applicable standard or OEM specification; the application note alone is not compliance evidence.
Bottom line
This note is most useful as a practical blueprint: capture the radar over the air, preserve time-frequency information, synchronize and calibrate the measurement chain, then test controlled interferers while measuring detection-level consequences. Its results show why chirp similarity, timing, frequency offset, power, and geometry matter, but they should remain example results—not universal thresholds or proof of vehicle-level failure.
Frequently Asked Questions
Is “Inference Tests” a separate radar-testing method?
No. The source title appears to contain a typo; the document’s sections and experiments concern radio-frequency interference tests.
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No. It demonstrates RF or receiver-processing degradation. Safety conclusions require detection, tracking, vehicle-function, and scenario-based evidence.
Can the 9 dB target loss be used as a general immunity limit?
No. It is an approximately 9 dB observation from one 76 GHz radar, interferer, geometry, trigger, and processing configuration.
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