Time Domain Measurements in automotive
Wolfgang WINTER #1, Markus HERBRIG #2
# emv GmbH
Wallberstrasse 7, 82024 Taufkirchen, Germany
Abstract—Time Domain Measurements are required to
analyze and interpret transient disturbing RF signals. In the area
of future automotive applications, transient RF disturbances are
becoming critical because comfort options like Bluetooth
connection, external devices with complex integrated RF
functionality (automotive WLAN, UMTS, GSM, WCDMA,
MIMO devices, multiband smart phones, Net-books) have to
interface with integrated car entertainment and control systems.
Due to the technical concept of such wireless communication
networks with digital modulation schemes, the interferences are
often short events with transient characteristics. The detection
and reproducible measurement of such signals is difficult
because of missing trigger signals and can be performed today
with Time Domain Measurement Systems using fast A/D
converters, digital filters and the Fourier Analysis to transform
the measured data into the frequency domain.
The analysis of time domain data in real time is necessary
to understand and to analyse the root cause of complex
electromagnetic interferences of modern electronic systems.
The international basic
measurements about Radio disturbance characteristics like the
CISPR 25 are due to the speed of technical developments in
electronics quite often not updated to deal with the latest RF
emitting or receiving interferences. The general application to
digitize analogue data and to use mathematical tools to obtain
frequency spectrum is quite common and moves back into the
1970s. For metrological purpose digitized time domain data
was captured with analogue digital converters and converted
into the frequency domain using the Fast Fourier
The use of digital oscilloscopes in high frequency
applications has become more popular in the last years due to
higher dynamic range and faster sampling speed in the range
of GHz. The disadvantage of such systems is the difficulty to
interpret the pure time domain data and to differentiate
between wanted and unwanted signals in the area of
measurement task requires additional hardware (preselection,
preamplifier, low pass filter, numerical processors for EMI
standards for automotive
(EMI). This special
detectors) and software capabilities to convert the time
domain data into the frequency spectrum. The first trials to
obtain the spectrum of a limited short time measured signals,
were done using a sample oscilloscopes with an integrated PC
platform to apply the Fast Fourier Transformation (FFT) to
convert the time domain data into the frequency domain .
This first experimental approach created the possibility to
look at transient signals in the environment of EMI
applications. It allowed a deeper look at the frequency nature
of the signals hidden in the time domain data. In case the
upper frequencies in the time domain data are not known and
to calculate the correct frequency spectrum, the use of a
frequency limiting low pass (anti aliasing filter) is required. It
will limit the upper frequency of the observed signal which
can enter the digital stage during the digitizing process. This
will ensure that the analogue signal can be reconstructed
correctly from the obtained set of digital data.
The disadvantage of sampling oscilloscopes is the limited
depth of fast memory to store a sufficient set of time domain
data. To be able to trace transient signals over a limited period
of time it is necessary to set trigger conditions, which are
difficult to define in EMI measurement applications. The
signals of interest are quite often hidden or at least
superimposed by ambient noise of other electronic
components of the device under test (DUT).
Microprocessor controlled bus systems in automotive
applications like CAN, LIN, FlexRay and all related sub
processor units in the external components (engine control,
car control systems, etc.) create conducted or radiated RF
emissions and its is a coincidence to capture the wanted RF
signals and disturbances.
To measure in real time the frequency spectrum of time
domain data, it requires other designs than commonly used for
typical oscilloscopes. High speed Field Programmable Gate
Arrays (FPGAs) are necessary to implement the FFT
algorithm in real or quasi real time to cope with the amount of
digital data if sampled in the magnitude of GHz.
1800 1900 2000 2100 2200 2300 2400 2500
Magnitude / dBµV
Frequency / MHz
EMI Test Receiver
Magnitude / dBµV
Time Domain Measuring Instrument
Fig.10. Comparison of measurement results for GSM, Bluetooth, WLAN
device inside a car: top EMI time domain measurement system, bottom: RF
EMI test receiver
This technical set up of several RF transmitting and
receiving devices inside vehicles creates RF disturbance
scenarios which are complex. In parallel, GSM 900, 1800,
UMTS 1.900 to 2.200 MHz, 2.400 to 2.483 MHz WLAN,
Bluetooth at 2.400 to 2.480 MHz are operating. Due to several
passengers per car, multiple mobile phones, note books with
WLAN, multiple Bluetooth RF devices could be on board of
the vehicle. This increases the likelihood for EMI disturbances
and interferences to other electronic components of the car
and vice versa.
The measurement results of both EMI measurement
systems are shown in Fig. 10 where the time domain
measurement system is capturing the time related signals more
clearly than the RF EMI receiver.
The commercially available EMI measuring instruments
(RF EMI Test receiver, Time Domain EMI analyser) have
been evaluated and measurements related to automotive
applications have been performed. The measuring results
show that the time domain EMI analyser is providing more
information for interpretation of modulated RF signals,
transient signals and time limited emissions. Measurement
time is significantly reduced. This enables the user to find the
root cause of disturbances related to automotive RF
transmitting and receiving devices faster and to interpret male
function of related electronic and electric components.
The authors would like to thank the Bayerische
Forschungsstiftung (BFS) for support.
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