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Backscattering modulation 101: VNA measurementsSimon Hemour1, Nicolas Barbot2
2023 IEEE International Conference on RFID Technology and Applications (RFID-TA) | 979-8-3503-3353-4/23/$31.00 ©2023 IEEE | DOI: 10.1109/RFID-TA58140.2023.10290457
1
Université de Bordeaux, IMS, 33405 Talence, France
Univ. Grenoble Alpes, Grenoble INP, LCIS, 26000 Valence, France
2
*
simon.hemour@u-bordeaux.fr
Abstract— Hardware tag characterization is one of the
most critical steps in RFID and zeropower IoT
development flow. However, the measurement process and
its respective setup can be long and cumbersome. This
paper describes how to use basic but uncharted VNA
modes to yield simple, fast and flexible measurements
without any spectrum analyser, SDR, oscilloscope, or
anechoic chamber.
A simple case study is presented, with transponders that
are based on simple rotating scatterers.
They are
handcrafted from metallic tape, so they can be easily
replicated by enthusiastic researchers. The VNA allows to
estimate both magnitude and phase of frequency
components around the illuminating carrier frequency (i.e.
the intermodulations) backscattered by the transponder. It
can also measure the modulated backscattered power as a
function of the carrier frequency. These modes of
operation can eventually be used to realize both
identification and sensing of linear time-variant (LTV)
transponders at a significant range.
Keywords— VNA, frequency offset, LTV transponders,
modulation, wireless, zeropower, IQ demodulation.
I. INTRODUCTION
Measurements are instrumental in the Research and
Development process of RadioFrequency Identification
transponders (RFID). Hence, tag activation distance is
commonly characterized in every scenario, while amplitude
(RSSI) and phase of backscattered signals can be estimated
using classical RFID readers. To push further measurement
capabilities, the RFID reader may sometimes be replaced or
augmented by a vector signal generator and a spectrum
analyser or a high speed (MS/s) downconverted IQ sampler to
observe and characterize signals on a given bandwidth and/or
in a given interval. This setup allows for additional
characterization, such as Power on Tag Forward (POTF)
Power on tag Reverse (POTR), delta RCS, and read range
over larger bandwidth [1-6]. Such a setup can also perform
chipless tag measurement. For example linear time-variant
(LTV) transponders are typically wirelessly characterized
using a vector signal generator and a real time spectrum
analyzer, both using the same 10MHz reference to achieve
coherent detection [4].
However, no instrument can match the Vector Network
Analyzers (VNA) in terms of flexibility, wide frequency
range, calibration-grade measurements, and optimum use of
couplers. VNA are classically used to measure S-parameters
in both magnitude and phase. Commercial instruments can
sweep over dozens of GHz bandwidth in a few milliseconds.
Thus, some VNA-based RFID measurements, such as radar
cross section are already known to yield high dynamic range
and accuracy. The Common limitation, however, is that the
measurement result is jeopardized by the multiple reflections
coming from the environment, so that additional
979-8-3503-3353-4/23/$31.00 ©2023 IEEE
measurements are necessary to remove the artefacts [7]
through multi-step calibration procedure.
An alternative way to circumvent the many reflections of the
illuminating frequency is to operate at its harmonic. For this
scenario, VNA in frequency offset mode has been used
previously for measuring harmonic transponders [8-9], but the
intermodulations have not yet been investigated.
The purpose of this paper is twofold. The first objective is to
show how well the vector architecture of the VNA matches
the advanced measurements required for the characterization
of the modulated signals backscattered by LTV transponders.
Two techniques will be highlighted, namely (i) the FFT
extraction (frequency domain) of Zero-Span ‘A’ receiver
measurements (Time domain) and (ii) the Frequency- Offset
‘A’ receiver measurement (shifted frequency domain). The
second objective is to show that robust linear time-variant
transponders can be easily built for a few dollars. These
rotating scatterers can be easily reproduced by enthusiastic
researchers using simple metallic tape. The testbench is able
to realize the operation of identification and sensing using the
proposed LTV transponders. Identification is performed from
a constant side-band measurement of a wideband
frequency-swept carrier (measured in frequency offset mode).
Sensing function is achieved from the measurement of the
magnitude and phase of the modulated power (zero span
mode) With the proposed setup, identification and sensing can
be done at a read range of several meters while maintaining
the read time below a few seconds.
II. VECTOR NETWORK ANALYSIS PRINCIPLE
As described in fig. 1-a, a VNA is by default assuming that
the device under test is a linear time-invariant system. The
S11-parameter (reflexion coefficient) is computed as the ratio
between one single tone seen as the ‘output’ (reflected wave
b1) and one single tone seen as ‘input’ (forward wave a1).
From an hardware standpoint, the two waves are coupled from
the transmission line connecting the generator to the Device
Under Test (DUT), down-converted, and eventually sampled
(Receiver A measuring the b1 wave image, and receiver R
measuring the the a1 wave image (diagram of fig. 1-b).
When a Linear Time Variant (LTV) or a non-linear time
invariant device is measured, the reflected b1 wave is no
longer a single tone, but contains many intermodulation
frequency components around the a1 initial “carrier”
frequency, and at its harmonics. If the intermodulation
frequencies are smaller than the IF bandwidth filter, they can
be captured by the ‘A’ receiver in a zero-span mode
(sometimes also named ‘CW mode’). In this mode, the
frequency sweep is disabled. The VNA does not plot the
frequency response to the swept carrier frequency, but traces
the magnitude (or phase) of the received signal as a function
of time. Of course, this data can be post-processed to visualize
the frequency content (see |A(t)| and FFT(|A(t)|) in fig. 1-b)).
Note however that since the carrier passes through the IF
bandwidth filter, available resolution is reduced
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RFID 2023
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Fig. 1 Comparison between classical mode, zero span mode and frequency offset mode of a VNA [10]
Since RFID engineers’ objective is to differentiate the
modulated reflections of the transponder (around the carrier
frequency) from the reflections of the environment (at the very
carrier frequency), the frequency offset mode of the VNA can
be used. In this configuration (Fig. 1-c) the receiver can
operate at a frequency which is different from the carrier
frequency. If the IF bandwidth is low enough, reflection by the
environment can be separated from the modulated signal and
the received signal is only a function of the LTV transponder.
The two latter modes will be investigated in this paper.
III. TAGS DESIGN
Linear time-variant transponders which do not use a chip
have been proposed in [11-13]. Note that all these
transponders can backscatter new frequency components
around the carrier frequency used by the reader when they are
in movement. This technique allows one to drastically
increase their read ranges compared to Linear Time-Invariant
(LTI) transponders. The proposed tags have been designed by
cutting an aluminum sheet. Different shapes have been
realized such as a rectangle, two triangles and a T-shape and
are presented in Fig. 2. Note that these designs can be easily
reproduced with simple metallic tape. These scatterers are
then placed on an adhesive support which can be rotated by a
motor. The rotational speed of the motor is controlled by an
Arduino. The microcontroller is programmed to generate a
Pulse Width Modulation (PWM) signal on a General Purpose
Input Output (GPIO) connected to an Electronic Speed
Control (ESC). This setup allows one to easily control the
rotational speed of the motor. Note that in all experiments, the
rotational speed has been set to approximately 60 tr/s.
Fig. 2 Photograph of the proposed transponders. Tags are placed on a rotating
support during the measurement.
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Fig. 3 Photograph of the measurement bench in a real environment.
Fig. 5 Fourier transform of the measured A parameter in zero span mode.
Carrier frequency has been set to 2.1 GHz.
One can also remark that in this setup, the A parameter is
measured as a function of the time and variation of amplitude
and/or phase of the received signal will directly impact the
received signal in the time domain.
Fig. 4 Measured A parameter in CW mode and an IF bandwidth of 3 kHz.
Sweep time is equal to 134 ms..
IV. VNA-BASED MEASUREMENT BENCH
The measurement bench is built around a VNA PNA N5222A
by Agilent. The bench uses a single UWB antenna (A.H.
Systems, inc. SAS-571) directly connected to Port 1
(monostatic configuration). The rotating tag is placed in front
of the antenna at a distance of 20 cm. The output power of the
VNA has been set at -5 dBm for all the measurements. The
whole bench is placed in a real environment. A picture of the
measurement bench is presented in Fig. 3. Anechoic
environment can be used to significantly reduce the multipath
propagation.
IV. ROTATIONAL SPEED SENSOR
For this analysis, the VNA is placed in Zero span mode. In
this mode the frequency sweeping is disabled and has been set
to 2.1 GHz. Note that any frequency can be used as long as the
considered transponders backscatter a power. If the scatterer is
resonant, the resonant frequency can be used to maximize the
backscattered power. The sampling rate at the receiver has
been set to 2 kS/s by setting the IF bandwidth to 3 kHz. Note
that with this IF bandwidth all frequency components
backscattered by the tag can be captured by the VNA. Number
of points has been set to 401 points which corresponds to an
acquisition time of 134 ms.
Fig. 4 presents the variation in magnitude of the A
parameter in the time domain. The acquisition time of 134 ms
(sampling time=0.5ms) allows one to capture about 7 periods
of the signal (fr=60Hz). Also note that the maximum
frequency of the measured signal should not be higher than 1
kHz to satisfy the Nyquist-Shannon theorem. The variation
seen in time corresponds to the amplitude modulation of the
backscattered signal induced by the rotation of the scatterers.
By applying a Fourier transform to the complex signals
presented in Fig. 4, one can obtain the spectral content of the
backscattered signal around the carrier sent by the VNA.
Fig. 5 presents the fourier transform of the (complex) A
parameter. Since the signal is periodic due to the rotation
movement, the spectrum is composed of peaks located at
multiples of the rotational frequency. Note that the frequency
components located at ± 2𝑓𝑟 are due to the polarization
modulation [13]. The components located at 𝑛 𝑓𝑟 are due to
the micro Doppler modulation [14]. The peak located at 0 Hz
is due to the reflection of the antenna and the objects present
in the environment. Finally the position of the peak
corresponding to the modulated signal allows one to realize an
accurate wireless sensor. From Fig. 5, the rotational speed of
the transponder has been measured at 116/2=58 Hz which is in
agreement with the analytic model [13] and the mechanical
setup.
IV. TAG IDENTIFICATION
This section presents a procedure allowing to extract the
modulated power as a function of the frequency. Results are
similar to the ones presented in [4, Fig. 8], [14, Fig. 12] and
[15, Fig. 3] (all obtained with a SVG and a RT SA) but can be
now fully realized by a single instrument. For this analysis,
the VNA is placed in linear frequency sweep between 500
MHz and 4 GHz using 401 points. A frequency offset of 110
Hz is applied on the receiver A. This offset corresponds
approximately to twice the rotational frequency of the tag (i.e.,
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REFERENCES
2 𝑓𝑟). More importantly, the IF bandwidth has been set to 50
Hz to efficiently filter the static reflections of the antenna and
the objects present in the environment (all located at 0 Hz).
With these settings the sweep time is equal to 7.1 s. Note that
since the rotation frequency and the frequency offset are not
perfectly equal, the measurement power seen on receiver ‘A’
beats at a frequency which is equal to the difference of these
two frequencies. Note that to obtain the total energy left by the
IF bandwidth filter, an averaging should be performed in the
time domain (AVERAGING PER POINTS). Similar results
can also be obtained using a Max Hold on the measured trace.
Fig. 6 shows the resulting magnitude as a function of the
offsetted frequency for all transponders and when no tag is
rotating (Empty). The energy below 800 MHz is due to the
reflection of the generator’s phase noise at the antenna port
(the antenna is not matched below this frequency). The peaks
located above 1 GHz are due to the modulated power
backscattered by the transponders. These variations in
modulated power depend on the considered scatterer and can
be used to identify the tag. Interestingly, this quantity is linked
to the delta RCS [4] of the scatterers. This quantity can also be
evaluated theoretically from the scattering matrix of the
scatterers [15] and can provide an estimator which is
independent from the distance. More importantly, and as
opposed to classical chipless measurement, all measurements
have been done without subtracting the empty measurement.
V. CONCLUSION
The paper introduces a flexible measurement bench allowing
to characterize the performance of any LTV transponder by
using a VNA. Backscattered signals can be extracted in time
and frequency domain based on the CW mode of the
instrument. Delta RCS can also be extracted over large
bandwidth by using the frequency offset of the instrument.
Finally we show that the sensitivity of the instrument is
sufficient and allow us to design LTV transponders using
simple metallic tape. We hope that results presented in the
paper can be reproduced by other researchers.
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