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SOURCE:
Institute of Telecommunications, Vienna University
of Technology, Vienna, Austria
A contactless evaluation Method on the powering of an
Active RFID Tag with On-Chip Antenna
Philipp K. Gentner, Guenter Hofer,
Arpad L. Scholtz, Christoph F. Mecklenbräuker
Gusshausstrasse 25 - 29 / 389
1040 Wien
AUSTRIA
Phone: +43 1 58801 78938
Fax: +43 1 58801 38999
Email: Philipp.Gentner@nt.tuwien.ac.at
A contactless evaluation Method on the
powering of an Active RFID Tag with On-Chip
Antenna
Philipp K. Gentner1 , Guenter Hofer2 , Arpad L. Scholtz1 , Christoph F. Mecklenbräuker1
1
2
Institute of Telecommunications, Vienna University of Technology
Gusshausstrasse 25/389, 1040 Vienna, Austria
Infineon Technologies Austria AG, Contactless and RF Exploration
Babenberger Strasse 10, 8020 Graz, Austria
philipp.gentner@nt.tuwien.ac.at
Abstract—A wireless method to power an active tiny
transmitter on a tag is inductive coupling, where the energy
to power the tag’s transmitter is drawn via a small antenna
from a radio frequency magnetic field. In this paper, we
present a contactless method to determine the voltage
delivered by the rectifier of an RFID circuit connected to an
on-chip antenna. The chip is designed to backscatter energy
with a modulation frequency proportional to the voltage
delivered by the rectifier. Hereby it becomes possible to
investigate inductive coupling between an excitation coil
which is part of a reader, and a coil on-chip, without the
need of bond wires or other physical connections to the
chip.
Index Terms—On-Chip Antenna, OCA antenna design
and characterisation
I. I NTRODUCTION
In an asymmetric or hybrid RFID communication
system, the tag consists of an ultra-wideband (UWB)
impulse radio transmitter and an UHF receiver [1]. A
reader station transmits an UHF carrier for powering and
synchronizing the tag. In general, the UWB transmitter
allows for higher data rates, while consuming less power.
This is an ideal scenario for RFID or wireless sensor
applications. If the size of the system is of importance,
on-chip antennas (OCAs) are an option. Considering a
typical size of a few square millimeters of the chip, the
highly reduced antenna efficiency of course yields to a
reduction of the feasible communication range.
The contactless powering method in this contribution
is inductive coupling. It is important for system design
to know the amount of power available for the functionality of the chip. There are three major aspects that
influence the power drawn by an integrated chip from the
electromagnetic field. These are the performance of the
on-chip antenna, the rectifier, and the performance of the
reader antenna exciting the magnetic field. We propose
an evaluation method to characterize the performance
of the OCA and rectifier by measurements, without the
need of wired connections to the chip. Thus, compared
to other methods we ensure that the electromagnetic field
of the excitation coil and the OCA itself is not affected.
The inductive coupling scenario is shown in Figure 1.
Also displayed is a microphotograph of the loop antenna
used (similar design as reported in [2] and [3]). The
silicon substrate carrying the OCA is placed at a fixed
distance in z-direction above the excitation coil. The coil
is a magnetic loop antenna printed on a standard FR4
PCB, fed by a sinusoidal signal at 850 MHz with variable
power level Pin . The FR4 substrate the loop antenna is
printed on has a thickness of 1.5 mm (copper thickness
35 µm). The ground planes on the top and bottom layers
are connected by vias to form a coplanar waveguide
feed. The loop consists of two turns of conductors
(width 0.5 mm). The overall diameter of the spiral coil
is 4.2 mm and the inner diameter is 1.5 mm.
The PCB is designed to make possible on-board
impedance matching with concentrated elements. For our
experiments, we preferred to use an external impedance
tuner. Therefore a jumper replaces the inline matching
element on the PCB.
II. D EVICE U NDER T EST
The loop OCA investigated has an overall size of
1 mm2 with four conductor turns. This type of on-chip
antenna is commonly used for inductive coupling and is
well known from literature (see [2] and [3]). Our antenna
is manufactured with standard 130 nm CMOS technology without any post processing. The conductor width
of each turn is 15 µm with a spacing of2.6 µm between
the turns. Two metal layers are utilised for the loop,
which are connected by vias. The metal stripes in the
loop’s inner section prevent the silicon from breaking.
The bonding pads inside the OCA (see photograph in
Fig. 1. Inductive coupling scenario. The silicon chip with the OCA is placed in close distance in z-direction directly above the center of the
excitation coil. On the right side, a microphotograph of the chip with a 4-turn coil antenna is shown.
Fig. 2. The backscattered signal of the chip in the frequency domain.
Fig. 3. Simulated proportionality between the modulation frequency
fmod and the induced voltage VDD , delivered by the rectifier.
Figure 1 are not required for the measurements presented
in this paper.
The circuit on the silicon chip features a rectifier [4],
a voltage controlled oscillator [5], and a logic part. The
rectifier powers the chip. The oscillator and the logic
part generate a modulated output signal that is fed to
the OCA. Figure 2 shows the corresponding measured
output spectrum. The peaks in the backscattered signals’ spectrum are offset from the sinusoidal carrier
at 850 MHz by multiples of the modulation frequency
fmod . The modulation frequency fmod is proportional to
the chip voltage VDD (induced by the excitation antenna)
after the rectifier.
The proportionality between fmod and the chips’ VDD
is calculated by transient simulation of the oscillator
(see Figure 3). The simulation, performed with Cadence,
incorporates the parasitic effects of the substrate. This
method to conclude from the measured fmod to VDD ,
allows to determine the performance of the OCA and
the rectifier without the need of wired connections.
III. M EASUREMENTS
To show the feasibility of our method several experiments were carried out. The setup for all measurements
consists of two styrofoam elements, one carrying the
excitation coil and the other carrying the device under
test. The excitation coil (see Figure 1) was matched with
a tuner to the RF source. The matching process was done
without the chip being in the excitation coil’s vicinity.
The chip was fixed with a double-faced adhesive tape
on one end of the dedicated long styrofoam board. The
other end was mounted on a multi axis platform, which
can be moved in x, y and z direction via micrometer
screws.
To get a first insight into the power transfer properties,
the following initial experiment was performed. For
constant input power Pin = +8.6 dBm at the excitation
coil and constant z-position, the chip was moved in
the x-y plane, until the measured fmod reached its
maximum. The coordinates such obtained are used as
the origin x = y = 0. Now, the chip was moved in
the x-y plane in equidistant steps of 0.5 mm. For each
of these steps the modulation frequency was measured,
and used to calculate the induced voltage VDD . This
measurement was carried out for two different z values
(0.5 mm and 1.5 mm). Figure 4 shows the corresponding
results. The induced voltage VDD decreases with distance
in z-direction as expected. Keeping in mind that chip
operation requires a certain minimum VDD level, the
area of sufficient induced power decreases with height,
as can be deduced from Figure 4 in accordance with
expectation.
Next, a distance sweep in z-direction at x = y = 0 mm
was carried out. For this range test, the styrofoam arm
with the chip under test was moved from z = 0.5 mm to z
= 2.75 mm, while the input power Pin was kept constant.
The VDD values resulting from the measured fmod are
shown in Figure 5. The induced voltage decreases with
increasing distance from the excitation coil, from 1.66 V
at z = 0.5 mm to 0.47 V at z = 2.75 mm.
Finally, the chip was brought into contact with the
excitation coil (x = y = z = 0 mm), and the input power
Pin of the excitation coil was varied from −6.4 dBm
to 5.6 dBm (see Figure 6). This was done to see how
much power can be transferred into the chip in an
ideal scenario. We observe that the induced voltage VDD
increases from 0.5 V to 2.13 V.
IV. C ONCLUSIONS
In this paper we presented a contactless method to
determine the voltage induced by an excitation coil to an
OCA. The modulation frequency of the signal backscattered from the tag is proportional to the induced voltage
VDD after the rectifier. This method was verified by
measurements on an RFID chip prototype with an OCA.
The tags operational area at constant heights above the
excitation coil, as well as the behavior with increasing
distance was successfully measured. Also, the voltage
provided by the rectifier in an ideal scenario, with zero
distance to the excitation coil has been measured. This
Fig. 5. Range test in z-direction with Pin = 8.6 dBm at max VDD
position.
Fig. 6.
Input power sweep at z = 0 mm.
information will help in optimally designing inductively
powered RFID chips with on-chip antennas.
ACKNOWLEDGEMENTS
This work was performed as part of the project
‘Smart Data Grain’ which is embedded into the program ‘Forschung, Innovation, Technologie - Informationstechnologie’ (FIT-IT) of the ‘Bundesministerium
für Verkehr, Innovation und Technologie’ (BMVIT).
This program is funded by the ‘Österreichische
Forschungsförderungsgesellschaft’ (FFG). This work is
partly supported by the Christian Doppler Laboratory
for Wireless Technologies for Sustainable Mobility. The
authors would like to thank the members for support
during manufacturing and measurements.
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Fig. 4.
Measured VDD for two different heights above the excitation coil. zleft = 0.5 mm and zright = 1.5 mm.
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