Changes PN532_Breakout board
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Changes PN532_Breakout board
TechnoCentrum - Radboud University Nijmegen
René Habraken
17 May 2011
1.0
Contents
Contents................................................................................................................................................. 3
1.Introduction......................................................................................................................................... 4
2.Matching components......................................................................................................................... 5
2.1Measurements............................................................................................................................... 5
2.2Return signal from antenna........................................................................................................... 6
2.3Results........................................................................................................................................... 8
3.Conclusion........................................................................................................................................... 9
2
1. Introduction
The PN532_Breakout board contains the PN532 chip of NXP Semiconductors for contactless
communication. Together with the pcb-antenna it forms an NFC reader for the ISO 14443 standard.
To increase the communication range of the PN532_Breakout board the antenna had to be matched
to the PN532 chip. Apart from that the signal coming back from the tag and picked up by the antenna
had to be optimized. This document describes the measurements and improvements done on the
PN532_Breakout board at the Techno Centrum of the Radboud University in Nijmegen.
Because the PN532 chip delivers a square wave of 13.56MHz a low-pass filter (see Figure ) follows the
output pins to end-up with a clean sine wave. In the reader–antenna setup below, most power is
send when the impedance of the antenna is matched to the impedance of the chip. The capacitors
C
1,2
transform the impedance of the antenna to a 50Ω impedance at the resonance frequency
(13.56MHz in this case). In order to receive the modulated signal from the tag the quality-factor (Q-
factor) of the antenna should not be too high. This factor is an indication how well the antenna
performs at its resonance frequency. Normally it should be as high as possible because most power is
radiated in that case. However, it acts like a filter and if the Q-factor is above 30 it cuts off the desired
frequencies send back by the tag. Resistors R
Q
reduce the Q-factor to the optimal value.
The measurements described in this document are done with the Agilent 4396B Network/Spectrum
analyzer. Together with the Agilent 16093A option it can be used to measure the impedance of an
antenna and the other parameters that are necessary for the calculations. Besides the network
analyzer a standard scope is used (Tektronix TDS3052) to measure the return signal from the antenna.
Together with a low capacitance probe (Tektronix P6243 1MHz- 1GHz, <1pF) the desired signals could
be measured with the necessary precision.
Figure : Reader-antenna setup
3
2. Matching components
2.1
Measurements
To tune the antenna and calculate the correct values for the components the following document is
used: NFC Transmission Module Antenna and RF Design Guide (AN144511) from NXP
Semiconductors. This document describes in detail the properties of the antenna for the PN532 chip
and it comes with a spreadsheet to calculate the correct matching components. It can be found using
the following link:
http://www.nxp.com/#/page/content=[f=/dynamic/applicationnotes/all/data.xml]
As a start the inductance of the antenna is calculated and it results in an inductance of 1.91μH. Later
on this value is confirmed by measurements done with the Agilent network analyzer. To calculate the
components of the matching circuit two equivalent antenna circuits must be used. The value for
resistor R
Q
(see: Figure ) follows from figure 2. The values of capacitors C
1,2
can be calculated from the
parallel circuit of figure 3
Figure : Series antenna circuit
Figure : Parallel antenna circuit
The measurements of the parameters that are necessary to obtain the component values are listed
below.
Parameter
Value
Comment
L
a
1.83μH
Series inductance
R
a
1.42Ω
Series resistance
R
pa
19.9kΩ
Resistance at f
res
f
res
38.3MHz
Resonance frequency antenna
Table : Measurements without R
Q
To reduce the Q-factor of the antenna two resistors (R
Q
) of 1.5Ω are placed in series with the antenna.
The calculation of the final values for the matching network must be done after a second
measurement with the series resistors in place.
Parameter
Value
Comment
L
pa
1.90μH
Series inductance
R
a
4.46Ω
Series resistance
R
pa
18.4kΩ
Resistance at f
res
f
res
37.3MHz
Resonance frequency antenna
Table : Measurements with RQ
Using the parameters from Table 2 the component values for the matching circuit are: C
0
=220pF,
C
1
=24pF and C
2
=103pF. (see: Figure : Reader-antenna setup). The closest available values in the
market for the capacitors are 220pF, 22pF and 100pF respectively. These components should result in
a matched antenna–reader system with a resonance frequency of 13.56MHz and a Q-factor of 30 or
less. For the results see paragraph: Results Results.
4
2.2
Return signal from antenna
The chip is protected from an over voltage by means of a voltage divider (see: Figure : Reader-
antenna setup). To calculate the necessary ratio the voltage on the antenna is measured with a low
capacitance probe. If you use a regular probe the antenna is detuned and the results are not correct.
The reason for this effect is mainly because of the added capacitance of the probe. For this purpose
the P6243 probe of Tektronix is used with a capacitance of <1pF.
Figure : Tektronix probe
Several other probes are tried including a high frequency probe of Agilent that must be used together
with the network analyzer. Also a standard scope probe is used with an added resistor to the probe
tip to reduce the capacitance. However these measurements showed to be less stable and
straightforward as was the case with the Tektronix probe.
Because the antenna generates an electromagnetic (EM) field continuously, it is important to keep a
short connection to ground i.e. close to the point where the measurement is done. If the ground wire
is in the EM-field a voltage is induced in the probe and the measurement becomes unreliable. Special
care is taken to reduce this problem to a minimum.
According to the datasheet of the PN321 chip the maximum allowed voltage on the receive pin of the
chip is AV
DD
+1V. The (analog) supply AV
DD
is 3.3V and this results in a maximum allowed voltage of
4.3V on pin RX (see: Figure : Reader-antenna setup). This must be valid for an RF-field that is switched
on continuously and for a chip in communication mode. If the antenna is detuned by a tag the voltage
increases on pin RX. Also in this situation the voltage may not exceed the 4.3V. To get to an optimal
result an iterative process is used to reach the largest communication distance without exceeding the
allowed voltage. Resistor R2 is varied from 1kΩ to 2k2Ω and the communication distance is
measured. Unfortunately this did not increase the communication distance significantly. A difference
from 74mm (2k2) to 77 (1k7) was found varying the resistor. In contrary to the specification with an
Urx higher than 4.01V the sine clips and is deformed. Values for R2 of 1k5 or less influence the
reading range negatively and should be prevented.
R2 [Ω]
Uc
0
[V]
Urx [V]
Urx [V]
(detuned)
Com. dist.
[mm]
5
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