NXP Semiconductors
AN11740
PN5180 Antenna design
AN11740
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Application note
COMPANY PUBLIC
Rev. 1.1 — 19 June 2018
345311
56 of 62
5.2 Impedance measurement
The impedance measurement, which is needed to tune the antenna circuit correctly, is a
S11 measurement, showing the normalized resistance and reactance of the antenna
circuit impedance in a smith chart:
Z = complex impedance
Θ = phase between voltage and current
R = real part, resistance
X = imaginary part, reactance
The smith chart shows the normalized impedance, typically normalized to 50Ω. Some
measurement and simulation tools allow a normalization to values different than
50Ω.
However, in this document the normalization is not changed.
The return loss (reflection coefficient), which can be directly derived from the S11
measurement (i.e. from the smith chart), is of no interest for the NFC Reader antenna
tuning, since no 50
Ω system is used. This return loss could only be used, if the chosen
target impedance is 50Ω. Anyhow the return loss does not show the phase or whether
the circuit is capacitive or inductive.
Typically, a frequency sweep from 10MHz to 20MHz is sufficient enough to do the
antenna tuning.
The standard VNA provides an unbalanced 50Ω measurement port, which is used here
to measure a balanced impedance. The mismatch due to this setup error can be
neglected.
The power level must be low enough (e.g. 0dBm or less), and the PN5180 must be
unpowered. Then the PN5180 can remain in the circuit, while measuring the S11
between TX1 and TX2.
Note:
Ensure the TVDD = 0V during the impedance measurement with a network
analyzer.
Note:
Ensure the power level of the network analyzer is small enough, otherwise the
OPN5180 might influence the measurement. Typically, an output level of -10dBm
should be used.
5.3 Calibration
The major part before the measurement itself is the calibration of the measurement tool
(VNA) and measurement
setup (cable and probe). A high end 50 Ω calibration kit for the
used connector types (N-connector or SMA) does not help much, since the measurement
itself needs to be done at the TX pins of the used NFC Reader IC as possible. Therefore,
the calibration must be done as close at these points as possible.
So, the better solution is to create a simple connection, i.e. using a standard 2.54mm pin
row connector (male and female) to connect the VNA. This can be used to measure the
antenna impedance at the correct points, but also to calibrate the VNA. A very simple but
highly efficient homemade connector and tool is shown in Fig 52.
jX
R
e
Z
Z
j
+
=
⋅
=
θ
(5)
