NXP Semiconductors
AN11740
PN5180 Antenna design
AN11740
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Application note
COMPANY PUBLIC
Rev. 1.1 — 19 June 2018
345311
28 of 62
Note:
The lower the Q, the better the stability and robustness of the antenna is.
Antennas with lower Q show less detuning. The higher the Q, the higher the field
strength is.
Note:
The final Q must be tuned with the pulse shape measurements, if the antenna
shall be fully optimized.
Note:
It might be helpful to slightly adapt the Q in the given Excel sheet calculation in
such a way that the resulting damping resistor R
Q
is calculated to be within an E-
series
of values (i.e. 2.7Ω or 3.3Ω, but not 2.845Ω). In such case the following
calculation is more accurate, i.e. the calculation result gets closer to the
measured result.
4.2.1.3 Define the EMC filter
The EMC filter can be a second order low pass filter as shown in Fig 18, and contains an
inductor (L0) and a capacitor (C0). The cut off frequency defines the overall detuning
behavior as well as the transfer function of the antenna circuit.
The inductor L0 needs to be capable to drive the full power into the antenna without
going into saturation. The Q-factor of this inductor should be as high as possible.
Typically, the inductance is in the range of
L0 = 330nH … 560nH
The cut off frequency typically should be far above the carrier frequency but far below the
second harmonic:
F
cutoff
= 14.5MHz … 22MHz
Due to the chosen cut off frequency basically two different antenna designs can be
chosen:
•
Symmetrical tuning (see Fig 24a), details refer to [14].
F
cutoff
= 14.5MHz …15.5MHz
•
Asymmetrical tuning (see Fig 24b)
F
cutoff
= 17MHz … 22MHz
