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Application note
COMPANY PUBLIC
Rev. 1.1 — 19 June 2018
345311
Note: For the symmetrical tuning the Rq might be much lower, since the EMC filter with
lower cut off frequency improves the transfer function. This can increase the
maximum possible field strength.
(1) Excelsheet from www.nxp.com
Fig 25. Antenna matching calculation (asymmetrical tuning)
The matching capacitors are calculated:
C1 = (C1A + C1B) = (C1C + C1D) = 41.6pF -> 33pF + 8.2pF (0.4pF less)
C2 = (C2A + C2B) = (C2C + C2D) = 125.6pF -> 68pF + 56pF (1.6pF less)
4.2.1.5 Simulate the matching
The measurement of the antenna coil itself typically is not very accurate. Therefore a
(fine) tuning of the antenna normally is required, which might become easier in
combination together with a simulation.
A simple matching simulation tool like e.g. RFSIM99 (refer to [10]) can be used to
support the antenna tuning.
The excel sheet supports the direct simulation: the path for the simulation must be set by
pressing the green button <Step 1: Set Path of RFSIM99>. Afterwards pressing the
green button <Step 2: Simulate> starts the simulation with the given and calculated
values.
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