AN851 Datasheet, PDF (20/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

AN851
Figure 14. Norton Equivalent of Antenna Interface with Module FE
The reactance part of the Norton impedance branch can be capacitive or inductive. The complex impedance of the
Norton branch depends on the following things:
(a) The front end series LC circuit (Ref designator C5 and L4)
(b) Type of antenna and ground plane, if any
(c) Length of the cable
(d) Band (FM/DAB)
(e) Frequencies within the band
The Figure 14 network can be further simplified into the simple parallel RLC circuit shown in Figure 15. The
transformation (series to parallel at single frequency) helps convert the Norton branch into its parallel equivalent
shown below. The network shown in Figure 15 is a simplified representation of parallel LC tank circuit with
inductors and capacitors in parallel. For a given frequency, using resonance frequency equation F=1/2LC, an
unknown quantity (inductor or capacitor) can be calculated.
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Rev. 0.2