AN851 Datasheet, PDF (12/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

AN851
5.3. Ferrite Loop Antenna Bill of Materials
Designator
ANT1
D1
Table 5. Ferrite Loop Antenna Bill of Materials
Description
Ferrite loop antenna, 180–450 µH
ESD diode, IC, SM, SOT23-3,
California Micro Devices, CM1213-01ST
Note
Optional; only needed if the there is any
exposed pad going to the AMI pin.
5.4. Ferrite Loop Antenna Layout
Place the chip as close as possible to the ferrite loop antenna feedline. This will minimize the trace going to the
ferrite antenna which in turn will minimize parasitic capacitance, and also will minimize the possibility of noise
sources coupling to the trace.
The placement of the AM antenna is critical, since AM is susceptible to noise sources causing interference in the
AM band. Noise sources can come from clock signals, switching power supply, and digital activities (e.g., MCU).
When the AM input is interfaced to a ferrite loop stick antenna, the placement of the ferrite loop stick antenna is
critical to minimize inductive coupling. Place the ferrite loop stick antenna as far away from interference sources as
possible. In particular, make sure the ferrite loop stick antenna is away from signals on the PCB and away from
even the I/O signals of the Si468x. Do not route any signal under or near the ferrite loop stick. Route digital traces
in between ground plane for best performance. If that is not possible, route digital traces on the opposite side of the
chip. This will minimize capacitive coupling between the plane(s) and the antenna.
To tune correctly, the total capacitance seen at the AMI input needs to be minimized and kept under a certain value.
The total acceptable capacitance depends on the inductance seen at the AM input. The acceptable capacitance at
the AM input can be calculated using the formula shown in Equation 1.
C_Total
=
-----------------------¹-------------------------
2f_max²L_effective
Equation 1. Expected Total Capacitance at AMI
Where:
C_Total = Total capacitance at the AMI input
L_effective = Effective inductance at the AMI input
f_max = Highest frequency in AM band
The total allowable capacitance, when interfacing a ferrite loop stick antenna, is the effective capacitance resulting
from the AMIP and AMIN input pins, the capacitance from the PCB, and the capacitance from the ferrite loop stick
antenna. The inductance seen at the AMI pins in this case is primarily the inductance of the ferrite loop stick
antenna. The total allowable capacitance in the case of an air loop antenna is the effective capacitance resulting
from the AMI input pins, the capacitance of the PCB, the capacitance of the transformer, and the capacitance of the
air loop antenna. The inductance in this case should also take all the elements of the circuit into account. The
formula shown in Equation 1 gives a total capacitance of 29 pF when a 300 µH ferrite loop stick antenna is used for
an AM band with 10 kHz spacing, where the highest frequency in the band is 1710 kHz.
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Rev. 0.2