AN400

Rev. 0.5

11

C2 (18 pF) is the ac coupling capacitor going to the FMI pin.

L2 (100 nH), L1 (120 nH), and L5 (150 nH) are the tuning inductors that provide dual peaking for FM and Weather

band. These inductors, together with the whip antenna capacitance (~22 pF at the center of FM band), 18 pF ac

coupling cap (C2), and 5 pF typical FMI pin input capacitance (C_FMI), resonate in the FM band as well as

Weather band. If either of the capacitance values changes, the inductor values have to be adjusted to achieve

peaking in both bands (desired peaking at 100 MHz and 162 MHz). The equivalent schematic model is shown

below. Si4740/41/44/45 do not support weather band. L5 can be replaced by a 0

 resistor, and the value of L2 can

be changed to 33 nH to support FM band only for Si4740/41/44/45.

U1 is a required ESD diode since the antenna is exposed. The diode should be chosen with no more than 1 pF

parasitic capacitance, such as the California Micro Device CM1213.

Figure 7. FM/WB Whip Antenna Schematic Model

3.3. FM/WB Whip Antenna Bill of Materials

Table 5. FM/WB Whip Antenna Bill of Materials

Designator

Description

WIP_ANTENNA Whip antenna

C2

Capacitor, 18 pF, 5%, COG

L1

Inductor, 0603, SM, 120 nH,

MURATA, LQW18ANR12J00D

L2

Inductor, 0603, SM, 100 nH,

MURATA, LQW18ANR10J00D

L5

Inductor, 0603, SM, 150 nH,

MURATA, LQW18ANR15J00D

U2

IC, SM, ESD DIODE, SOT23-3, California

Micro Devices, CM1213-01ST