SI4721 datasheet(44/69 Pages) SILABS

44 / 69 page

AN383

Rev. 0.8

9.3. Air Loop Antenna Bill of Materials

9.4. Air Loop Antenna Layout

Place the chip and the transformer as close as possible to the air loop antenna feedline. This will minimize the

trace going to the air loop antenna which in turn will minimize parasitic capacitance and the possibility of noise

coupling.

When an air loop antenna with a transformer is used with the Si473x, minimize inductive coupling by making sure

that the transformer is placed away from all sources of interference. Keep the transformer away from signals on the

PCB and away from even the I/O signals of the Si473x. Do not route any signals under or near the transformer.

Use a shielded transformer if possible.

9.5. Air Loop Antenna Design Checklist

 Select

a shielded transformer or a torroidal shape transformer to prevent noise pickup from interfering sources

 Select

a high-Q transformer with coupling coefficient as close to 1 as possible

 Use

multiple strands Litz wire for the transformer winding

 Place

the transformer away from any sources of interference and even away from the I/O signals of the Si473x.

Please make sure that the AM antenna is as far away as possible from circuits that switch at a rate which falls in

the AM band (520–1720 kHz).

 Route

traces from the transformer to the AMI input via the ac coupling cap C1 such that the capacitance from

the traces and the pads is minimized.

 Select

ESD diode D1 with minimum capacitance.

Table 20. Air Loop Antenna Bill of Materials

Designator

Description

Note

LOOP_ANTENNA

Air loop antenna

Transformer, 1:6 turns ratio

AC coupling capacitor, 0.47 µF, 10%, Z5U/X7R

ESD diode, IC, SM, SOT23-3,

California Micro Devices, CM1213-01ST

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