References

AN4169

12/14

Doc ID 023704 Rev 1

impedance is not critical. As a result, their recommended width is the smallest possible.

In this way, the parasitic capacitances to ground can be minimized.

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The choke inductor (L0) must be placed as close to the TX pin of the RF IC as possible.

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Inductors must be put perpendicular to each other to improve filter attenuation at higher

harmonic frequencies. If this is not possible, put the inductances in perpendicular, do

not place the inductors in parallel to each other.

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Put the L7 and L8 inductors in perpendicular mode to avoid coupling. Put the smaller

capacitor of the battery capacity of SMPS filtering as close as possible to the choke

inductor (L0).

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Put a ground plane internally to the SMPS loop to reduce the coupling between the

traces of the SMPS.

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The smaller VDD bypass capacitor should be kept as close to the VDD pin as possible.

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The exposed pad of the SPIRIT IC should be connected to the ground layer below with

the maximum number of vias possible. The exposed pad should also be connected to

the top layer ground metal to further improve RF grounding using also diagonal trace

connections where possible.

●

The crystal should be placed as close as possible to the SPIRIT IC. The crystal

capacitors should also be placed near to the crystal pads. This to ensure that wire

parasitic capacitances are kept as low as possible in order to reduce the frequency

offsets that may occur. Use only COG capacitors to stabilize the crystal frequency, due

to their poor value variation effected by the temperature variation.

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Add large and continuous ground metallization on the top layer. To provide a good RF

ground, the RF voltage potential should be equal along the entire ground area because

this helps maintain good VDD filtering.

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A three or four-layer board is strongly recommended. Put the ground layer very close to

the top layer to obtain a good ground plane reference. A thickness between the top

layer and ground layer of 0.2 mm or 0.3 mm is suggested.

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If it is not possible to use a three or four-layer board, it is necessary to fill the area under

the RF part of the board (RF IC, TX matching and filtering network, RX balun and

SMPS section) with ground metal to reduce or eliminate radiation emissions. Board

routing and wiring should not be placed in this region to prevent coupling effects and to

ensure a good ground reference plane to the RF parts.

●

Connect all the ground metallization and/or layers with as many vias as possible. It is

also recommended that the ground return path between the ground vias of the TX and

RX networks and the ground vias of the SPIRIT IC should not be blocked in any way:

the return current should see a clear unhindered pathway through the ground plane to

the back of the SPIRIT IC.

5

References

1.

SPIRIT1 datasheet.