AN4169

Design recommendations when using SPIRIT1 RF IC

Doc ID 023704 Rev 1

11/14

Figure 7.

SPIRIT1 direct tie type application board layout

Here below is a guide on how to realize the layout of the SPIRIT1 application board.

●

The discrete balun of the RX path must be placed as close to the RX pins as possible.

●

The traces that connect the two RX pins to the balun network (differential traces)

should be of equal length. If the two differential signals are unbalanced, common-mode

problems can be generated.

●

Route differential traces closely together. Differential receivers are designed to be

sensitive to the difference between a pair of inputs, but also to be insensitive to a

common-mode shift of those inputs. Therefore, if any external noise is coupled equally

into the differential traces, the receiver is insensitive to this (common mode coupled)

noise. The more closely differential traces are routed together, the more equal any

coupled noise is on each trace, therefore the better the rejection of the noise in the

circuit is.

●

The parallel inductors in the RX path (and in general) should be mutually perpendicular

to avoid mutual couplings. If no perpendicular position is possible, turn away their

interposing capacitors or resistors.

●

The TX and RX section should be separated by a ground plane in the top layer to

reduce the coupling.

●

Connect the TX and RX section using 50

Ωgrounded coplanar lines wherever possible.

This type of line structure reduces radiation and coupling effects. Also the characteristic

impedance of the track is less sensible to the PCB thickness variation.

●

The interconnections between the elements are not considered transmission lines

because their lengths are much shorter than the wavelength and, therefore, their

RX balun network

V

Crystal

section

Spirit

RF IC

V

SMPS filtering

TX filtering

and matching

Decoupling

capacitors

Choke inductor

PCB vias

AM14762V1