Page 6

BOARD LAYOUT CONSIDERATIONS

If you are at all familiar with RF devices you may be concerned about

specialized layout requirements. Fortunately, because of the care taken in

design, integrating an RM transmitter into your design is very straightforward.

By adhering carefully to a few basic design and layout rules, you can enjoy a

trouble-free path to RF success.

Figure 6 shows the suggested PCB

footprint for the RM transmitter.

1) A

ground-plane

(as

large

as

possible) should be placed directly

under the RM transmitter. This

ground-plane can also be critical to

the performance of your antenna.

2) The RM transmitter should, as much

as reasonably possible, be isolated from all other components on your PCB.

Specifically, high-frequency circuitry such as crystal oscillators should be

kept as far away as possible from the transmitter.

3) The module may be mounted vertically or bent horizontal to the product’s

PCB. If the transmitter is to be mounted parallel to the board, it should be

bent over so that the side without components is closest to the PC board

and the component side is facing away from the PC board.

4) No conductive items should be placed within .25 in. of the module’s

component side to prevent detuning.

5) The trace from the transmitter to the antenna should be kept as short as

possible. A simple trace is suitable for runs up to 1/4 inch for antennas with

wide bandwidth characteristics. For longer runs or to avoid detuning a high-

Q narrow bandwidth antenna such as a helical, use a 50-ohm coax or 50-

ohm microstrip transmission line as shown in figure 7.

Depending on the type

antenna being used,

the output power of the

RM module

may

be

significantly higher than

FCC regulations allow.

The output power of the

module is intentionally

set high since many

designers

pair

the

module

with

an

inefficient antenna in

order to realize cost or

space savings. Since

attenuation is commonly

required a designer may want to place locating pads in-line with the antenna

trace as shown. In the event that a T-pad is required the antenna trace can

be cut and the pads populated. For further details on T-pads please refer to

Linx application note #00150.

figure 6: Recommended PCB footprint

figure 7: Microstrip transmission line