MDEV-900-TT datasheet(24/29 Pages) LINX

24 / 29 page

–

Do not route PCB traces directly under the module. There should not be

any copper or traces under the module on the same layer as the module,

just bare PCB. The underside of the module has traces and vias that could

short or couple to traces on the product’s circuit board.

The Pad Layout section shows a typical PCB footprint for the module. A

ground plane (as large and uninterrupted as possible) should be placed on

a lower layer of your PC board opposite the module. This plane is essential

for creating a low impedance return for ground and consistent stripline

performance.

Use care in routing the RF trace between the module and the antenna

or connector. Keep the trace as short as possible. Do not pass it under

the module or any other component. Do not route the antenna trace on

multiple PCB layers as vias add inductance. Vias are acceptable for tying

together ground layers and component grounds and should be used in

multiples.

Each of the module’s ground pins should have short traces tying

immediately to the ground plane through a via.

Bypass caps should be low ESR ceramic types and located directly

adjacent to the pin they are serving.

A 50-ohm coax should be used for connection to an external antenna.

A 50-ohm transmission line, such as a microstrip, stripline or coplanar

waveguide should be used for routing RF on the PCB. The Microstrip

Details section provides additional information.

In some instances, a designer may wish to encapsulate or “pot” the

product. There are a wide variety of potting compounds with varying

dielectric properties. Since such compounds can considerably impact

RF performance and the ability to rework or service the product, it is

the responsibility of the designer to evaluate and qualify the impact and

suitability of such materials.

Microstrip Details

A transmission line is a medium whereby RF energy is transferred from

one place to another with minimal loss. This is a critical factor, especially

in high-frequency products like Linx RF modules, because the trace

leading to the module’s antenna can effectively contribute to the length

of the antenna, changing its resonant bandwidth. In order to minimize

loss and detuning, some form of transmission line between the antenna

and the module should be used unless the antenna can be placed very

close (<1/8in) to the module. One common form of transmission line is a

coax cable and another is the microstrip. This term refers to a PCB trace

running over a ground plane that is designed to serve as a transmission line

between the module and the antenna. The width is based on the desired

characteristic impedance of the line, the thickness of the PCB and the

dielectric constant of the board material. For standard 0.062in thick FR-4

board material, the trace width would be 111 mils. The correct trace width

can be calculated for other widths and materials using the information in

Figure 36 and examples are provided in Figure 37. Software for calculating

microstrip lines is also available on the Linx website.

Trace

Board

Ground plane

Figure 36: Microstrip Formulas

Example Microstrip Calculations

Dielectric Constant

Width / Height

Ratio (W / d)

Effective Dielectric

Constant

Characteristic

Impedance (Ω)

4.80

1.8

3.59

50.0

4.00

2.0

3.07

51.0

2.55

3.0

2.12

48.8

Figure 37: Example Microstrip Calculations

MDEV-900-TT Datasheet(PDF) 24 Page - Linx Technologies

MDEV-900-TT Datasheet(HTML) 24 Page - Linx Technologies

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Datasheet Download

Link URL