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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 connection can be made in very close proximity (<1/8in.) to

the module. One common form of transmission line is a coax cable, another is

the Microstrip. This term refers to a PCB trace running over a groundplane 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 below. Handy software for calculating microstrip lines is also

available on the Linx website (www.linxtechnologies.com).

Dielectric Constant

Width/Height (W/d)

Effective Dielectric

Constant

Characteristic

Impedance

4.8

1.8

3.59

50.0

4

2

3.07

51.0

2.55

3

2.12

48.0

Trace

Board

Groundplane

Figure 16: Microstrip Formulas (Er = dielectric constant of PC board material)

BOARD LAYOUT GUIDELINES

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

board layout requirements. Fortunately, because of the care taken by Linx in

designing the modules, integrating them is very straightforward. Despite this

ease of application it is still necessary to maintain respect for the RF stage and

exercise appropriate care in layout and application in order to maximize

performance and ensure reliable operation. The antenna can also be influenced

by layout choices. Please review this manual in its entirety prior to beginning

your design. By adhering to good layout principles and observing some basic

design rules you well along on the path to RF success.

The adjacent figure shows the suggested PCB

footprint for the module. The actual pad

dimensions are shown in the Pad Layout

section of this manual. A groundplane (as large

as possible) should be placed on a lower layer

of your PC board opposite the module. This

groundplane can also be critical to the

performance of your antenna, which will be

discussed later in the manual. There should not

be any ground or traces under the module on

the same layer as the module, just bare PCB.

During prototyping, the module should be soldered to a properly laid-out circuit

board. The use of prototyping or "perf" boards will result in horrible performance

and is strongly discouraged.

No conductive items should be placed within 0.15in of the module’s top or sides.

Do not route PCB traces directly under the module. The underside of the module

has numerous signal-bearing traces and vias that could short or couple to traces

on the product's circuit board.

The module’s ground lines should each have their own via to the ground plane

and be as short as possible.

AM/OOK receivers are particularly subject to noise. The module should, as much

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

especially high-frequency circuitry such as crystal oscillators, switching power

supplies and high-speed bus lines. Make sure internal wiring is routed away from

the module and antenna, and is secured to prevent displacement.

The power-supply filter should be placed close to the module's Vcc line.

In some instances, a designer may wish to encapsulate or "pot" the product.

Many Linx customers have done this successfully; however, there are a wide

variety of potting compounds with varying dielectric properties. Since such

compounds can considerably impact RF performance it is the responsibility of

the designer to carefully evaluate and qualify the impact and suitability of such

materials.

The trace from the module to the antenna should be kept as short as possible.

A simple trace is suitable for runs up to 1/8 inch for antennas with wide

bandwidth characteristics. For longer runs or to avoid detuning narrow bandwidth

antennas, such as a helical, use a 50-ohm coax or 50-ohm microstrip

transmission line as described in the following section.

GROUNDPLANE

GROUNDPLANE

ON LOWER LAYER

ON LOWER LAYER

GROUNDPLANE

ON LOWER LAYER

Figure 15: Suggested PCB Layout