TRM-915-R250 datasheet(33/39 Pages) LINX

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Interference Considerations

The RF spectrum is crowded and the potential for conflict with unwanted

sources of RF is very real. While all RF products are at risk from

interference, its effects can be minimized by better understanding its

characteristics.

Interference may come from internal or external sources. The first step

is to eliminate interference from noise sources on the board. This means

paying careful attention to layout, grounding, filtering and bypassing in

order to eliminate all radiated and conducted interference paths. For

many products, this is straightforward; however, products containing

components such as switching power supplies, motors, crystals and other

potential sources of noise must be approached with care. Comparing your

own design with a Linx evaluation board can help to determine if and at

what level design-specific interference is present.

External interference can manifest itself in a variety of ways. Low-level

interference produces noise and hashing on the output and reduces the

link’s overall range.

High-level interference is caused by nearby products sharing the same

frequency or from near-band high-power devices. It can even come from

your own products if more than one transmitter is active in the same area.

It is important to remember that only one transmitter at a time can occupy

a frequency, regardless of the coding of the transmitted signal. This type of

interference is less common than those mentioned previously, but in severe

cases it can prevent all useful function of the affected device.

Although technically not interference, multipath is also a factor to be

understood. Multipath is a term used to refer to the signal cancellation

effects that occur when RF waves arrive at the receiver in different phase

relationships. This effect is a particularly significant factor in interior

environments where objects provide many different signal reflection paths.

Multipath cancellation results in lowered signal levels at the receiver and

shorter useful distances for the link.

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 77 and examples are provided in Figure 78. Software for calculating

microstrip lines is also available on the Linx website.

Trace

Board

Ground plane

Figure 77: 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 78: Example Microstrip Calculations

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