MDEV-868-PRC datasheet(46/55 Pages) LINX

46 / 55 page

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Power Supply Requirements

The module does not have an internal

voltage regulator, therefore it requires a clean,

well-regulated power source. The power supply

noise should be less than 20mV. Power supply

noise can significantly affect the module’s

performance, so providing a clean power supply

for the module should be a high priority during

design.

A 10

Ω resistor in series with the supply followed by a 10µF tantalum

capacitor from V

cc to ground helps in cases where the quality of supply

power is poor (Figure 88). This filter should be placed close to the module’s

supply lines. These values may need to be adjusted depending on the

noise present on the supply line.

Antenna Considerations

The choice of antennas is a

critical and often overlooked

design consideration. The range,

performance and legality of an RF

link are critically dependent upon the

antenna. While adequate antenna

performance can often be obtained

by trial and error methods, antenna

design and matching is a complex

task. Professionally designed antennas such as those from Linx (Figure

89) help ensure maximum performance and FCC and other regulatory

compliance.

Linx transmitter modules typically have an output power that is higher

than the legal limits. This allows the designer to use an inefficient antenna

such as a loop trace or helical to meet size, cost or cosmetic requirements

and still achieve full legal output power for maximum range. If an efficient

antenna is used, then some attenuation of the output power will likely be

needed.

It is usually best to utilize a basic quarter-wave whip until your prototype

product is operating satisfactorily. Other antennas can then be evaluated

based on the cost, size and cosmetic requirements of the product.

Additional details are in Application Note AN-00500.

Ω

µF

Vcc IN

Vcc TO

MODULE

Figure 88: Supply Filter

Figure 89: Linx Antennas

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.

MDEV-868-PRC Datasheet(PDF) 46 Page - Linx Technologies

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