TRM-915-R250 datasheet(32/39 Pages) LINX

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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 75) 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 may 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.

Helpful Application Notes from Linx

It is not the intention of this manual to address in depth many of the issues

that should be considered to ensure that the modules function correctly

and deliver the maximum possible performance. We recommend reading

the application notes listed in Figure 76 which address in depth key areas

of RF design and application of Linx products. These applications notes

are available online at www.linxtechnologies.com or by contacting the Linx

literature department.

Figure 75: Linx Antennas

Helpful Application Note Titles

Note Number

Note Title

AN-00100

RF 101: Information for the RF Challenged

AN-00126

Considerations for Operation Within the 902–928MHz Band

AN-00130

Modulation Techniques for Low-Cost RF Data Links

AN-00140

The FCC Road: Part 15 from Concept to Approval

AN-00500

Antennas: Design, Application, Performance

AN-00501

Understanding Antenna Specifications and Operation

Figure 76: Helpful Application Note Titles

Typical Applications

Figure 73 shows a circuit using the 250 Series transceiver.

The transceiver UART is connected to a microcontroller UART for

communication of configuration data and data to be sent over the air. The

microcontroller is connected to the CMD-RSP, EX, CMD, BE and CTS

lines to monitor the current state of the module. It monitors the RSSI line to

monitor the strength of the incoming RF signal.

There is no need for buffering or other circuitry between the transceiver and

microcontroller provided that both are operating on the same voltage.

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 74). 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.

GND

VCC

GND

TXD

RESET

C2D

CMD_RSP

RSSI

CMD

CTS

RXD

GND

ANT

GND

RXD

GPIO

Figure 73: 250 Series Transceiver Basic Application Circuit

Ω

µF

Vcc IN

Vcc TO

MODULE

Figure 74: Supply Filter

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