User's Manual

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The transceiver includes a U.FL connector as well as a line for the
antenna connection. This offers the designer a great deal of flexibility in
antenna selection and location within the end product. Linx offers cable
assemblies with a U.FL connector on one end and several types of
standard and FCC-compliant reverse-polarity connectors on the other end.
Alternatively, the designer may wish to use the pin and route the antenna to
a PCB mount connector or even a printed loop trace antenna. This gives
the designer the greatest ability to optimize performance and cost within
the design.
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 26 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.
Power Supply Requirements
The transceiver incorporates a precision
low-dropout regulator which allows operation
over a wide input voltage range. Despite this
regulator, it is still important to provide a supply
that is free of noise. 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 10Ω 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 24). 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
25) 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. This can easily be accomplished by using the LVL_ADJ line.
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.
+
10Ω
10µF
Vcc IN
Vcc TO
MODULE
Figure 24: Supply Filter
Figure 25: Linx Antennas
Note: Either the connector or the line can be used for the antenna, but
not both at the same time.
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
RG-00103 TT Series Transceiver Command Data Interface Reference Guide
Figure 26: Helpful Application Note Titles