Linx Technologies TRM-xxx-NT - Manual (Page 15)

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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 10 Ω resistor in series with the supply followed by a 10µF tantalum
capacitor from V
CC
to ground will help in cases where the quality of supply
power is poor (Figure 30). 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
31) will 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 (Figure 47).
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.
Protocol Guidelines
While many RF solutions impose data formatting and balancing
requirements, the transparent modes of Linx RF modules do not encode
or packetize the signal content in any manner. The received signal will be
affected by such factors as noise, edge jitter and interference, but it is not
purposefully manipulated or altered by the modules. This gives the designer
tremendous flexibility for custom protocol design and interface.
Despite this transparency and ease of use, it must be recognized that there
are distinct differences between a wired and a wireless environment. Issues
such as interference and contention must be understood and allowed for in
the design process. To learn more about protocol considerations see Linx
Application Note AN-00160 (Figure 47).
Errors from interference or changing signal conditions can cause corruption
of the data packet, so it is generally wise to structure the data being sent
into small packets. This allows errors to be managed without affecting large
amounts of data. A simple checksum or CRC could be used for basic error
detection. Once an error is detected, the protocol designer may wish to
simply discard the corrupt data or implement a more sophisticated scheme
to correct it.
+
10 Ω
10 µF
Vcc IN
Vcc TO
MODULE
Figure 30: Supply Filter
Figure 31: Linx Antennas
Note: Either the connector or the line can be used for the antenna, but
not both at the same time.