Linx Technologies TRM-915-R250 - Manual (Page 32)

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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
GND
GND
NC
1
TXD
2
NC 3
NC 4
NC
5
RESET
6
C2D 7
NC
8
CMD_RSP
9
EX
10
GND
11
GND
12
GND
13
RSSI
14 CMD
15
BE 16
NC 17
NC
18 CTS
19
RXD 20
GND 21
ANT
22 GND
23 VC
C
24
VC
C
25
VC
C
26
RXD
GPIO
GPIO
GPIO GPIO GPIO GPIO GPIO TX
D
µ
Figure 73: 250 Series Transceiver Basic Application Circuit
+
10 Ω
10 µF
Vcc IN
Vcc TO
MODULE
Figure 74: Supply Filter