Linx Technologies TXM-xxx-LC - Manual (Page 8)

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Adjusting the Output Power
Depending on the type of antenna being used and the duty cycle of the
data, the output power of the LC Series transmitter module may be higher
than FCC regulations allow. The output power of the module is intentionally
set high to compensate for losses resulting from inefficient antennas that
may be used to realize cost or space savings. Since attenuation is often
required, it is generally wise to provide for its implementation and allow the
FCC test lab to easily attenuate the transmitter to the maximum legal limit
for your product.
Two methods of attenuation are available using the LC Series transmitter
module. First, a resistor may be placed between Pin 4 (LADJ) and
ground to achieve up to a 7dB reduction in output power. The resistor
value is easily determined from Figure 9 on page 6. Do not exceed
the resistance values shown as transmitter instability may result. This
method can also be used to reduce the transmission range and power
consumption.
Another method commonly used to achieve attenuation, particularly at
higher levels, is the use of a T-pad attenuator. A T-pad is a network of three
resistors that allows for variable attenuation while maintaining the correct
match to the antenna. It is usually prudent to allow space for the addition
of a T-pad. An example of a T-pad attenuator layout is shown in Figure 14.
For further details on T-pad attenuators, please refer to Application Note
AN-00150.
CIRCUIT TYPICAL LAYOUT
WITH PROVISION FOR ATTENUATION
GND
ANT OUT
PADS FOR SMD
RESISTORS
PADS FOR SMD
RESISTORS
ANT
ANT
R1
R1
R2
GROUND PLANE
ON LOWER LAYER GROUND PLANE
ON LOWER LAYER
GROUND GROUND GROUND GROUND
TXM-xxx-LC
LOT CTxxxx
Transferring Data
Once a reliable RF link has been established, the challenge becomes how
to effectively transfer data across it. While a properly designed RF link
provides reliable data transfer under most conditions, there are still distinct
differences from a wired link that must be addressed. The LC Series is
intended to be as transparent as possible and does not incorporate internal
encoding or decoding, so a user has tremendous flexibility in how data is
handled.
If the product transfers simple control or status signals such as button
presses or switch closures and it does not have a microprocessor on board
(or it is desired to avoid protocol development), consider using a remote
control encoder and decoder or a transcoder IC. These chips are available
from a wide range of manufacturers including Linx. They take care of all
encoding and decoding functions, and generally provide a number of data
pins to which switches can be directly connected. In addition, address bits
are usually provided for security and to allow the addressing of multiple
units independently. These ICs are an excellent way to bring basic remote
control / status products to market quickly and inexpensively. Additionally,
it is a simple task to interface with inexpensive microprocessors, IR, remote
control or modem ICs.
It is always important to separate the types of transmissions that are
technically possible from those that are legally allowable in the country
of intended operation. Linx Application Notes AN-00125, AN-00128
and AN-00140 should be reviewed, along with Part 15, Section 231 of
the Code of Federal Regulations for further details regarding acceptable
transmission content in the US All of these documents can be downloaded
from the Linx website at www.linxtechnologies.com.
Another area of consideration is that the data structure can affect the
output power level. The FCC allows output power in the 260 to 470MHz
band to be averaged over a 100ms time frame. Because OOK modulation
activates the carrier for a ‘1’ and deactivates the carrier for a ‘0’, a data
stream that sends more ‘0’s has a lower average output power over
100ms. This allows the instantaneous output power to be increased, thus
extending range.
Figure 14: A T-Pad Attenuator Layout Example