TXM-418-LC データシートの表示（PDF） - Unspecified
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TXM-418-LC Datasheet PDF : 11 Pages
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PROTOCOL GUIDELINES
While many RF solutions impose data formatting and balancing requirements,
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 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, we suggest you read Linx
Application Note AN-00160.
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.
INTERFERENCE CONSIDERATIONS
The RF spectrum is crowded and the potential for conflict with other unwanted
sources of RF is very real. While all RF products are at risk from interference, its
effects can be minimized by better understanding its characteristics.
Interference may come from internal or external sources. The first step is to
eliminate interference from noise sources on the board. This means paying
careful attention to layout, grounding, filtering, and bypassing in order to
eliminate all radiated and conducted interference paths. For many products, this
is straightforward; however, products containing components such as switching
power supplies, motors, crystals, and other potential sources of noise must be
approached with care. Comparing your own design with a Linx evaluation board
can help to determine if and at what level design-specific interference is present.
External interference can manifest itself in a variety of ways. Low-level
interference will produce noise and hashing on the output and reduce the link’s
overall range.
High-level interference is caused by nearby products sharing the same
frequency or from near-band high-power devices. It can even come from your
own products if more than one transmitter is active in the same area. It is
important to remember that only one transmitter at a time can occupy a
frequency, regardless of the coding of the transmitted signal. This type of
interference is less common than those mentioned previously, but in severe
cases it can prevent all useful function of the affected device.
Although technically it is not interference, multipath is also a factor to be
understood. Multipath is a term used to refer to the signal cancellation effects
that occur when RF waves arrive at the receiver in different phase relationships.
This effect is a particularly significant factor in interior environments where
objects provide many different signal reflection paths. Multipath cancellation
results in lowered signal levels at the receiver and, thus, shorter useful distances
for the link.
Page 8
TYPICAL APPLICATIONS
The LC Series transmitter is ideal for the transmission of remote control /
command data. One of the easiest way to transmit on / off data or switch
closures is to use an encoder and decoder. These ICs provide a number of data
lines that can be connected to switches or buttons or even a microcontroller.
When a line is taken high on the encoder, a corresponding line will go high on
the decoder as long as the address matches. The figure below shows an
example using the Linx MS Series encoder.
1 GND
GND 8
2 DATA
VCC 7
3 GND
GND 6
750 4 LADJ/GND ANT 5
TXM-xxx-LC
0
0
OPEN
100k
100k
1 D6
2 D7
3 SEL_BAUD0
4 SEL_BAUD1
D5 20
D4 19
D3 18
D2 17
100k
100k
100k
100k
5 GND
VCC 16
6 GND
7 GND
8 TX_CNTL
VCC 15
D1 14
D0 13
100k
100k
9 DATA_OUT
SEND 12
220
220
10 MODE_IND CREATE_ADDR 11
100k
LICAL-ENC-MS001
Figure 12: Typical Remote Control Example
This circuit uses the LC Series transmitter and the MS Series encoder to transmit
button presses. The MS Series has eight data lines, which are connected to
buttons that will pull the line high when pressed. When not used, the lines are
pulled low by 100kΩ resistors. The encoder will begin a transmission only when
the SEND line is taken high. Diodes are used to pull this line high when any data
line is pulled high while isolating the data lines from each other.
The MS Series Encoder Data Guide explains this circuit and the many features
of the encoder in detail, so please refer to that document for more information.
A 750Ω resistor is used on the LADJ line of the transmitter to reduce the output
power of the transmitter. This is appropriate for some antennas, but may need to
be adjusted depending on the design. Typically, a resistor pad will be placed on
the board and a potentiometer used by the FCC test lab to adjust the output
power to the maximum legal limit. The potentiometer value would then be
measured and the closest standard value resistor placed for final testing.
If the level adjust resistor does not provide enough attenuation, a T-pad
attenuator can be placed between the transmitter and antenna. This is a network
of three resistors that will provide a set amount of attenuation while maintaining
a 50Ω match between the antenna and the transmitter. Application Note
AN-00150 gives the formulas for calculating the resistor values. If not needed,
the series resistors can be zero ohms or shorted and the parallel one not placed.
Page 9
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