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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.

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 100k

Ω 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 750

Ω 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 50

Ω 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.

100k

100k

220

100k

100k

100k

100k

100k

100k

100k

LICAL-ENC-MS001

D6

D7

SEL_BAUD0

SEL_BAUD1

GND

GND

GND

TX_CNTL

DATA_OUT

MODE_IND

D5

D4

D3

D2

VCC

VCC

D1

D0

SEND

CREATE_ADDR

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

220

TXM-xxx-LC

1

2

3

45

6

7

8

GND

VCC

GND

ANT

GND

DATA

GND

LADJ/GND

750

00

OPEN

Figure 12: Typical Remote Control Example

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.