Page 7

Page 6

POWER SUPPLY REQUIREMENTS

The module does not have an internal voltage regulator; therefore it requires a

clean, well-regulated power source. While it is preferable to power the unit from

a battery, the unit can also be operated from a power supply as long as noise is

less than 20mV. Power supply noise can significantly

affect the transmitter modulation; therefore, providing

a clean power supply for the module should be a high

design priority.

A 10

Ω resistor in series with the supply followed by a

in cases where the quality of supply power is poor.

These values may need to be adjusted depending on

the noise present on the supply line.

TRANSMITTING 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. Since the LC Series modules do not

incorporate internal encoding or decoding, a user has tremendous flexibility in

how data is handled.

If you want to transfer simple control or status signals, such as button presses or

switch closures, and your product does not have a microprocessor on board, or

you wish to avoid protocol development, consider using an encoder and decoder

IC set. These chips are available from a 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,

such as the Microchip PIC, or one of many IR, remote control, or modem ICs.

It is always important to separate what types of transmissions are technically

possible from those that are legally allowable in the country of intended

operation. While the LR Series is ideally suited to the long range transfer of

control and command information, it can also be used with great success for the

transfer of true variable data such as temperature, pressure, or sensor data.

However, the 260 - 470MHz band in which the module operates is regulated by

Part 15, Section 231 of the FCC regulations. Many types of transmissions,

especially those involving automatic transmissions or variable data, may need to

be periodic. You may wish to review Application Notes AN-00125 and AN-00140

along with Part 15, Section 231 of the FCC regulations for further details on

acceptable transmission content in the Unites States.

Another area of consideration is that of data structure or protocol. The data

should be formatted in a predictable way and should be able to deal with errors

due to interference. This will ensure that the data is received and interpreted

correctly. If you are not familiar with the considerations for sending serial data in

a wireless environment, you will want to review Application Note AN-00160.

10

Ω

10

μF

Vcc IN

Vcc TO

MODULE

Figure 11: Supply Filter

THE DATA INPUT

A CMOS / TTL level data input is provided on Pin 2. This line is normally supplied

with a serial bitstream input directly from a microprocessor, encoder, or UART.

During standby, or the input of a logic low, the carrier is fully suppressed and the

transmitter consumes less than 2µA of current. During a logic high, the

transmitter generates a carrier to indicate to the receiver the presence of a logic

‘1’. The applied data should not exceed a rate of 5,000bps. The data input line

should always be driven with a voltage common to the supply voltage present on

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 7 on Page 3. 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 the figure below. 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

PADS FOR SMD

RESISTORS

RESISTORS

PADS FOR SMD

RESISTORS

ANT

ANT

R1

R1

R2

GROUND PLANE

GROUND PLANE

ON LOWER LAYER

ON LOWER LAYER

GROUND PLANE

ON LOWER LAYER

GROUND

GROUND

GROUND

GROUND

GROUND

GROUND

Figure 10: A T-Pad Attenuator Layout Example