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POWER SUPPLY REQUIREMENTS

The receiver module 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 and ‘hash’ is less than 20 mV. A

10 resistor in series with the supply followed by a 10µF

tantalum capacitor from V

where the quality of supply power is poor. Please note that

operation from 4.7 to 5.2 volts requires the use of an

external 200 resistor placed in series with V

THE DATA OUTPUT

A CMOS-compatible data output is available on pin 8. This output is normally used to

drive directly a digital decoder IC or a microprocessor that is performing the data

decoding. The receiver’s output is internally qualified, meaning that it will only

transition when valid data is present. In instances where no carrier is present the

output will remain low. Since a UART utilizes high marking to indicate the absence of

data, a designer using a UART may wish to insert a logic inverter between the data

output of the RXM-LC-S and the UART.

It is important to realize that the data output of the receiver may be subject to some

pulse stretching or shortening. This is caused by a combination of oscillator start-up

time on the transmitter and ring-down time in the receiver’s ceramic filter. It is

important to consider this effect when planning protocol. To learn more about protocol

considerations for the LC series we suggest you read Linx applications note #00232.

RECEIVING 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 RXM-LC-S modules do not

incorporate internal encoding/decoding, a user has tremendous flexibility in how data

is handled.

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

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

You may wish to review application notes #00125 and #00140 along with Part 15

Sec. 231 for further details on acceptable transmission content.

Another area of consideration is that of data structure or protocol. If unfamiliar with

the considerations for sending serial data in a wireless environment, you will want

to review Linx application note #00232 (Considerations for sending data with the LC

series). These issues should be clearly understood prior to commencing a

significant design effort.

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 your

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

decoder IC set. These chips are available from a wide range of manufacturers

including: Microchip (Keeloq), Holtek (available directly from Linx), and Motorola.

These chips take care of all encoding, error checking, 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 receivers independently. These IC’s are an excellent way

to bring basic Remote Control/Status products quickly and inexpensively to market.

Additionally, it is a simple task to interface with inexpensive microprocessors such

as the Microchip PIC or one of many IR, remote control, DTMF, and modem IC’s.

Figure 15: Supply Filter

10R

Band Select

Filter

pre-

amplifier

AM Detector

SAW Local Oscillator

Gilbert Cell

Mixer/Amp

Limiting Amp

Data Slicer

DATA

50

Ω RF IN

(Ant.)

10.7 Mhz

Bandpass Filter

10.7 Mhz

Ceramic Filter

Figure 13: LC Series Receiver Block Diagram

MODULE DESCRIPTION

The RXM-LC-S is a low-cost, high-performance Surface Acoustic Wave (SAW)

based Carrier-Present Carrier-Absent (CPCA) receiver, capable of receiving

serial data at up to 5,000 bits/second. Its exceptional sensitivity provides

outstanding range at the maximum data rate. While oriented toward high-volume

automated production, the LC-S’s compact surface-mount package is also

friendly to prototype and hand production. When combined with a Linx LC series

transmitter, a highly reliable RF link capable of transferring digital data over line-

of-sight distances in excess of 300 feet (90m) is formed.

THEORY OF OPERATION

The RXM-LC-S is designed to recover

data sent by a CPCA transmitter. This

type of AM modulation is often referred

to by other designations including CW

and OOK. As the CPCA designation

suggests, this type of modulation

represents a logic low ‘0’ by the

absence of a carrier and a logic high ‘1’

by the presence of a carrier. This

modulation method affords numerous

benefits. Two most important are: 1) Cost-effectiveness due to design simplicity

and 2) Higher output power and thus greater range in countries (such as the US)

which average output power measurements over time. Please refer to Linx

application note #00130 for a further discussion of modulation techniques

including CPCA.

The LC series utilizes an advanced single-conversion superhet design which

incorporates a SAW device, high IF frequency and multi-layer ceramic filters.

The SAW device has been in use for more than a decade but has only recently

begun to receive the widespread acclaim its outstanding capabilities deserve. A

SAW device provides a highly accurate frequency source with excellent

immunity to frequency shift due to age or temperature. The use of SAW devices

in both the LC transmitter and receiver modules allows the receiver’s pass

opening to be quite narrow, thus increasing sensitivity and

reducing

susceptibility to near-band interference. The quality of components and overall

architecture utilized in the LC series is unusual in a low-cost product and is one

of the primary reasons the LC receivers are able to outperform even far more

expensive products.

Data

Data

Carrier

Carrier

Figure 14: CPCA (AM) Modulation