MODULE DESCRIPTION
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 VCC to ground will help in cases
where the quality of supply power is poor. Please note that
operation from 4.7 to 5.2 volts requires the use of an
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.
external 200 resistor placed in series with VCC
.
Figure 15: Supply Filter
THE DATA OUTPUT
50 Ω RF IN
(Ant.)
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.
Gilbert Cell
Mixer/Amp
10.7 Mhz
Bandpass Filter
Band Select
Filter
DATA
pre-
amplifier
10.7 Mhz
Limiting Amp Ceramic Filter
AM Detector
Data Slicer
SAW Local Oscillator
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.
Figure 13: LC Series Receiver Block Diagram
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.
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.
Daattaa
Caarrrriieerr
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.
Figure 14: CPCA (AM) Modulation
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.
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.
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.
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