ion for an OOK (on-off keying) transmitter and receiver
in the periodic operation part 15 band from 260 to 470 MHz is described
in this application note.
The receiver is designed to track and lock to the carrier of a SAW
(surface acoustical wave) resonator or crystal-controlled PLL
transmitter. The simplest form of ASK (amplitude shift keying) is OOK in
which the carrier is simply switched on and off by the PCM (pulse code
modulation) waveform. Generally, a low-cost OOK transmitter consists
of a SAW resonator and a transistor used as a Colpitts oscillator.
Frequency stability is only fair, compared to a crystal-controlled source,
offering excellent frequency stability. Because of poor frequency stability
due to time and temperature effects, the frequency of a 300-MHz SAW
resonator oscillator typically shifts
±
150 kHz. A standard OOK receiver
is designed with a wideband response to capture the drifted transmitter
at the expense of sensitivity and selectivity.


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© Freescale Semiconductor, Inc., 2004. All rights reserved. Application Note
AN1833
2
The OOK receiver and transmitter presented in this application note offer
greater range of operation and more robust data recovery due to
improved receiver sensitivity and the receivers ability to tune
automatically to the received carrier from the low-power crystal-
controlled transmitter. The transmitter has high-frequency stability and
low-harmonics power from unintentional radiators.
NOTE:
Throughout this document, numbers appearing in square brackets
([1], [2], etc.) refer to the information in
References
.
Application Overview
In the United States, the Federal Communications Commission (FCC)
has allocated the 260- to 470-MHz band for periodic part 15 operation.
The FCC regulations are found in the Code of Federal Regulations
(CFR), title 47, part 15 (paragraph 15.231).
NOTE:
It is strongly recommended that this document be reviewed before
pursuing an application in this band.
In the United States, most of the consumer applications are regulated
under part 15, which covers non-licensed intentional radiators. CFR title
47 [1] can be found at most libraries in the reference section or may be
obtained from the U.S. Government Printing Office or the Federal
Communications Commissions Web site.
Periodic operation refers to the amount of time a system is active. A
manually operated transmitter will employ a switch that will deactivate
the transmitter automatically within five seconds. Likewise, a transmitter
that is activated automatically will cease transmission within five
seconds after activation. Periodic transmissions at regular intervals are
not allowed. However, polling to determine the integrity of the
transmitters used in security or safety applications is allowed if the
periodic rate of transmission does not exceed one transmission of more
than one second duration per hour for each transmitter in the system.
The field strength of emissions from intentional radiators from 260 to
470 MHz is limited to 12,500 microvolts per meter measured at three



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Freescale Semiconductor, Inc.
For More Information On This Product,
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. Application Note
Application Overview
AN1833
3
meters. At band edges, the tighter limit of 3750 microvolts per meter
applies. Field strength of spurious emissions are 1/10 of the fundamental
frequency of the intentional radiator. The bandwidth of the emission shall
not be wider than 0.25 percent of the center frequency. Bandwidth is
determined at the points 20 dB down from the modulated carrier.
Although continuous transmissions such as voice, video, and data are
not permitted, the transmission of recognition codes is allowed. These
codes are used to identify a sensor or switch that is activated or to
identify the particular component as being part of the system. The
intentional radiator is restricted to the transmission of control signals
such as those used with security alarm systems, door openers, keyless
entry, remote switches, and remote sensing/telemetry, etc.
The general system may consist of a number of transmitters used to
provide control signals from remote sites. The receiver demodulates the
signal and recognizes the remote sites; then the system may initiate a
response in several ways such as sounding an alarm, making an
automatic distress call, turning on a light, opening a door, and turning on
sprinklers, etc. Each site may use a coded address that would be
broadcast to indicate the intended receiver. Some applications are more
critical than others, such as a security alarm or distress calling systems.
It is essential that the system is dependable.
The receiver has the job of extracting and reproducing information from
the modulated RF (radio frequency) signal that has been corrupted by
noise in the channel. Consistent recovery and error-free replication of
the modulating signal requires a receiver which has reliable
performance.
The two main classes of radio frequency receivers are: Tuned radio frequency (TRF) Superheterodyne receivers
The TRF receiver may consist of several cascaded high gain RF
amplifier and tunable bandpass filter stages followed by an appropriate
detector (envelope detector, product detector, FM (frequency
modulation) detector, etc.). A reliable TRF receiver at UHF (ultra-high
frequency) frequencies is difficult to achieve because appreciable



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Freescale Semiconductor, Inc.
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. Application Note
AN1833
4
parasitic feedback between the output and input of the high gain RF
amplifier chain creates oscillation at the center frequency. Reducing the
gain in the RF amplifier chain may prevent oscillations but lower gain will
reduce the sensitivity of the receiver. These receivers generally use
discrete semiconductors since integrated circuits are not available for
low-voltage, low-power applications.
The more reliable and stable receiver described in this application note
is a superheterodyne type receiver. In this receiver, the RF carrier is
filtered and amplified using an RF preamplifier and is down converted
using a mixer and local oscillator (LO) to a convenient, lower frequency
called the intermediate frequency (IF). The IF signal is amplified and
filtered and then demodulated by using an appropriate detector.
The OOK receiver shown in
Figure 1a
has many advantages over a
TRF receiver. It utilizes two monolithic integrated circuits (IC), the
MC13144 [2] as a cascade low-noise amplifier (LNA), and the
MC13158 [3] as the main building block components of the receiver.
This receiver offers very stable operation with more than 115 dB of
stable gain. Typical sensitivity is 100 dBm which allows for a greater
operating distance between transmitter and receiver. By utilizing a
varactor-controlled LO locked by an external automatic frequency
control (AFC) circuit, the receiver has the ability to lock onto the
transmitter. Thus, this receiver system has no need to use a more
expensive crystal oscillator for the LO.
The OOK transmitter (
Figure 1b
) is implemented by using a versatile
1-chip, fixed frequency PLL (phase-locked loop) system IC, the
MC13176D [4] which operates under an inexpensive crystal reference
source from a microcontroller unit, the MC68HC705J1A [5].
Baseband processing functions are performed by MCUs in both the
remote receiver and handheld transmitter.



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Freescale Semiconductor, Inc.
For More Information On This Product,
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. Application Note
Application Overview
AN1833
5
Figure 1a. OOK Receiver Block Diagram
20
19
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
ANTENNA
LNA
MC13144 OR
MRFIC0916
MC13158FTB
SINGLE-CONVERSION RECEIVER
SWITCHING
4-MHz
TTL DATA OUT
V
CC
OUTPUT
FUND
XTAL
10 M V
SS
V
DD
V
DD
V
DD
V
SS
MC34164
MC68HC705J1A



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