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Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 3395
Maxim > Design Support > Technical Documents > Application Notes > Wireless and RF > APP 3395
Keywords: Remote Keyless Entry, RKE, 315 MHz, 433 MHz, 433.92 MHz, 434 MHz, RKE transmitter,
RKE TX, RKE RX, RKE receiver,
APPLICATION NOTE 3395
Requirements of Remote Keyless Entry (RKE)
Systems
Feb 16, 2005
Abstract: Remote keyless entry (RKE) has captivated automobile buyers, as evidenced by the popularity
of RKE on new automobiles and as an after-market item. This application note provides an overview of
RKE systems and discusses how they meet requirements such as range, battery life, reliability, cost, and
regulatory compliance. It shows some circuits and design approaches and offers some predictions for
future systems, which will include two-way communications.
Remote keyless entry (RKE) systems have become extremely
popular. The installation rate for RKE systems in new vehicles is
more than 80% in North America and more than 70% in Europe.
Besides the obvious advantages of convenience, RKE-actuated
vehicle-immobilization technology minimizes car theft. European
automakers are incorporating the technology in vehicles in
cooperation with insurance companies, who in turn, require it as a
condition for acquiring auto insurance. That trend began in
Germany, and is expected to spread throughout Europe within a
few years.
Click here for an overview of the wireless
components used in a typical radio
transceiver.
Most of these systems employ one-way (simplex) communications. But second- and third-generation
systems may talk back to the key, telling you that the car needs gas or more pressure in the left front
tire.
An RKE system consists of an RF transmitter in the keyfob (or key) that sends a short burst of digital
data to a receiver in the vehicle, where it is decoded and made to open or close the vehicle doors or the
trunk via receiver-controlled actuators. The wireless carrier frequency, is currently 315MHz in the
US/Japan and 433.92MHz (ISM band) in Europe. In Japan the modulation is frequency-shift keying
(FSK), but in most other parts of the world, amplitude-shift keying, or ASK is used. The carrier is
amplitude modulated between two levels: To save power, the lower level is usually near zero, producing
complete on-off keying (OOK).
Detailed RKE Description and Design Objectives
Typical RKE systems (Figure 1) include a microcontroller in the key or keyfob. You unlock the car by
pressing a pushbutton on the key that wakes up the microcontroller. The microcontroller sends a stream
of 64 or 128 bits to the key's RF transmitter, where it modulates the carrier and is radiated through a
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simple printed-circuit loop antenna. (Though inefficient, a loop antenna fabricated as part of the PC
board is inexpensive and widely used.)
Figure 1. An RKE system consists of a keyfob circuit (lower diagram) transmitting to a receiver in the
vehicle (upper diagram).
In the vehicle, an RF receiver captures that data and directs it to another microcontroller, which decodes
the data and sends an appropriate message to start the engine or open the door. Multibutton keyfobs
give the choice of opening the driver's door, or all doors, or the trunk, etc.
The digital data stream, transmitted between 2.4kbps and 20kbps, usually consists of a data preamble, a
command code, some check bits, and a "rolling code" that ensures vehicle security by altering itself with
each use. Without this rolling code, your transmitted signal might accidentally unlock another vehicle or
fall into the hands of a car thief who could use it to gain entry later.
Several major objectives govern the design of these RKE systems. Like all mass-produced automotive
components, they must offer low cost and high reliability. They should minimize power drain in both
transmitter and receiver, because replacing batteries in a keyfob is a nuisance and recharging the car
battery is a major nuisance. In addition to these requirements, the RKE system designer must juggle
receiver sensitivity, carrier tolerance, and other technical parameters to achieve maximum transmission
range within the constraints imposed by low cost and minimum supply current.
Design constraints include those defined by local regulations for short-range devices, such as FCC
regulations in the US. While the use of short-range devices does not require a license, the products
themselves are governed by laws and regulations that vary from country to country. For the US, the
relevant document is the Code of Federal Regulations (CFR), Title 47, Part 15, which includes the
260MHz to 470MHz band (Section 15.231) and the 902MHz to 928MHz band (Section 15.249). (See
http://www.access.gpo.gov/nara/cfr/waisidx_01/47cfr15_01.html).
Here are some examples of how FCC regulations impose limits on an RKE design.
Section 15.231 allows the device to transmit command or control signals, ID codes, and radiocontrol signals during emergencies, but not voice or video, toy-control signals, or continuous data.
Transmission times must not exceed five seconds, and periodic transmissions of one second (max)
at regular intervals are allowed only if the rate of such transmissions is less than one per hour.
Maximum field strength at three meters from the transmit antenna shall be linearly proportional to
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the fundamental frequency (260-470MHz), giving a range of 3750µV/m to 12500µV/m.
Bandwidths at points 20dB down from the carrier shall not exceed 0.25% of the center frequency,
and spurious emissions shall be attenuated by 20dB of the fundamental.
The following sections explore some of the issues associated with RKE system design, beginning with
generation of the carrier frequency.
Carrier Generation
First-generation RKE circuitry included surface acoustic-wave (SAW) devices for generating an RF
carrier in the transmitter and a local-oscillator (LO) frequency in the receiver. Unfortunately, the initial
frequency uncertainty of a typical SAW device is at least ±100kHz, and its frequency stability vs.
temperature is relatively poor. At the receiver, an IF bandpass wide enough to admit the carrier also
admits excessive noise, which in turn limits the range at which the vehicle can respond to a keyfob
signal.
A current alternative to SAW devices is the crystal-based phase-locked loop, or PLL. The transition to
PLLs is encouraged by increasingly strict regulation of RF emissions, especially in Europe and Japan. A
crystal-based PLL transmitter costs a bit more than SAW resonators, but is typically ten times more
accurate. The receiver can therefore have a narrower IF bandwidth, which in turn extends the
transmission distance by raising the S/N ratio.
Earlier SAW devices positioned their nominal frequency at the midpoint of the 1.74MHz-wide 433MHz
band (433.05MHz to 434.79MHz) to ensure reliable operation over the expected process and
temperature variations. Thus, the nominal carrier frequency for 433MHz applications is now 433.92MHz,
and PLL crystals must be selected accordingly.
Modern receiver and transmitter chips integrate the PLL circuitry so one need only connect a suitable
crystal between two terminals on the chip. (See Sidebar below, ICs for RKE.) The MAX1470 PLL, for
instance, includes a divide-by-64 block and a 10.7MHz IF with low-side injection. (The chip can operate
at 433.92MHz, but its image-rejection capability is optimized for 315MHz.) The required crystal frequency
for 315MHz operation (in megahertz) is fXTAL = (fRF-10.7)/64 = 4.7547. You must select a crystal that is
specified to oscillate at 315MHz when loaded with the 5pF capacitance presented by chip terminals
XTAL1 and XTAL2. For details on how to trim the crystal frequency, see Application Note 1017: How to
Choose a Quartz Crystal Oscillator for the MAX1470 Superheterodyne Receiver.
Power Conservation
Because battery life is so important in an RKE system, the system must use every way possible to
minimize operating current and "on time." The voltage-controlled oscillator (VCO) in the receiver PLL
offers a good example of this attention to detail. The receiver must check almost constantly to avoid
missing a demand for entry to the vehicle. To save power the receiver attempts to shut down as often as
possible, even during the brief intervals between checks.
A keyfob transmitter usually issues four 10ms data streams in succession (about 40ms total) to ensure
that the receiver captures at least one of them. The receiver polls every 20ms, working to decode at least
two data streams as a margin against timing errors and noise. It needs 0.75ms of decoding time (enough
for 7 or 8 received bits) to determine whether the data is of interest.
In addition to decoding time, the polling operation must first allow time for the receiver circuits to "wake
up" and stabilize. Most amplifier circuits can wake up quickly, but the VCO's crystal is an
electromechanical component that requires time to begin oscillating and more time to stabilize at the
desired frequency. Conventional superheterodyne recievers require 2ms to 5ms for that purpose. But the
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MAX1470 VCO does it in only 0.25ms, by supplying just enough power to maintain vibration in the
crystal. Thus, the MAX1470 detects keyfob transmissions by waking up for only 1ms (0.75ms for
decoding plus 0.25ms for stabilizing) during every 20ms (Figure 2). The fast-wakeup MAX1470 also
operates on 3.3V instead of 5V, for a net energy savings that extends battery life (compared to
conventional superhet receivers) by a factor of four or five.
Figure 2. To monitor keyfob transmissions, an RKE receiver must allocate time to wake up and stabilize
before decoding the incoming signal.
RKE is strictly a short-range technology, up to 20 meters, or 1 to 2 meters for passive RKE systems.
Ensuring even a short transmission distance on low power and a low-cost design budget can be
challenging for the RF circuitry. For simplicity, the transmit and receive antennas consist of a circular or
rectangular loop of copper trace on a small PC board, with a simple LC network to match the antenna
impedance to the transmit or receive chip. (See Application Note 1830: How to Tune and Antenna Match
the MAX1470 Circuit.
Add a Low-Noise Amplifier (LNA)?
The low transmit power imposed by FCC regulations, the small battery capacity, and the uncertainty in
orientation of the transmit antenna demand maximum sensitivity at the RKE receiver chip. One way to
enhance receiver sensitivity is to add an external low-noise amplifier (Figure 3), but the restriction in
dynamic range associated with that approach may be unacceptable in your application. Consider the
following analysis based on the MAX1470 superheterodyne receiver.
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Figure 3. Adding an external LNA (the MAX2640) increases receiver sensitivity, but lowers the thirdorder intercept point.
A receiver's sensitivity depends on its noise figure, the minimum S/N ratio required for detection of the
carrier modulation, and thermal noise in the system:
S = NF + n 0 + S/N, Equation 1
where S is the minimum required signal level in dBm, NF is the receiver's noise figure in dBm, n 0 is the
receiver's thermal noise power in dBm, and S/N is the output signal-to-noise ratio in dBm required for
adequate detection (usually based on the acceptable bit-error rate).
For simplicity we estimate S/N at 5dB, based on an assumption of Manchester-encoded data. By
definition:
n 0 = 10log10 (kTB/1E-3),
where k is Boltzmann's constant (1.38E-23), T is the temperature in degrees Kelvin, and B is the system
noise bandwidth. At room temperature (T = 290°K) over a 1Hz bandwidth, n 0 = -174dBm/Hz. Over a
300kHz IF bandwidth, n 0 = -119dBm.
Assuming the system sensitivity (S) is -109dBm, use Equation 1 to calculate NF = 5dB. The relationship
between noise figure (NF) and noise factor (F) is (NF)dB = 10logF, where F = 10(NF dB /10). Thus, F =
3.162. For a cascade of several 2-port devices, the noise factor is
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FTotal = F1 + (F2-1)/G1 + (F3-1)/(G1*G2) + . . . Equation 2
Equation 2 lets you calculate the new noise factor after adding an external LNA to the system. For the
MAX2640 LNA from Maxim, NF = 1dB and gain = 15dB (i.e., F1 = 1.26 and G1 = 31.62). Noise factor
for the original system was 3.162, so FTotal = 1.327, which is 1.23dB. Substituting in Equation 1:
S = 1.23 - 119 + 5 = -112.77dB.
We assumed the original sensitivity was -109dBm, so we gained only 3.77dB in that category. Now, note
the effect on dynamic range as indicated by the third-order intercept point (IIP3). The MAX1470 has an
internal LNA gain of 16dBm and an internal mixer IIP3 of -18dBm, for an overall IIP3 of -34dBm. Adding
the external LNA with its gain of 15dB lowers this number to -49dBm. Thus, the addition of an external
LNA improved sensitivity by almost 4dB, but reduced the system dynamic range by 15dB! For a given
application, you must decide whether such a trade-off is acceptable.
Prognostications
The next development in RKE systems is 2-way (half-duplex) communications, which first appeared as
the "passive RKE" already available in some high-end automobiles. With the key in your pocket. you
simply walk up to the car, where a transmitter is continually polling to detect your arrival. When you
come within range (one or two meters), the key and vehicle establish 2-way communications and open
the door for you. Current 2-way systems include the usual acknowledgment functions (yes, door is
locked), in addition to a remote-start function that allows the user to warm the car engine before leaving
the house.
Future developments may also include the technology for tire-pressure sensing (TPS). Like passive RKE,
TPS is available at this time only for some trucks and luxury automobiles. TPS systems have much in
common with RKE. Circuitry very similar to that of an RKE keyfob resides in the valve stem of each tire,
along with a sensor for tire pressure and temperature. Regular transmissions from each tire to a receiver
in the vehicle (quite similar to an RKE receiver) then provide the driver with an early warning of any
problem developing with the tires. TPS and RKE have so much in common (short range, simple
modulation, need to conserve power, etc.), that future systems will probably save cost by sharing and
consolidating circuit functions.
RKE may, or may not, evolve into a half-duplex system that informs the driver about the state of the car
and its need for gas, oil, etc—all before the door is opened. It is more likely that RKE, if proven
sufficiently robust and reliable, will eventually obsolete the key and its associated door hardware.
CMOS ICs for RKE
Maxim is one of several manufacturers producing special-purpose integrated circuits for the RKE market.
For the keyfob, it offers the world's smallest transmitter of its type—the 300MHz to 450MHz MAX1472,
which comes in a tiny 3mm by 3mm, 8-pin SOT23 package. Its 2.1V to 3.6V supply voltage range
enables the device to operate from a single lithium cell, drawing only 5nA of supply current in the
standby mode.
During transmission of Manchester-encoded data, the MAX1472 supports data rates up to 100kbps and
draws between 3.0mA and 5.5mA of supply current while delivering -10dBm to +10dBm of power to a
50Ω load. Its crystal-based phase-locked loop produces an accurate carrier frequency that enhances
transmission range by allowing a tighter IF bandwidth in the receiver. To minimize power consumption,
the internal oscillator starts quickly. It requires only 220µs startup time following an enable signal.
For vehicle receivers, consider the MAX1473 300MHz to 450MHz superhet ASK receiver. It offers -
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114dB sensitivity, and 50dB of RF image rejection in its fully differential internal mixer. The MAX1473 is
optimized for either 315MHz or 433MHz operation. It operates on 3.3V or 5V and includes a low-noise
amplifier (LNA), a crystal-based PLL for the local oscillator, and a 10.7MHz IF limiting amplifier with
received signal-strength indicator (RSSI). An internal data filter and data slicer provide the digital data
output. As an alternative, you can choose the MAX1470 receiver, which is similar to the MAX1473 but
optimized only for 315MHz. It operates on a supply voltage of 3.0V to 3.6V.
Related Parts
MAX1470
315MHz Low-Power, +3V Superheterodyne Receiver
Free Samples MAX1471
315MHz/434MHz Low-Power, 3V/5V ASK/FSK
Superheterodyne Receiver
Free Samples MAX1472
300MHz-to-450MHz Low-Power, Crystal-Based ASK
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315MHz/433MHz ASK Superheterodyne Receiver with
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300MHz to 450MHz Low-Power, Crystal-Based +10dBm
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Free Samples MAX7030
Low-Cost, 315MHz and 433.92MHz ASK Transceiver
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Free Samples MAX7032
Low-Cost, Crystal-Based, Programmable, ASK/FSK
Transceiver with Fractional-N PLL
Free Samples MAX7033
315MHz/433MHz ASK Superheterodyne Receiver with
AGC Lock
Free Samples MAX7042
308MHz/315MHz/418MHz/433.92MHz Low-Power, FSK
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300MHz to 450MHz High-Efficiency, Crystal-Based
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Free Samples More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 3395: http://www.maximintegrated.com/an3395
APPLICATION NOTE 3395, AN3395, AN 3395, APP3395, Appnote3395, Appnote 3395
Copyright © by Maxim Integrated Products
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