Remote-keyless-entry (RKE) capability has captivated automotive buyers, with an RKE installation rate of more than 80 percent for new vehicles in North America and more than 70 percent in Europe. Most RKE systems employ one-way (simplex) communications (from the key to the door lock, for example), although second- and third-generation RKE systems may incorporate duplex operation with communication back to the key. Designing an effective RKE system involves understanding emissions limits set by the Federal Communications Commission (FCC) as well as technical capabilities of integrated circuits (ICs) and supporting circuitry.
An RKE system consists of an RF transmitter in the key fob (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 trunk via receiver-controlled actuators. The wireless link is simply a carrier frequency, currently 315 MHz in the United States and Japan, and 433.92 MHz of the Industrial-Scientific-Medical (ISM) band in Europe. Japanese RKE systems employ frequency-shift-keying (FSK) modulation, but in most other parts of the world amplitude-shift-keying (ASK) modulation is used, in which the carrier is amplitude modulated between two levels. To save power, the lower level is usually near zero, producing complete on-off-keying (OOK) modulation.
Typical RKE systems (Fig. 1) include a microcontroller in the key or key fob. A car is typically unlocked by pressing a pushbutton on the key that wakes up the microcontroller and sends a 64- or 128-b data stream to the key's RF transmitter, where it modulates the carrier and is radiated via a simple printed-circuit loop antenna. (Although inefficient, a loop antenna fabricated as part of the printed-circuit board (PCB) is inexpensive and widely used.) 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. Key fobs with multiple buttons provide choices of opening the driver's door, or all doors, or the trunk, etc.
The RKE digital data stream, transmitted between 2.4 and 20 kb/s, 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. (Otherwise, a transmitted signal might accidentally unlock another vehicle, or fall into the hands of a car thief who could use it to gain entry later on.)
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 key fob is a nuisance and recharging the car battery is a major nuisance. With one eye on these requirements, the designer of an RKE system must also 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.
Further constraints include those defined by local regulations for short-range devices (such as FCC regulations in the US). The use of short-range devices does not require a license, but 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 260-to-470-MHz band (Section 15.231) and the 902-to-928-MHz band (Section 15.249).1
The following provides some guidelines as to how the FCC regulations impose limits on an RKE design: Section 15.231 allows the device to transmit command or control signals, identification (ID) codes, and radio-control 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 maximum 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 should be linearly proportional to the fundamental frequency (260 to 470 MHz), giving a range of 3750 to 12,500 µV/m. Bandwidths at points 20 dB down from the carrier should not exceed 0.25 percent of the center frequency, and spurious emissions should be attenuated by 20 dB of the fundamental-frequency signal.
First-generation RKE circuitry includes surface-acoustic-wave (SAW) source 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 ±100 kHz, and its frequency stability versus temperature is relatively poor. At the receiver, an intermediate-frequency (IF) pass band wide enough to admit the carrier also admits excessive noise, which in turn limits the range at which the vehicle can respond to a key fob signal.
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A current alternative to SAW devices is crystal-based phase-locked-loop (PLL) oscillator. (The transition to PLLs is encouraged by increasingly strict regulation of RF emissions, especially in Europe and Japan.) A crystal-based PLL transmitter costs only $0.50 (US) more than one with a SAW resonator, but is typically 10 times more accurate. The receiver can therefore have a narrower IF bandwidth, which in turn extends the transmission distance by raising the signal-to-noise ratio (SNR).
Earlier SAW devices were designed to operate at the midpoint of the 1.74-MHz-wide 433-MHz band (433.05 to 434.79 MHz) to ensure good system reliability even with expected process and temperature variations. Thus, the nominal carrier frequency for 433 MHz applications is now 433.92 MHz, and PLL crystals must be selected accordingly.
Modern receiver and transmitter ICs feature integrated PLL circuitry requiring only an external crystal resonator for effective RKE signal generation (see sidebar). The MAX1470 PLL from Maxim Integrated Products, for example, includes a divide-by-64 block and a 10.7-MHz IF with low-side injection. (The chip can operate at 433.92 MHz, but its image-rejection capability is optimized for 315 MHz.) The required crystal frequency for 315 MHz operation (in MHz) is fXTAL = (fRF − 10.7)/64 = 4.7547. The IC is designed for use with a crystal that is specified to oscillate at 315 MHz when loaded with the 5-pF capacitance presented by chip terminals XTAL1 and XTAL2. For details on how to trim the crystal frequency, refer to Application Note 1017 available at the company's website.2
Because maximum battery life is important, RKE systems employ various techniques 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, and to save power it attempts to shut down as often as possible, even during the brief intervals between checks.
A key fob transmitter usually issues four 10-ms data streams in succession (about 40 ms total) to ensure that the receiver captures at least one of them. The receiver performs a polling operation every 20 ms, seeking to decode at least two data streams as a margin against timing errors and noise. About 0.75 ms of decoding time (enough for receiving 7 or 8 data bits) is required 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 receivers require 2 to 5 ms for that purpose. The MAX1470's VCO does it in only 0.25 ms by supplying just enough power to maintain vibration in the crystal. Thus, the IC detects key fob transmissions by waking up for only 1 ms (0.75 ms for decoding plus 0.25 ms for stabilizing) during every 20 ms (Fig. 2). The fast-wakeup MAX1470 also operates on +3.3 VDC instead of +5 VDC, for a net energy savings that extends battery life (with respect to conventional superheterodyne receivers) by a factor of four or five.
RKE is strictly a short-range technology (up to 20 m for active systems or 1 to 2 m for passive RKE systems), but 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 PCB, with a simple inductive-capacitive (LC) network to match the antenna impedance to the transmit or receive chip.3
The low transmit power imposed by FCC regulations, small battery capacity, and uncertainty in orientation of the transmit antenna demands maximum sensitivity at the RKE receiver chip. One way to enhance receiver sensitivity is to add an external low-noise amplifier (Fig. 3), but the restriction in dynamic range associated with that approach might be unacceptable in a given application. For example, consider the following analysis based on the MAX1470 superheterodyne receiver.
A receiver's sensitivity depends on its noise figure, the minimum SNR required for detection of the carrier modulation, and the thermal noise in the system:
where:
S = the minimum required signal level (in dBm);
NF = the receiver noise figure (in dB);
n0 = the receiver's thermal noise power in dBm; and
S/N = the output SNR (in dB) required for required for adequate signal detection (usually based on the acceptable bit-error rate).
For simplicity, an SNR of 5 dB is estimated, based on an assumption of Manchester-encoded data. By definition,
n0 = 10log10(kTB/1 × 10−3),
where:
k = Boltzmann's constant (1.38 × 10−23);
B = the system noise bandwidth; and
T = the temperature (in °K).
At room temperature (T = 290°K) over a 1-Hz bandwidth n0 = −174 dBm/Hz. Over a 300-kHz IF bandwidth, n0 = −119 dBm.
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Assuming the system sensitivity (S) is −109 dBm, Eq. 1 can be used to calculate a noise figure (NF) of 5 dB. The relationship between noise figure (NF) and noise factor (F) is (NF)dB = 10logF, where F = 10(NFdB/10). Thus, F = 3.162. For a cascade of several two-port devices, the noise factor is:
where:
F1, F2, F3 = the noise factors of system stages one, two, and three, respectively, and
G1, G2, G3 = the gain (numerical voltage) of system stages one, two, and three, respectively.
Equation 2 makes it possible to calculate the new noise factor after adding an external LNA to the system. For the MAX2640 LNA from Maxim, NF = 1 dB and gain = 15 dB (i.e., F1 = 1.26 and G1 = 31.62). The noise factor for the original system was 3.162, so FTotal = 1.327, which is 1.23 dB. Substituting this value into Eq. 1 yields:
S = 1.23 − 119 + 5 = −112.77 dB
Assuming that the original sensitivity was −109 dB, only 3.77 dB has been gained by the addition of the LNA. Note the effect on dynamic range as indicated by the third-order intercept point (IIP3). The MAX1470 has an internal LNA gain of 16 dB and an internal mixer IIP3 of −18 dBm, for an overall IIP3 of −34 dBm. Adding the external LNA with its gain of 15 dB lowers this number to −49 dBm. Thus, the addition of an external LNA improved sensitivity by almost 4 dB, but reduces the system dynamic range by 15 dB! For a given application, such a trade-off must be considered.
Beyond simplex systems, the next RKE advancement involved two-way, half-duplex systems which first appeared as the "passive RKE" already available in some high-end automobiles. The vehicle's transmitter is continually polling to detect the proximity of an operator's key fob. Within range (1 to 2 m), the key fob and vehicle establish two-way communications and open the door. Current two-way systems include the usual acknowledgment functions in addition to a remote-start function that allows an operator to start the engine from a distance.
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 key fob 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 (very 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 costs by sharing and consolidating circuit functions.
REFERENCES
- For more information on the Federal Communications Commission and RF devices, go to the website at www.access.gpo.gov/nara/cfr/waisidx_01/47cfr15_01.html.
- Application Note 1017, Maxim Integrated Products, Sunnyvale, CA, www.maxim-ic.com.
- Application Note 1830, Maxim Integrated Products, Sunnyvale, CA, www.maxim-ic.com.