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.