| RKE = REMOTE KEYLESS ENTRY
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 - the lower level is usually near zero, producing complete on-off keying (OOK).
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.
Modern receiver and transmitter chips integrate the PLL circuitry so one need only connect a suitable crystal between two terminals on the chip.
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.
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.
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.
LOW NOISE AMP
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 50#937; 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 -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.