Maxim Integrated Products (Sunnyvale, CA) is one of several manufacturers producing special-purpose integrated circuits (ICs) for the RKE market. For the key fob, it offers the world's smallest transmitter of its type: the 300-to-450-MHz MAX1472, which is supplied in a tiny 8-pin SOT23 package only 3 × 3 mm. Its supply voltage range (+2.1 to +3.6 VDC) enables the device to operate from a single lithium cell, drawing only 5 nA of supply current in standby mode.
During transmission of Manchester-encoded data, the MAX1472 supports data rates to 100 kb/s and draws between 3.0 and 5.5 mA of supply current while delivering −10 to +10 dBm of power to a 50-(omega) load. Its crystal-based PLL source 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 220 µs startup time following an enable signal.
The MAX1473 300-to-450-MHz superheterodyne amplitude-shift-keying (ASK) receiver, on the other hand, is ideal for use in vehicle RKE receivers. It offers −114 dBm sensitivity and 50 dB RF-image rejection in its fully differential internal mixer. The IC is optimized for either 315- or 433-MHz operation and operates on +3.3 or +5 VDC. The receiver includes a low-noise amplifier (LNA), crystal-based PLL LO, and a 10.7-MHz IF limiting amplifier with received-signal-strength-indicator (RSSI) circuitry. An internal data filter and data slicer provide the digital data output. As an alternative, the MAX1470 receiver is similar to the MAX1473 but optimized only for 315-MHz operation. It runs on a supply voltage of +3.0 to +3.6 VDC.