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RF/Microwave??

Rolling data codes and fast oscillator trigger remote keyless entry

Posted: 16 Jul 2004 ?? ?Print Version ?Bookmark and Share

Keywords:rf? data transmission? superheterodyne receiver? rke? telematic?

A common application for simple RF data transmission is the remote keyless entry (RKE) system found in most vehicles today. Those systems control the locking and unlocking of vehicle doors, the opening of trunks and security alarms. Features planned for the future include the location and starting of remote vehicles.

The operation of an RKE system is straightforward. It consists of a key-fob transmitter and a vehicle-based receiver. The frequency of operation is usually 300MHz to 450MHz, but some of the systems in Europe are considering a frequency allocation of 868MHz in the ISM band. Communications are simplex, meaning that data flows only from transmitter to receiver. Of the many reasons that justify this architecture, the most cited are low cost and extended battery life for the key fob.

An RKE can be built on the MAX1470, a low-power CMOS superheterodyne receiver. It uses amplitude-shift-keyed (ASK) data in the 315MHz band. With few required external components and a low-current power-down mode, it supports many cost- and power-sensitive applications in the automotive and consumer markets. The chip consists of a 315MHz low-noise amplifier, an image rejection mixer, a 315MHz PLL, a 10.7MHz IF limiting amplifier stage and analog baseband data-recovery circuitry.

To initiate action, the user depresses a button on the key fob, thereby waking an internal MCU that immediately outputs a data stream into the RF transmitter. The data stream includes a data preamble, the actual command, a rolling code for vehicle-to-vehicle security to ensure that your key fob does not unlock another vehicle.

Complete data packets are usually transmitted at a rate between 2.4kHz and 20kHz, with RF modulation in the form of ASK or on/off keying. These modulation schemes minimize cost and extend battery life for the key fob.

For the key-fob transmitter, long battery life minimizes battery replacement by the user. The ideal transmitter battery would last the life of the vehicle, and such a battery is possible today, but you probably wouldn't want to carry the resulting large key fob in your pocket. A small key fob is more convenient, but not if you must change batteries every two months. Most of today's products fall in the middle, offering a reasonable key fob size with battery life in the 2-to-5-year range.

Equally important is battery life for the receiver. The receiver battery must always be on, because a user can issue commands at any time. The RKE receiver is powered by the vehicle's battery. If power consumption in the receiver is too high, the battery won't have enough power to start the vehicle!

Vehicle manufacturers therefore size their batteries accordingly. For an RKE system, battery size is directly proportional to the product of power consumed by the receiver times the number of days it is powered. So, be forewarned if your vehicle is stored for longer than about 30 days. Back to the title of this article - how does the fast start-up of an oscillator in the superheterodyne receiver affect battery life?

To simplify calculations, we use some mid-range values. Recalling the discussion of data packets and transmission speeds, assume a 100bit data packet and 10kHz data rate. To save power in the receiver, we "time-slice" its operation by leaving it just long enough to determine if there is a valid transmission. This "on time" value usually produces a duty cycle of approximately 10 percent.

Because the receiver is time sliced, we need to provide additional transmissions to make sure that the receiver detects one of the requested actions. Normally, the key-fob transmission is repeated three more times, for a total of four transmissions. Total transmission time for the key fob is four times 10ms, or 40ms. For the receiver to act, it must completely decode at least one of the 100bit transmissions.

To catch at least one full transmission, we must poll the receiver to determine if valid data is present. A given 40ms transmit packet may not be repeated, so one must poll the receiver often enough to catch at least one complete 10ms transmission. That requirement imposes a maximum time between receiver polls of 30ms.

But that interval may be too infrequent, allowing a command to be missed. The system timing may be a bit off, or there may be interference or other noise that corrupts the data. To be conservative, the system should be set up to catch at least two complete transmissions. Hence, we set the receiver time-slice circuit to 20ms. Every 20ms, the receiver wakes up and attempts to decode the transmission. If valid data is present, the receiver decodes it; otherwise it goes back to sleep for another 20ms.

To detect valid data, the receiver needs 0.75ms of time to decode the information. That condition determines whether the transmitter is sending data at a frequency and format in which we are interested. Thus, the receiver needs to wake up for 0.75ms or so every 20ms. Unfortunately, only a perfect receiver can accomplish that feat.

Normal superheterodyne receivers can start and stabilize within 2ms to 5ms. We assume 2.25ms for our discussion. Adding 0.75ms for the data decode, they need a 3ms "on time" every 20ms to detect the key-fob transmission. The MAX1470 superheterodyne receiver, on the other hand, includes a fast-startup oscillator that minimizes turn-on time by maintaining vibration in the crystal, thereby reducing turn-on times from the normal 2.25ms to a fast 0.25ms. Adding the 0.25ms turn-on time to the 0.75ms data decode time, we need only 1ms every 20ms to detect a key-fob transmission. Thus, the MAX1470 saves power by performing the same measurement function in one third the time.

Most high-performance superheterodyne receivers draw 5mA at 5V when operating. MAX1470 receivers offer their best receiver sensitivity while drawing 5ma from a supply voltage of only 3.3V. Power saved at the lower supply voltage is substantial: normal superheterodyne receivers require 25mW; the MAX1470 requires 16.5mW. Adding the time function for each 20ms poll cycle produces an energy requirement of 25mW*3ms = 75?J for the normal superheterodyne receiver vs. 16.5mW*1ms = 16.5?J for the MAX1470. The energy savings gained with a fast-wakeup receiver can therefore extend battery life by a factor of four or five.

- Ken Lenk

Maxim Integrated Products Inc.





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