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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING On the noise immunity of automobile radio locks. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms In the 90s of the last century in the UK, it often happened that the owner of the car could not use it, because the door lock did not respond to the radio signals sent by the key fob. The reason for the blockage was radio interference, the sources of which the media hastened to name the parasitic radiation of amateur radio stations operating in the 70 cm band (430 ... 440 MHz). Indeed, some of the frequencies allocated for radio locks (418 and 433,92 MHz) lie close to and even within this range, but the true cause of the accidents is not the poor quality of amateur transmitters, but poor selectivity and extremely poor noise immunity of radio lock receivers. Many of them, especially those installed by various service centers, are the simplest super-regenerators with a bandwidth of several megahertz. They are sensitive even to interference with frequencies far outside the amateur band. More expensive factory-installed superheterodyne radio lock receivers are also affected by interference. Many of them, having an intermediate frequency of 500 kHz, are not equipped with any filters to suppress the image reception channel. And it lies in close proximity to the operating frequency band of British amateur repeater transmitters (433,0 ... 433, 375 MHz). In some vehicles (for example, Vauxhall Vectra), a door with a locked radio lock can still be opened with a regular key, although an alarm will sound. After the key is inserted into the ignition lock and turned, the alarm will turn off and the immobilizer will allow the engine to be started. There are no special difficulties here if the driver knows what to do. But sometimes (for example, on Rover cars), a locked lock creates a lot more problems for the driver. To start the car, you need not only to remember the secret code, but also to know a rather complicated way to enter it. Some aftermarket immobilizers cannot be disabled at all if the radio lock is jammed. In 1996, the RAKE organization was created in the UK, which included representatives of all interested parties, including radio amateurs. Its long-term goal is to achieve the allocation of new frequencies for radio locks in interference-free bands. The improvement of locks operating at a frequency of 433,92 MHz is declared as a medium-term goal. A brochure has been released with recommended specifications for their receivers. The requirements that must be met for reliable operation near amateur radio stations (immunity to interference with a field strength of not more than 1 mV / m in the 433,921 MHz band and 100 mV / m outside it) turned out to be not too stringent, and already in 1997 it was the issue is said to be resolved. It proved more difficult to deal with interference from professional systems, which were assigned frequencies on a primary basis that lie close to those allocated for radio locks. There is a known case of mass blocking of locks with an operating frequency of 418 MHz in a parking lot near a supermarket, near which a base station of a TETRA digital trunk communication system with transmitters operating in the 420 ... 425 MHz band was installed. The antennas of similar stations and amateur repeaters in the UK are often installed on the same masts, and the first suspicion always falls on radio amateurs. The selectivity required for collaboration with professional radio systems, manufacturers of car radio locks, promised to achieve by the year 2000. In the summer and autumn of 1998, British motorists and radio amateurs tested the noise immunity of radio locks with an operating frequency of 433,92 MHz. Interference was created by a transmitter with a power of 32 mW, the antenna of which was located at a distance of 3 m from the driver's window. In accordance with the British standard, the transmitter signal was modeled in amplitude (80% depth) by a sinusoidal signal with a frequency of 1 kHz. The fact of blocking was recorded if the lock could not be opened, even by bringing the key fob close to the window. It turned out that the radio locks of only eight out of 19 checked cars of different models produced in 1997-1998. meet the requirements of RAKE 1997, the remaining 11 may well be blocked by a nearby 70 cm amateur radio station. Only the Nissan QX and Volvo V2000 radio locks exceeded the 70 RAKE requirements. The receiver of the first of them has the narrowest bandwidth tested, in the second - frequency, rather than amplitude keying of the carrier is used to transmit the code.
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