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Understanding Alarm Sensor RFI Immunity

After reading about RFI immunity features in sensor specifications, I understand these designs are resistant to rouge wireless signals but still don’t understand how these signals would impact on an alarm sensor.

A: If powerful radio waves propagate onto electronic circuits they may interfere with normal operation and, in the case of alarm sensors, the impact of this interference might be false alarms caused when RFI energy throws off the signals of a sensor’s electronic circuits.

Many sensors have protection in the 10MHz to 1000MHz range, with sensors able to handle the impact of RF energy at 30V/m (volts per metre) generally accepted to be RFI immune. In terms of overall understanding, consider that the 10MHz to 1000MHz range represents the mid to upper end of the radio frequency band and it’s in this area that most RFI related problems occur.

Also important to consider is the nature of this energy itself. In free space, all electromagnetic waves, regardless of their speed of vibration, move at just under 300,000km per hour and they scarcely slow down as they pass through air, water and solids. Unconstrained, these waves can’t be observed and it’s only when they encounter objects that there are noticeable effects.

When a stray electromagnetic wave contacts a copper wire, a voltage is induced into that wire – the stronger the wave, the higher the voltage generated. It’s this abnormal voltage, and the electrical signal it generates, that interferes with operation of electronic circuits. This voltage is generated by passing radio waves in a copper cable greater than 1m in length. V/m is the standard unit for measuring the strength of an induced electrical field.

Most manufacturers of quality passive infrared and dual technology sensors use an RF test chamber (TEM Cell) to bombard their boards with energy. Use of the TEM Cell to challenge sensor boards allows sensor design and tuning to be adjusted under workshop conditions, while boards are subjected to the fiercest RF exposure they’re ever likely to face.

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