| 摘要 |
A loop (1) or strip (2) of conducting foil is attached to an object to be protected. The strip or loop is connected to one end of a high inductance coil (3), the other end being grounded. Another coil (5) of high inductance is wound co-axially of coil (3). Coil (5) is connected via a diode (6) to an oscillator (7) operating at about 80 KHz. The oscillator output is coupled to a means which produces a signal which is frequency modulated (FM) by proximity of e.g. an intruder to the protected object. Intruder induced frequency variations tend to fall within a fixed frequency band, and thus can be distinguished from variations induced by other causes. The intruder induced variations are detected by using a band-pass frequency discriminating amplifier (10) and a comparator (11) which triggers (14) an alarm (13). The high inductance coils and use of FM reduce interference. |
