Patent ID: 6019208
Filing Date: 2000-02-01
Classification: G07D

Abstract:
A device for validating the authenticity of a document having an electrically conductive security thread extending across the document, said device comprisinga drive arrangement for moving the document in a lengthwise manner along a predetermined path of the device,a generator producing a high frequency time varying oscillator signal,an elongate oscillator electrode being electrically conductive and connected to said generator which applies said time varying oscillator signal thereto, said elongate oscillator electrode being positioned to extend generally across said path,a lead elongate measuring electrode being electrically conductive and positioned in front of said oscillator electrode and extending across said path,a trailing elongate measuring electrode being electrically conductive and positioned behind said oscillator electrode and extending across said path,a reference ground electrode,a signal processing arrangement connected to said measuring electrodes and receiving the outputs thereof and producing a measuring signal, said signal processing arrangement also receiving said time varying oscillator signal as a reference signal, said signal processing arrangement processing said measuring signal relative to said reference signal to detect a phase shift therebetween caused by a conductive security thread passing by said electrodes, and wherein said processing arrangement includes a synchronous detector which compares a differential signal derived from said measuring electrodes to said reference signal and detects said change in phase.