Quantitatively checking magnetised areas on object esp. security strip in banknote

摘要

The checking involves passing the object to be checked through an air gap in a magnetic field of a resonance circuit (1) a pulsed frequency being generated with a pulse width t1. The pulse commences at a specific time and ends when an output pulse n/f(osc) formed from the resonance circuit output voltage U(osc) ends. The pulse width serves to evaluate the change in frequency caused by the magnetised area. The output voltage U(osc) can be integrated over a number of counting periods and the pulse frequency f(vco) can be controlled to obtain a pulse width in accordance with the equation: t1 = n/f(osc) - 2m/f(vco) where t1 = pulse width; n = number of periods over which U(osc) is integrated; f(osc) = resonant circuit frequency; f(vco) = pulse frequency; m = pulses per counting period.