CA2320670A1 - Apparatus for interrogating magnetic tags - Google Patents

Abstract

Apparatus for interrogating a magnetically coded tag, which apparatus is characterised by: a) means for generating a magnetic field, the magnetic field comprising a first region at which the component of the magnetic field resolved in a first direction is zero, and being characterised in that within regions contiguous with the first region the component of the magnetic field resolved in the first direction is sufficient to saturate the, or a part of the, tag; (b) an electrical circuit which comprises a signal source at frequency 2f and output means for providing an electrical output, the source being coupled to the output means by (i) means for halving the frequency of the source signal; (ii) first filter means arranged substantially to reject signal at frequency 2f; (iii) tuned circuit means tuned to energy at frequency f and including a transmitter coil adapted to radiate energy towards a magnetically coded tag positioned so as to be influenced by said radiated energy and a receiver coil (which may be the same coil as the transmitter coil) for receiving energy radiated by the tag in response to the received energy; and (iv) second filter means arranged substantially to reject energy at frequency f and to pass energy at frequency 2f, and wherein the output means is arranged to provide an output which is a function of the amplitude of the output of the second filter means and the phase difference between the source and the output of the second filter means; wherein the magnetic field generating means is associated with said transmitter and receiver coils such that, in use, a magnetically coded tag moving through the first region will also interact with the transmitter and receiver coils.