| 摘要 |
A tag reader (10, fig.1) in a near-field communication (NFC) system must demodulate a signal, for example a load-modulated signal, in which the modulation created by a passive device such as an RFID tag (50, fig.1) may be small in comparison with the high-level signal produced by the tag reader. The invention provides for demodulation by first applying upper and lower clip levels 14a,b to a signal received in the reader antenna 12. The clipping circuit 14 is arranged to provide a clipped signal (solid line, fig.3) corresponding to a combination of a first portion of the signal below the lower clip level and a second portion of the signal above the upper clip level. After clipping, the modulated part of the signal corresponds to a greater portion of the overall signal, and thus the requirements placed on an analogue-to-digital converter for the clipped signal are reduced. Prior to reception of a load-modulated signal, the clip levels may be set in a feed-forward manner based on preset values or on a detected change in the overall signal; the clip levels may subsequently be set by feedback. |
