An Implementation Method for 100% ASK Modulation Applied to NFC Tags

Abstract

Passive near field communication (NFC) tags rely on the carrier-provided clock for operation. They can receive 100% amplitude shift keying (ASK)-modulated information but are unable to respond using 100% ASK modulation. This limitation restricts the tag’s resistance to interference and its communication range. This article proposes a design approach that enables passive NFC tags to employ 100% ASK modulation (termed “strong modulation” in this article, while non-100% ASK modulation is referred to as “typical modulation”) for response. Addressing the critical issue where the tag’s clock is lost due to the antenna carrier being turned off during the low signal bits of the modulation signal, preventing the tag from continuing to function, this article introduces a high-precision recovery clock circuit as a solution. The recovery clock circuit consists of a digitally controlled oscillator (DCO) circuit composed of 12 sets of current mirrors and a logic circuit DCO calibrator. The design feasibility was validated through the postlayout parasitic extraction and the AMS mixed-signal simulation in Cadence Virtuoso, ensuring correct communication between the tag and the reader. By implementing the tag’s strong modulation response, the anti-interference capability of the tag’s returned signal can be significantly enhanced, effectively reducing the difficulty of demodulation at the receiving end and improving the tag’s poor long-distance communication capabilities. Comparatively, the minimum antenna coupling coefficient k required for response under strong modulation is only 40.91% of that needed for typical modulation, enabling the tag to operate in weaker electromagnetic fields and exhibit better long-distance communication capabilities.