An Implementation Method for 100% ASK Modulation Applied to NFC Tags
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% true 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 long 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 postural parasitic extraction and the AMS mixed-signal simulation in Cadence Virtuoso, confirming correct communication between the tag and the reader. By implementing the tag’s strong modulation response method, the communication range was returned to the typical modulation range, effectively reducing the difficulty of increasing the load modulation depth and improving the tag’s poor long-distance communication capabilities. Compared with tags whose minimum antenna coupling coefficient k required for response is 0.15, the proposed tag has k = 0.02 as that needed for typical modulation, enabling the tag to operate in weaker electromagnetic fields and showing better long-distance communication capabilities.
Index Terms — Amplitude shift keying (ASK), digitally controlled oscillator (DCO), ISO/IEC14443-A, load modulation, near field communication (NFC) tag.
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