Active load modulation (ALM) and wider test coverage for NFC devices — phase drift immunity testing

Background: smaller antennas and the limits of passive load modulation

Most NFC applications follow ISO/IEC 14443 Type A and B, where a reader and an ID-1 sized transponder communicate through inductive coupling. The transponder draws power from the reader's field and returns data by switching a passive load connected to its antenna, modulating the power transfer. Because the response is synchronised to the reader's RF field, the reader decodes it easily.

As ISO/IEC 14443 Type A and B became the general-purpose contactless interface for consumer devices, antenna requirements shrank from Class 1, the ID-1 credit card format, to the much smaller Class 6, and smaller still — below Class 6 — in phones with metal unibodies. The smaller the antenna, the lower the coupling factor, which is the strength of the electromagnetic link. Energy transfer and operating distance fall with it until communication no longer works. The transponder's ability to answer is constrained in the same way.

What active load modulation is

If the antenna cannot be made any larger, the answer is to change how the transponder is powered. A battery-powered device can generate a signal with the same characteristics as passive load modulation and transmit it actively to the reader. If that signal is coherent with the reader's RF field, destructive interference produces synchronous modulation, and the reader receives the same amplitude-modulated signal it would from a conventional battery-less card.

The problem is phase drift

When ALM is implemented and tuned correctly, the reader cannot tell passive from active. When it is not fully under control, it causes interoperability problems. The main cause is the phase of the load modulation relative to the field, and the drift of that phase. ALM synchronises to an external clock — the reader field — typically with a PLL, but switches to free running while data is transmitted, so a long command can lose synchronisation. The internal clocks of reader and transponder also never match exactly, even within the ISO tolerance, so the response is asynchronous and the received signal drifts in phase.

Demodulated against the reader signal, an asynchronous response appears as a shift from the I signal to the Q signal, and at baseband as a change in modulation amplitude. In some cases the load modulation even looks like envelope modulation with zero amplitude.

This is not limited to NFC mobile devices using ALM. Passive cards also vary in initial phase from unit to unit — chip-on-module construction differs from a conventional card, for example — so sweeping the phase conditions also improves interoperability with passive cards.

The Quest PCD Phase Drift Immunity tool

PCD Phase Drift Immunity, part of the KEOLABS Quest environment, is a characterisation tool for testing how well a reader tolerates every combination of initial phase and phase drift a transponder might present. Results appear on a Shmoo plot designed for ALM, giving an overview of the whole test campaign while still allowing any specific condition to be examined in detail.

Setup

Five items are needed: ProxiLAB Quest as the contactless test station, Active ISO Reference PICC, a demodulation probe, an oscilloscope, and a PC to run Quest.

In ISO/IEC 10373-6, the Reference PICC is defined to test whether a device can generate a field strength between Hmin and Hmax, transmit a modulated signal to the transponder, receive load modulation at the minimum level, and tolerate EMD signals from the transponder. ISO/IEC 10373-6 Edition 4 defines a new reference card for phase drift immunity testing, the Active Reference PICC. The Active Reference PICC can only be used together with a capable test tool such as ProxiLAB that can generate the modulated signal.

Using it — three steps

Step 1: set the parameters. Choose the Tx coil (Active PICC or a plain ID-1 coil), the specification (ISO/IEC 14443 or EMVCo), and the phase drift type — several protocols and patterns are supported, including Type A conditions A1/A2 and A3/A4, Type B conditions B1/B2, and Type F at 212 and 424 kbit/s. Then set the start, stop and step for both phase drift and initial phase, the DUT timeout, the number of repetitions per measurement point, and whether digital and analogue traces are captured.

Step 2: run the measurement campaign. Every configured condition is tested to see whether the reader handles the transponder response correctly. Depending on the number of measurement points, the repetitions, whether traces are captured and how the device behaves, this takes anywhere from seconds to hours.

Step 3: analyse the results. Results are presented in three views: phase drift (Shmoo), I/Q, and constellation.

Reading the results

On the Shmoo plot, the horizontal axis is phase drift and the vertical axis is initial phase. Red is a 0% transaction success rate, amber is 1–99% where two or more repetitions were run, and green is 100%. The blue rectangle marks the ISO tolerance limit of ±30° phase drift. Double-click any cell to see the captured analogue waveform for that condition.

The I/Q view shows the in-phase and quadrature components of the PICC signal, demodulated against the reader field, over time. The constellation view is the same I/Q data in polar form, and gives visual proof that the configured phase drift conditions were actually applied during the test.

In summary

The limited space in a mobile device forces a very small antenna, and ALM is an effective answer to the communication problem that creates. But because the transponder cannot observe the reader's RF field, the frequencies lose synchronisation — phase drift — which can lead to reception and decoding errors. The Phase Drift Immunity tool lets you establish whether the reader conforms to the ISO limit, how much margin there is beyond that limit, and whether there are non-working regions inside it. Looking at the reader's behaviour as a whole, rather than testing only the limit values, is what gives confidence in the device.