About the Author:
Dr. Michael Gebhart owns the company RFID-Systems Gebhart GmbH and works as a consultant and in new technology development. He graduated from Graz University of Technology in Electrical Engineering and was with Philips/NXP for twelve years, pioneering NFC chip technology development from birth to maturity. Afterwards, he spent six years in sensor and system development. Michael is guest lecturer for the RFID Systems course at Graz UT and the Integrated Circuit Design course at University of Applied Sciences Upper Austria. He has more than sixty scientific publications and several patents, and he is a co-author of the well-known RFID Handbook by Klaus Finkenzeller.
This article explores Near Field Communication (NFC) at 13.56 MHz, highlighting its applications in short-range wireless communication and contactless power transmission for e-payment and access control. Key design considerations for addressing electromagnetic compatibility issues are also discussed.
Contactless Near Field Communication (NFC) is quite a different technology compared to classical short-range radios. And this is not only due to an operating carrier frequency that is not in the crowded UHF (ultra high frequency) band but rather in the HF (high frequency) band, at 13.56 MHz. Originally this was an inspiration for the name NFC, as this technology operates in the physical near-field range. Contactless transmission technologies are wireless but are intended to operate only over distances of a few centimeters. This range limitation is imposed by the nature of the system and cannot be significantly extended. In addition to communication at rates of a few kilobits up to a few megabits per second, the system can provide contactless power transmission.
The combination of contactless energy and data transmission makes NFC technology suitable for battery-less system-on-chip applications, which today have a large market in e-payment (contactless bank and credit cards), e-government (passports, health cards, driving licenses in some countries), and access control. But beyond this, the technology is useful for applications in which it is combined with sensors and actuators, e.g., NFC door locks. NFC actually has become an overarching standard for a number of previously existing contactless communication standards in the 13.56 MHz band for industrial, scientific, and medical (ISM) use; thus, it consists of several communication protocols, data rates, and power levels from which the most suitable can be selected for a given application.
Since NFC is implemented in most modern mobile phones, numerous people have access to this technology, which makes it quite attractive. However, it also requires that developers understand certain issues related to EMC. Aiming to support engineers with new designs, we will discuss these special aspects in this series of articles dedicated to a practical understanding of NFC.
If you switch on the NFC function in your mobile phone, the integrated NFC interface will initially listen to the only available channel, at 13.56 MHz. If there is already a reader transmitting a continuous-wave (CW) carrier signal—e.g., if the phone is close to an active payment terminal reader—the NFC interface is allowed to become active only in the so-called Card Mode, meaning it acts as a transponder. This “listen before talk“ rule prevents two NFC devices from interfering with each other. The criterion for a quiet channel is an alternating magnetic field (or H-field) amplitude below 0.185 A/m (RMS) over a few milliseconds, as detected by the NFC interface in the mobile phone. If the channel is quiet, and the function is supported, the integrated NFC interface can switch on in Reader Mode.
Fig. 1. Polling Loop: Initialization of NFC communication as implemented in a mobile phone.
In Reader Mode, the interface will activate and transmit a continuous-wave carrier for a few milliseconds, which is sufficient for a battery-less NFC transponder to boot and prepare for reception of reader commands. Then the reader will attempt to find out if there are any NFC transponders (or interfaces operating in Card Mode) in its communication range.
As there are several protocols and data rates available in the NFC system, the next task is to attempt this with all these protocols. The polling loop is how an integrated NFC reader initiates its operation. The following sequence is applied:
The most important protocols are NFC-A, NFC-B (both previously defined in ISO/IEC 14443), NFC-F (previously defined as FeliCa in Japan), and ISO/IEC 15693. Figure 1 depicts reader commands in these protocols. Each protocol has an initialization sequence and starts at a defined base data rate. Also, the timing is defined for each protocol. The NFC reader asks for transponders supporting one protocol after another. NFC transponders in the operating range respond, providing their Unique Identification (UID) number. Thus, after this polling loop, the reader will know all NFC transponders that are in reach, and it can then initiate individual communication.
The implementation of this polling loop (meaning which protocol is requested first, or how often it is requested during the loop) can vary according to geographic region or the phone’s manufacturer. There is also a trade-off between response time and power consumption, which must be balanced, since charging interval will be influenced by a device’s battery capacity.
Observing a system’s behavior is helpful when developing NFC applications, and we will demonstrate a way to easily accomplish this.
An oscilloscope is the typical instrument for visualizing signals in the time domain; oscilloscopes are available as USB devices at an affordable cost. An oscilloscope should have a sampling rate of 1 GS/s, or at the very least 100 MS/s, in order to observe the 13.56 MHz carrier and analyze communication. Voltage is usually measured with the probe tip touching a conducting point and referenced to ground (GND) potential. Typically, the connection to a GND node is made by means of a short, flexible cable and a crocodile clip. Many oscilloscope probes offer bandwidth that is more than sufficient for NFC signal measurements.
Fig. 2. Oscilloscope probe GND loop acts as a simple H-field probe, to detect NFC signals.
An oscilloscope probe can also be used to detect and visualize the H-field RF NFC signal at the air interface between a reader (or mobile phone NFC interface in Reader Mode) and a transponder. Simply connect the GND clip to the tip, such that the GND cable creates a small loop, known as a “spy coil“ in the context of NFC. This method is effective for observing signals, but because the shape and the area of this loop are not well defined, voltage amplitudes cannot be precisely related to H-field strength. The principle here is found in the Maxwell–Faraday equation:
ui = - dφ dt (Eq. 1)
The induced voltage (ui) corresponds to the derivative of magnetic flux (ϕ) with respect to time (t).
This method, by the way, is also useful for testing any unwanted alternating H-field emission in a circuit. This can be done by simply moving the small coil loop slowly over operational circuitry. Emission sites can be identified by the signal amplitude on the oscilloscope, and also the signal frequency can be measured, which sometimes helps to identify the root causes of EMC issues.
Actually, this is just a very rough implementation of a professional H-field probe. Professional probes consist of a conductive loop but also have very well-defined, stable geometry that allows them to be calibrated and thus provide a voltage that accurately corresponds to H-field strength. This concept was implemented as a so-called calibration coil in the Proximity Card Test Setup, basically a predecessor and one of the protocol constituents of NFC.
Fig. 3: Calibration coil (above) from ISO/IEC 10373-6, and improvised connection to a probe tip.
The Proximity Test Setup calibration coil has a single turn loop antenna with the size of a typical card; it is a rectangular shape with rounded edges of 5 mm corner radius and 72 × 42 mm dimensions (approximately 3000 mm² area), with 0.5 mm track width and 35 µm copper layer thickness. Measuring the induced voltage with this coil allows us to calculate the H-field amplitude as well, in addition to representing the RF communication signals. We can modify the above formula to calculate the magnitude of the induced voltage amplitude Ui as a function of the magnetic field strength H (RMS).
|Ui| = (2πf)μ0HA (Eq. 2)
In this case, the frequency f is 13.56 MHz, the area A is approximately 3000 mm², and the magnetic field constant is 4π × 10-7 V·s / A·m. So we get 0.32 V (RMS) or 0.45 V (peak) from an H-field of 1 A/m (RMS).
It should be noted that there are a few assumptions when working with this method. The calibration coil can measure only the average H-field over its area (not any peak values at very small locations, which may appear if a reader loop antenna is very small), so we must speak of the equivalent homogenous H-field. But this is basically what a card with the same loop antenna size will see. Also, it measures the H-field that is perpendicular to the calibration coil plane, so the orientation from coil to H-field is important.
A critical aspect in measuring the voltage amplitude at the coil is maintaining the defined area, since the H-field is also present where a probe tip should connect. A flexible GND cable on the probe cannot be used, as the resulting conductive loop would add area. Instead, the GND of the probe should be connected a short distance from the tip connection. A practical way to implement this is to use bare wire of, for example, 0.5 mm diameter; 3 to 5 turns of this wire can be wound around the probe’s GND cylinder below the tip (this is shown in Fig. 3). The wire should not be too flexible, so that it can maintain shape. Be careful not to damage the probe tip when winding! One end of the wire can then be very close to the tip, and both the tip and the end of the wire should fit into a typical 2.54 mm female socket connector strip, which can be soldered to the termination of the calibration coil. Some probe manufacturers offer the same interface via special connectors for their probe tips.
With these practical means for measurement, you are equipped for initial inspections of NFC signals at the air interface; you can identify reader command protocols, analyze signal timing, and measure approximate H-field amplitude in the NFC system. Visualizing the results of an implementation can be especially helpful for software developers.