electronics Article Design of Multi Standard Near Field Communication Outphasing Transmitter with Modulation Wave Shaping Žiga Korošak 1, Nejc Suhadolnik 1 and Anton Pleteršek 2,* 1 STMicroelectronics d.o.o., Tehnološki Park 21, 1000 Ljubljana, Slovenia; [email protected] (Ž.K.); [email protected] (N.S.) 2 Faculty of Electrical Engineering, University of Ljubljana, Tržaška Cesta 25, 1000 Ljubljana, Slovenia * Correspondence: [email protected]; Tel.: +386-41-961-342 Abstract: The aim of this work is to tackle the problem of modulation wave shaping in the field of near field communication (NFC) radio frequency identification (RFID). For this purpose, a high- efficiency transmitter circuit was developed to comply with the strict requirements of the newest EMVCo and NFC Forum specifications for pulse shapes. The proposed circuit uses an outphasing modulator that is based on a digital-to-time converter (DTC). The DTC based outphasing modulator supports amplitude shift keying (ASK) modulation, operates at four times the 13.56 MHz carrier frequency and is made fully differential in order to remove the parasitic phase modulation compo- nents. The accompanying transmitter logic includes lookup tables with programmable modulation pulse wave shapes. The modulator solution uses a 64-cell tapped current controlled fully differential delay locked loop (DLL), which produces a 360◦ delay at 54.24 MHz, and a glitch-free multiplexor to select the individual taps. The outphased output from the modulator is mixed to create an RF pulse width modulated (PWM) output, which drives the antenna. Additionally, this implementation is fully compatible with D-class amplifiers enabling high efficiency. A test circuit of the proposed differential multi-standard reader’s transmitter was simulated in 40 nm CMOS technology. Stricter Citation: Korošak, Ž.; Suhadolnik, pulse shape requirements were easily satisfied, while achieving an output linearity of 0.2 bits and N.; Pleteršek, A. Design of Multi maximum power consumption under 7.5 mW. Standard Near Field Communication Outphasing Transmitter with Keywords: RFID; EMVCO; NFC; outphasing; PWM; DLL; TX wave shaping Modulation Wave Shaping. Electronics 2021, 10, 188. https://doi.org/10.3390/electronics 10020188 1. Introduction Received: 17 December 2020 Radio frequency identification (RFID) has gained prominence in recent years as one of Accepted: 11 January 2021 the dominant technologies in banking [1,2], access control [3], ticketing [4,5], tracking [6] Published: 15 January 2021 and in the Internet of things [7]. The technology can broadly be split into devices that operate in the following frequency bands: low frequency (LF), high frequency (HF) and Publisher’s Note: MDPI stays neutral ultra-high frequency (UHF). HF RFID devices that operate at 13.56 MHz are also called near with regard to jurisdictional claims in field communication (NFC) devices. These come in two categories. The first category is published maps and institutional affil- readers or proximity coupling devices (PCD), which are larger, more complex devices that iations. act as masters in the communication. They transmit a magnetic field that powers the tags or proximity inductively coupled cards (PICC), which are the second category of devices. They are usually simpler devices that can be entirely powered from the reader. They act as slaves in the communication and respond to reader’s commands by modulating reader’s Copyright: © 2021 by the authors. magnetic field. Licensee MDPI, Basel, Switzerland. Several standards exist in the field. In the HF frequency range, the most promi- This article is an open access article nent standards are ISO/IEC 14443 A/B, ISO/IEC 15693 and JIS: X6319-4/FeliCa [8–10]. distributed under the terms and A comparison between standards is shown in Table1. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Electronics 2021, 10, 188. https://doi.org/10.3390/electronics10020188 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 188 2 of 17 Electronics 2021, 10, x FOR PEER REVIEW 2 of 17 Table 1. Standards comparison. ASK stands for amplitude shift keying, OOK stands for on–off keying, NRZ-L stands for Table 1. Standards comparison. ASK stands for amplitude shift keying, OOK stands for on–off keying, NRZ-L stands for non-return to zero level, BPSK stands for binary phase shift keying. non-return to zero level, BPSK stands for binary phase shift keying. Standard ISO/IEC 15693 JIS: X6319-4/FeliCa ISO/IEC 14443 A ISO/IEC 14443 B Standard ISO/IEC 15693 JIS: X6319-4/FeliCa ISO/IEC 14443 A ISO/IEC 14443 B CarrierCarrier frequency 13.56 13.56 MHz ± ± 7 kHzkHz 13.56 MHzMHz± ±50 50 ppm ppm 13.56 MHzMHz± ±7 7 kHz kHz SubcarrierSubcarrier frequency 423.75/484.28 423.75/484.28 kHzkHz Not usedused 847.5 847.5 kHz, kHz, 1.695 1.695 MHz,3.39 3.39 MHz, MHz, 6.78 6.78 MHz MHz 106106 kbps, kbps, 212 212 kbps, kbps, 424 424 kbps, kbps, 848 kbps, 848 kbps, 1.7 Mbps, 1.7 DataData Rate 6.67 6.67 kbps, kbps, 26.69 kbpskbps 212 212 kbps, 424424 kbps kbps Mbps,3.4 3.4 Mbps, Mbps, 6.8 Mbps 6.8 Mbps ASKASK Modulation Modulation indexindex 10%,10%, 100% 8–14%8–14% 8–100% 8–14% 8–14% (PCD(PCD to PICC)PICC) DataData coding coding (PCD(PCD to to Modified Miller, PulsePulse position position codingcoding Manchester Modified Miller, NRZ-L NRZ-LNRZ-L PICC)PICC) NRZ-L ModulationModulation type (PICC(PICC OOKOOK OOKOOK OOK, BPSKBPSK BPSK BPSK toto PCD) Data Coding (PICC to Manchester, Data Coding (PICC to Manchester Manchester Manchester, NRZ-L PCD) Manchester Manchester NRZ-L NRZ-L PCD) NRZ-L InIn addition to the standards there also exist specificationsspecifications released by organizations and industry industry consortia. consortia. The The main main organizations organizations are are EMVCo EMVCo [11], [11 which], which is an is organization an organiza- responsibletion responsible for payment for payment cards, cards, and NFC and NFCForum Forum [12], [which12], which is a technology is a technology consortium consor- workingtium working to further to further the use the of use HF ofRFID HF RFIDtechnology, technology, branded branded under under their own their name own nameNFC. BothNFC. organizations Both organizations release release their own their versions own versions of specifications, of specifications, which which include include previously previ- mentionedously mentioned standards standards but also but add also their add theirown fu ownnctionality, functionality, certification certification procedures procedures and requirements.and requirements. The recent recent version version of of EMVCo EMVCo specification specification Level Level 1 Specifications 1 Specifications of Payment of Payment Systems Sys- Contactlesstems Contactless Interface Interface Specification Specification Version Version 3.0 [13] 3.0 and [13 Contactless] and Contactless Terminal Terminal Level 1 LevelType Approval1 Type Approval Proximity Proximity Coupling Coupling Devices Devices (PCD) (PCD)Analogue Analogue Test Bench Test Bench and Test and TestCase Case Re- quirementsRequirements version version 3.0a 3.0a [14], [14 as], well as well as newer as newer versions versions of the of specification, the specification, introduce introduce more stringentmore stringent requirements requirements for modulation for modulation pulse pulseshapes. shapes. Similarly, Similarly, the recent the recent version version of NFC of ForumNFC Forum Analog Analog Technical Technical Specification Specification Version Version 2.1 [15] 2.1 [and15] andNFC NFC Forum Forum Test Test Cases Cases for Analogfor Analog Version Version 2.1 2.1[16] [ 16also] also specify specify stricter stricter requirements requirements for for pulse pulse shapes. shapes. Examples Examples of generalof general on–off on–off keying keying (OOK) (OOK) and and amplitude amplitude shift shift keying keying (ASK) (ASK) modulation modulation pulses pulses are displayed in Figure1. Each of the specifications has its own definition of the modulation displayed in Figure 1. Each of the specifications has its own definition of the modulation pulse wave shape. Overshoot, undershoot and ringing level are highlighted in the figure. pulse wave shape. Overshoot, undershoot and ringing level are highlighted in the figure. Modulation pulses in NFC are measured by the spy coil, which is present in the RF field Modulation pulses in NFC are measured by the spy coil, which is present in the RF field alongside the PCD and proximity inductively coupled cards (PICC) coils. The size and alongside the PCD and proximity inductively coupled cards (PICC) coils. The size and shape of the spy coil is specified by each of the specifications. shape of the spy coil is specified by each of the specifications. Figure 1. Visualization of general purpose OOK and ASK modulation pulses. Overshoot, undershoot and ringing phe- Figure 1. Visualization of general purpose OOK and ASK modulation pulses. Overshoot, undershoot and ringing nomena are highlighted on both types of pulses. phenomena are highlighted on both types of pulses. Electronics 2021, 10, x FOR PEER REVIEW 3 of 17 Modulation pulse wave shapes created by the modulator are distorted by the trans- Electronics 2021, 10, 188mission path, which includes an electromagnetic compatibility filter (EMC), matching cir- 3 of 17 cuit, antenna coil and proximity coupled devices loading the antenna. Due to magnetic coupling between the PICC and PCD, the quality and resonance frequency of the antenna coil resonance circuit are affected by the antenna load. The shape of the pulses is thus changed with theModulation distance and pulse the wavenumber shapes of PICCs created present. by the The modulator most relevant are distorted require- by the transmis- ment affectingsion the path, reader’s which transmitter includes an is electromagnetic the overshoot compatibilityand undershoot filter requirement, (EMC), matching circuit, which is displayedantenna in coil Figure and 2. proximity The x axis coupled in the devicesfigure represents loading the the antenna.