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Modulation and Data Transfer

This article explores the principles of modulation and data transfer in RFID (Radio Frequency Identification) systems, specifically focusing on the communication between RFID readers and tags. It describes how modulation techniques like Amplitude Shift Keying (ASK) and Frequency Shift Keying (FSK) are employed to transmit digital data across analog carrier signals. The document also highlights key standards such as ISO 15693 and EPC GEN2. Readers will gain insights into encoding methods, including Manchester coding and pulse position modulation, which enable effective data transmission in RFID technology.

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Modulation and Data Transfer

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  1. Modulation and Data Transfer February 21, 2002

  2. References • http://www.technovelgy.com/ct/Technology-Article.asp?ArtNum=2 • http://electronics.howstuffworks.com/smart-label1.htm • http://www.answers.com/topic/iso-15693 • http://focus.ti.com/docs/prod/folders/print/ri-i02-114a-01.html#technicaldocuments

  3. Reader • The scanning antenna transmits radio-frequency signals in a relatively short range. • It provides a means of communicating with the transponder or the RFID tag. • For passive tags it provides the RFID tag with the energy to communicate.

  4. Modulation (from Wikipedia) • … the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Normally a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.

  5. Reader (or Interrogator) • modulates

  6. ISO 15693 Smart Tags • Inductively coupled • EPC GEN2 standard uses ISO 15693 IDs • Uses ASK from Reader to Tag • Uses ASK or FSK from Tag to Reader • Two encoding methods: 1 of 4 and 1 of 256

  7. Amplitude Shift Keying - ASK • Sends digital data across analog carrier by changing the amplitude of a carrier signal in time with data • video

  8. Frequency Shift Keying - ASK • Sends digital data across analog carrier by changing the frequency of a carrier signal in time with data • video

  9. Encoding • How the tag and reader will interpret the analog carrier to represent digital data. Think Morse code…

  10. 1 of 256

  11. ASK 0 8 pulses of 423,75 kHz unmodulated time of 18,88 µs (256/ fc 1 unmodulated time of 18,88 µs (256/ fc 8 pulses of 423,75 kHz FSK 0 8 pulses of 423,75 kHz 9 pulses of 484,28 kHz 1 9 pulses of 484,28 kHz 8 pulses of 423,75 kHz Data Transmission

  12. ASK SOF an unmodulated time of 56,64 µs (768/ fc), 24 pulses of 423,75 kHz a logic 1 Data encoded in using Manchester code EOF a logic 0 24 pulses of 423,75 kHz an unmodulated time of 56,64 µs FSK SOF 27 pulses of 484,28 kHz 24 pulses of 423,75 kHz a logic 1 Data encoded in using Manchester code EOF A logic 0 24 pulses of 423,75 kHz 27 pulses of 484,28 kHz Data Transmission to Reader

  13. Manchester Code

  14. Data Transmission to Card • Data encoded in using 1 of 4 or 1 of 256 pulse position modulation (PPM) • PPM is signal modulation in which M message bits are encoded by transmitting a single pulse in one of 2M possible time-shifts. This is repeated every T seconds • Transmitted bit rate is M/T bits per second

  15. 1 of 256 • Values from 0-255 are encoded by the timing of a pulse • 512 timing slots per frame where a pulse may occur • Value, v, is encoded by 2*v + 1 = slot location • Example: value 24 would be in slot location 49

  16. 1 of 4 • How many slots per frame? • How many bits transmitted per frame?

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