Modulation and Data Transfer February 21, 2002
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
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.
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.
Reader (or Interrogator) • modulates
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
Amplitude Shift Keying - ASK • Sends digital data across analog carrier by changing the amplitude of a carrier signal in time with data • video
Frequency Shift Keying - ASK • Sends digital data across analog carrier by changing the frequency of a carrier signal in time with data • video
Encoding • How the tag and reader will interpret the analog carrier to represent digital data. Think Morse code…
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
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
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
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
1 of 4 • How many slots per frame? • How many bits transmitted per frame?