Passive RFID Basics.pdf

Key Components:
- Reader (Interrogator): A microcontroller-based unit with an output coil, peak detector hardware, comparators, and firmware designed to transmit energy, read data, and detect backscatter modulation from tags.
- Tag: An RFID device consisting of a silicon memory chip, an antenna coil, a rectification bridge, and other RF front-end components. It has no internal power source and derives energy from the reader's field.
- Carrier Signal: A time-varying electromagnetic radio frequency (RF) wave transmitted by the reader to energize the tag and communicate with it. Frequencies used include 125 kHz, 13.56 MHz, and 2.45 GHz.

Operation and Communication:

- Reader-Tag Handshake: The process begins with the reader generating an RF carrier. When a tag enters the field, it divides down the carrier, clocks data from its memory, and modulates the coil to transmit information back to the reader.
- Backscatter Modulation: Tags communicate by shunting their coils, causing momentary fluctuations in the reader's carrier amplitude. The reader peak-detects these changes to interpret the data.
- Data Encoding: Before transmission, data is encoded using various algorithms (NRZ, Differential Biphase, Manchester) for error correction, synchronization, and bandwidth optimization.
- Data Modulation: While backscatter modulation provides the carrier amplitude variations, additional techniques like Direct Modulation and FSK are used to encode 1s and 0s onto the carrier.

Encoding Methods:
- NRZ (Non-Return to Zero): Direct data transfer with no encoding; 1s and 0s are represented by logic levels.
- Differential Biphase: Ensures a transition at every clock edge, aiding synchronization and providing basic error correction.
- Biphase_L (Manchester): Similar to Differential Biphase but with less stringent timing requirements.

Modulation Techniques:
- Direct Modulation: Amplitude modulation of the carrier to represent data; offers high data rates but low noise immunity.
- FSK (Frequency Shift Keying): Modulates the frequency of the carrier to encode data, providing better noise immunity.