SCBA020A October   1999  – March 2020 TMS3705

 

  1.   Integrated TIRIS™ RF Module TMS3705 Introduction to Low-Frequency Reader
    1.     Trademarks
    2. 1 Document Overview
    3. 2 Introduction
    4. 3 Key Features of the TMS3705
      1. 3.1 PLL for the System Clock
      2. 3.2 Ceramic Resonator for Reference Oscillator
      3. 3.3 Full-Bridge Driver Stage
      4. 3.4 Fully Integrated Bandpass Amplifier
      5. 3.5 Multifunctional 2-Wire Interface
    5. 4 Sequence Control
      1. 4.1 Initialization
    6. 5 Circuit Description of the Sample RF Module
      1. 5.1 Oscillator System
      2. 5.2 Antenna Circuit
      3. 5.3 Pre-Amplifier
      4. 5.4 Interface Circuit to the Control Module
      5. 5.5 Antenna Coil
    7. 6 Regulatory Notices and Ordering
      1. 6.1 Regulatory Notices
      2. 6.2 Ordering
  2.   Revision History

5.2 Antenna Circuit

The antenna circuit is formed by the resonance capacitor C1 and the antenna coil that is connected to the connector J1. The antenna circuit is driven by the push-pull output stages at the terminals ANT1 and ANT2, which can be seen as fast digital switches. These produce harmonic interference. To minimize this interference, each driver stage is fed to the antenna circuit through a low-pass RC filter (R3/C3 and R4/C2). These filters are particularly helpful if the antenna coil and the RF module are not combined. In such a case, the line to the antenna coil could operate as antenna for high-frequency interference.

Because resistors R3 and R4 are inline with the antenna circuit, they reduce the quality factor of the resonance circuit so that the unloaded quality factor of the coil must be higher to achieve the same reading performance. Using additional resistors inline with antenna coil wound with copper wire can stabilize the quality factor of the antenna circuit during temperature fluctuations.