near-field magnetic induction communication

Discussion in 'Wireless & RF Design' started by chryso, Mar 23, 2015.

  1. chryso

    Thread Starter New Member

    Jun 21, 2011
    Hello everybody,

    I’d like to ask you who are more experienced with ferrite antennas and LF transmissions for a help with the following problem. I have a circuit for near-field magnetic inductive communication consisting of a microcontroller generating a square-wave signal (carrier, 131 kHz). An amplitude modulation (on/off keying) is used for the communication. A schematic is given in the figure 1 (schematic_transceiver.png).
    The signal is fed into a non-inverting op amp U1A. The output from the op amp drives a series LC circuit, the inductor L1 being a ferrite antenna. Receiver is similar in that it is a parallel LC circuit whose output is fed to another op amp U1B. Both op amps have an asymetric power supply (Vdd = 1.8V and GND).
    The signal transmitted is a pulse of 128 cycles of the carrier frequency. A received signal is shown in the figure 2 (signal.png).
    The horizontal axis correspond to samples taken by an ADC. The vertical one is an arbitrary number corresponding to voltage (the higher the number on the axis the lower the voltage; GND = 5450, Vdd = 3500). The signal is rectified by the second op amp, therefore it is a half sine wave. I would expect to see an envelope of the received signal similar to the transmitted one (a square pulse) except for an exponential attack and decay of the envelope. However, as can be seen in the figure, there are other oscillations at roughly 1/12 of the carrier frequency, mainly at the beginning of the pulse. Could anybody provide an explanation of the fenomena and/or suggest what should I do to remove/decrease these dramatic changes in the amplitude of the envelope? Any suggestion is welcome.

    Thank you.
  2. skeptic

    Active Member

    Mar 7, 2010
    What is the sample rate of your ADC?
  3. chryso

    Thread Starter New Member

    Jun 21, 2011
    About 8 samples per carrier period, i.e. 1MHz