08-05-2012, 01:19 PM
High Level Hardware Design:
[attachment=21552]
Before we start with actual circuit design, it is neccessary to understand the principals behind the technology that this project has set out to harness; passive wireless power communications. Passive wireless power receiver work in such a way that they are actually powered by an external signal, which, in most cases is the carrier signal from the Antenna circuit. These receiver are fairly simple and are comprised of merely an L-C antenna (similar to the one shown in the block diagram below) and the circuitry necessary to modulate this carrier signal once powered on. The receiver communicate using magnetic coupling since their respective antennas can sense changes in magnetic field, which is observed as a change in voltage in the receiver circuit.
Specific Circuit Elements:
Transmit Stage: RF Choke and Power Amplifier:
The circuit of Figure 3 below is an RF choke followed by a current buffer and half-bridge amplifier. The RF choke is used to filter out most, if not all of the upper harmonic frequencies found in the square wave output from the MCU, leaving the fundamental frequency, 125 KHz, as a sine wave to be amplified. The square wave generator seen in the figure below is, in actuality, the output from the MCU and a set of inverters to ramp up the current. Diodes are used in the half bridge to help reduce crossover distortion caused from differing points of either transistor in the half bridge turning on and off. In our design we used the 2N3904 and 2N3906 NPN and PNP BJT transistors from the lab since they were cheap and convenient. In order to get better amplifier gain, and thus increase read range of our circuit, we could have used power MOSFETS instead for the half-bridge, but we found the BJT's gave us a mostly acceptable level of gain, especially once the circuit was tuned.
Resonant Antenna Circuit:
While this portion of our circuit is only comprised of two components, it is also arguably the most important hardware element; if it performs poorly then our security system performs poorly. Because this design was recommended for proximity solutions from the Microchip® guide, we decided to go with a series L-C resonant curcuit as opposed to one where the resistor and inductive antenna were in parallel.
[attachment=21552]
Before we start with actual circuit design, it is neccessary to understand the principals behind the technology that this project has set out to harness; passive wireless power communications. Passive wireless power receiver work in such a way that they are actually powered by an external signal, which, in most cases is the carrier signal from the Antenna circuit. These receiver are fairly simple and are comprised of merely an L-C antenna (similar to the one shown in the block diagram below) and the circuitry necessary to modulate this carrier signal once powered on. The receiver communicate using magnetic coupling since their respective antennas can sense changes in magnetic field, which is observed as a change in voltage in the receiver circuit.
Specific Circuit Elements:
Transmit Stage: RF Choke and Power Amplifier:
The circuit of Figure 3 below is an RF choke followed by a current buffer and half-bridge amplifier. The RF choke is used to filter out most, if not all of the upper harmonic frequencies found in the square wave output from the MCU, leaving the fundamental frequency, 125 KHz, as a sine wave to be amplified. The square wave generator seen in the figure below is, in actuality, the output from the MCU and a set of inverters to ramp up the current. Diodes are used in the half bridge to help reduce crossover distortion caused from differing points of either transistor in the half bridge turning on and off. In our design we used the 2N3904 and 2N3906 NPN and PNP BJT transistors from the lab since they were cheap and convenient. In order to get better amplifier gain, and thus increase read range of our circuit, we could have used power MOSFETS instead for the half-bridge, but we found the BJT's gave us a mostly acceptable level of gain, especially once the circuit was tuned.
Resonant Antenna Circuit:
While this portion of our circuit is only comprised of two components, it is also arguably the most important hardware element; if it performs poorly then our security system performs poorly. Because this design was recommended for proximity solutions from the Microchip® guide, we decided to go with a series L-C resonant curcuit as opposed to one where the resistor and inductive antenna were in parallel.