125KHz RFID emulator

Discussion in 'General Electronics Chat' started by jbillington, Mar 25, 2011.

  1. jbillington

    Thread Starter New Member

    Mar 25, 2011
    Let me start by giving some background into my experience. I can solder decently, route working PCBs in EAGLE, perform home-etching, source components, easily assemble kits, design basic circuitry, etc. I am lacking, however, in a proper electrical education and have been studying The Art of Electronics to gain a better mathematical and conceptual understanding of components.

    I recently decided that I wanted to test myself and build something to emulate a 125KHz RFID tag. It would require all of my skills and beyond to finish this, but I'm still confident I can do it. Please don't tell me about how much easier it would be to use a chip, I know, I know. I am looking to gain experience in basic RF design by starting from scratch. I have done a bunch of research and come up with the following subassemblies I need to create on the transmitting side:
    • square wave generator (f = 125KHz)
    • active bandpass filter (fo = 125KHz)
    • impedance matching amplifier
    • frequency-matching antenna

    The receiving side would involve another active filter and then an interface to a microcontroller, then the rest is software.

    What I've got so far is that I'll need to create the following:
    • 555 with a bypass diode to generate a 50% duty cycle square wave @ 125KHz
    • Sallen-Keys/Butterworth MFB active opamp filter using, for example, an MC33078P
    • push/pull mosfet pair as an antenna amplifier

    Only thing is, I've been trying a variety of circuits from a multitude of sources but have merely run into issues. For example, the 555 won't adjust with two pots as expected, the active filter's output is just a noisy 5v, and I have no clue how to get started with a push/pull mosfet pair.

    Anybody with some experience? My first goal is to at least create a carrier wave. Am I going about this in at least the right direction?