125KHz RFID emulator

Thread Starter


Joined 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?