Electronics > Beginners
NFC field generator
judge:
I knocked up a LTSPice model from the AO4294 datasheet and this web site, and the results of the simulation were pretty spectacular, so I need to make up a bread-board friendly adapter for it and try it out.
judge:
This is what I have so far. Naturally it doesn't work. If I connect the source resistor (R3) the signal at pin 8 of the 74LS00 disappears. If I increase the value of R3, the amplitude of the signal at pin 8 will increase too.
Before I abandon this and explore other oscillator circuits, I would really like to understand why connecting the source of the MOSFET has this effect.
PA0PBZ:
--- Quote from: judge on December 28, 2019, 03:50:07 pm ---I would really like to understand why connecting the source of the MOSFET has this effect.
--- End quote ---
The input capacitance of your mosfet is about 2.4nF or 5 Ohm at 13.5 MHz
judge:
On to a Pierce oscillator, or at least that what this is supposed to be. The oscillation dies down after about 10s, but until then I am getting good range from the coil (L1), with the LED tags lighting up at about 1cm from the coil.
The values of RFC, C2 and C3 are pure guesswork. 'RFC' is not really an RFC, it is just an inductor I happened to have lying around. C2 I set to 2x the load capacitance, minus a bit for stray capacitance. I also tried 1x the load capacitance and I also tried nothing - it doesn't make any difference. C3 was just a capacitor I happened to have lying around.
So I have a couple of questions:
* How do I calculate what values these things should have?
* Why do the oscillations die out?
judge:
I think the answer to (2) is that the mosfet overheats...
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