Originally Posted by
edeca
Thanks for your reply - apologies it took a while to respond but I haven't had much time to experiment. Attached are some pictures as I can't include them yet.
The transponder is hanging from a piece of string 18 inches from the table, where a large pickup loop of stranded wire is fixed. A scope probe (with GND lead shorted to tip) is hung around the inductor.
The yellow trace is off the large pickup loop, fed through a simple RC filter. The blue trace is from the probe. I don't get quite as high Vpp from a distance but it seems fairly clean - and 18 inches is much further than I'd hope for.
Right now I'm just running some C code on the PIC to toggle periodically, I haven't tried to match it to the required 4 cycles yet. I'm using a 12F1840 with 4x PLL though so this wont be hard.
You can see that I have switched to using a slightly different inductor, I wrapped a number of turns around a ferrite rod. This measures around 3.5uH using an LCR meter and because of this my capacitors are much larger than yours. I tried this as I'm hoping to use it on karts rather than RC cars and this seems to be how the kart transponders work - I assume the magnetic permeability of the ferrite material helps it to propagate further. I know that the AMB kart transponders operate on a different frequency but as I said previously I am not worried about compatibility.
Does the scope screenshot look roughly correct to you? My next step is to build your loop amplifier and see if I can get it to work nicely.
The waveforms look very nice! I might suggest using a little lower Q so the waveform "rings up" a bit quicker, but it's probably not necessary.
The ferrite core increases the effective aperture of the transponder's "antenna" loop, and of course increases the inductance. I don't think it will be necessary for what you are doing, but if it intrigues you, go for it! Also, let us know if if you discover that there are other advantages to it as you experiment further.
If you don't need compatibility with AMB, then life will be much easier for you, as there won't be any need to replicate data trains from existing transponders.