Digitally tunable 125kHz RFID circuit
I would like to be able to digitally tune the series capacitance to keep the resonance at 125kHz as the antenna's inductance varies due to proximity to metal surfaces (so I can read tags on laptops, phones etc).
I have measured antenna inductance varying from 350uH to 700uH. That implies that I need to be able to select between 2 to 4nF.
Any ideas on what I could use to switch in/out different parallel capacitances in a resonating circuit? I would like to be able to control it from the microcontroller.
Too bad you can't get nanofarad varicaps.
You can use MOSFET switches. As a first step, determine the required switch voltage rating (capacitor Vpp).
The EM4095 runs off 5V so it drives the antenna loop at -2.5 to 2.5V.
Yes, means what?
More exactly, the voltage swing at each push-pull antenna driver terminal is 5 Vpp. The voltage across the inductor and capacitor will be considerably higher due to resonance effect, depending on circuit Q. I guess at least several 10 Vpp, may be up to 100 Vpp and more.
Suitable switch transistors should be searched in the 100 to 300 V Vds range.
See page 13 in http://store.comet.bg/download-file.php?id=807
There is a schematic for changing the frequency with single transistor. You can add more transistors if you need to change the resonance in wider range.
I'm not sure if the suggested topology (switching an additional parallel capacitor to ground) will work well for the intended 2:1 variation range. But due to it's high gate impedance, MOSFET switches are also suitable as "floating" switches with slightly increased circuit complexity.
Ah yes I forgot about that. According to the calculations I made with a Q of 40 I get ~200V, does that sound right?
So something as simple as a MOSFET will do?
Thanks for finding that. I found a user guide for Atmel's U2270B evaluation board and saw it mentioned being able to change the frequency but couldn't find the schematic.