Digitally tunable 125kHz RFID circuit

[Leith Bade]

I am building a 125kHz RFID reader using a EM4095 IC.

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.

 

[FvM]

You can use MOSFET switches. As a first step, determine the required switch voltage rating (capacitor Vpp).

 

[Leith Bade]

The EM4095 runs off 5V so it drives the antenna loop at -2.5 to 2.5V.

 

[FvM]

The EM4095 runs off 5V so it drives the antenna loop at -2.5 to 2.5V.

Yes, means what?

- - - Updated - - -

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.

 

[luben111]

See page 13 in https://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.

 

[FvM]

There is a schematic for changing the frequency with single transistor.

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.

 

[Leith Bade]

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.

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?

See page 13 in https://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.

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.