Electronic article surveillance (EAS) system receiver antenna.. mixer?

[nergal]

Hello,

I've been a lurker on this forum for quite some time, and a big fan of Dave's videos. I am coming to you now with my first question.

I've been working on reverse engineering the receiving block of an 8.2Mhz EAS system working with soft tags.

I am having a problem understanding how the small module below functions. (photo and circuit attached below)

The idea is this:

I have one coil that emits discreet frequencies around 8.2Mhz. And 2 coils that check if there is something resonating in the proximity (soft tag or hard tag). The two signals from the receiving coils are mixed with the circuit above into one signal, but i do not understand how it is done. The two coils are connected to the 4 pin terminal and the two pin terminal is sent directly to the sensor board. (everything after R33 is on the receiver board)

I measured the values of the coils with a network analyzer and from my calculations they do not resonate even close to the emitted freq (8.2Mhz). So I believe there is something more subtle going on but cant put my finger on it. The transformers are 1:1 from the measurements. The values are actual values on the board.


This being said I am happy to hear your opinions, and sorry for the English mistakes, I am not a native speaker.