**Background**

As a fun / learning project I wanted to build a circuit which can tell me if a given RFID reader on a wall is 125Khz or 13.56Mhz ( by lighting LEDs).
My approach is to build two filters which lead to diode detectors - one filter + detector for 125khz, and one for 13.56Mhz.

I was hoping to use a length of wire or an unmeasured coil of wire as the input (pickup).

**Problem**

I can't figure out how to calculate values for the tuned circuit - I havent found any online calculator or sensible derivation (I don't understand the math/electronics enough to do it from first principles, I know Ohms law + basic AC theory + putting numbers into formulas).

Complication: I cant give an exact value of the 'drive' (input impedance?) of the input coil, nor can I work out the impedance of the diode envelope detector on the amps output. Can we just guess these?

**The whole circuit so far, including the 125Khz section**

[![Full circuit](https://i.stack.imgur.com/eXWz7.png)](https://i.stack.imgur.com/eXWz7.png)

Why cant HF be as easy as 125Khz? Cheap op-amps work at 125Khz.

**The tuned amplifier schematic so far**

[![Tuned amp](https://i.stack.imgur.com/UMfGm.png)](https://i.stack.imgur.com/UMfGm.png)