Hello everyone! Please help me to find out how to add a zero at higher frequency to this transfer function (the first one). One solution I found is to insert Rx in series with that capacitor but someone already did it I would like to discover new ways. If what you think of adds another pole too, it's not a problem unless it's at an even higher frequency like 2 decades after.

Hello,
I dont think you get another zero by placing a cap in series with the first resistor in the first diagram.
However, i think you do get another zero if you place a cap in parallel to either the first resistor or the third resistor.
I think the idea is to get it to behave more 'high pass like'.
Also note that either of those caps would 'bridge' either a source and cap or two caps so probably no extra pole.
We could verify with some formulas.
In the numerator, s*a+1 is one zero, (s*a+1)*(s*b+1) is a second zero, so you should get two factors like that in the numerator.
The transfer function of the original circuit is:
Vout/Vin=(s*C2*R3+1)/(s^2*C1*C2*R1*R3+s*C2*R3+s^2*C1*C2*R1*R2+s*C2*R2+s*C2*R1+s*C1*R1+1)
and note only one factor in the numerator.
We could go over this in detail if you like and change components and check.
And yes that extra resistor in series with the bottom (second) resistor creates another zero too.