100MHz noise filter doesn't attenuate much
Consider preventing noise pollution on the 12V supply in the first place.







simulate this circuit &ndash; Schematic created using CircuitLab
Keep the loop area of the noise as small as possible - ensuring C1 is (a) adequate for U2's HF current demand and (b) as close as possible to its supply pins. And ensure L1 (or ferrite bead and/or feedthrough cap) provides enough impedance at 100MHz to prevent noise reaching your 12V supply.

Of course you can add further decoupling on the 12V supply - L2 and its impedance at 100MHz form a voltage divider, attenuating the noise on U2's 12V input.