### Pi Attenuator
___a___ is attenuation in dB
___Zin___ is desired input impedance in Ω
___Zout___ is desired output impedance in Ω
___L = 10a/20___
___A = (L+1) / (L−1)___
___R2 = (L−1) / 2·√(Zin · Zout / L)___
___R1 = 1 / (A/Zin − 1/R2)___
___R3 = 1 / (A/Zout − 1/R2)___