Bridged-Tee Attenuator Calculator
The bridged-tee attenuator is a useful, but (for "RF") rarely used circuit element. The principal benefit is the ability to vary the attenuation without affecting the circuit impedance. This is achieved by adjusting only two resistors, R2 and R3, whereas other circuits (namely the "tee" and "pi" attenuators) require at least 3 adjustments to be made.
Whilst "Tee" and "Pi" attenuators are often used for impedance matching, the bridged-tee is not used in this way. Source and load impedances (Zo) should be identical. R1a and R1b will be equal in value to Zo.